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Unsignalized Intersection Safety StrategiesU.S. Department of Transportation July 2008Slide 1Unsignalized Intersection Safety StrategiesNCHRP Report 500, Volume 5: A Guide for Addressing Unsignalized Intersection Collisions[Speaker Name Goes Here]
Slide 2NCHRP Report 500
Slide Notes Slide 3Explanation of Time, Cost, and Effectiveness
Slide Notes This slide contains a brief explanation of the time frame, cost estimate, and effectiveness terms used on the countermeasure slides. Slide 4Strategy Categories
Slide Notes These are the 9 strategic categories for addressing collisions at unsignalized intersections. You can click on any of them to hyperlink to that portion of the file. Or you can simply go through one by one. Each slide has a "home" icon in the bottom right corner of the slide that will bring you back to this slide. [Home] Slide 5Implement Driveway Closures/RelocationsNCHRP Report 500 • Vol. 5 • Strategy A1
Where to use:Unsignalized intersections with high crash frequencies related to driveways adjacent to the intersection. Generally, driveways within 250 feet of the intersection are the greatest concern. Key to success:Work with owners of properties to assure them that some restriction of access to their properties will improve safety and will not affect their ability (or their customers') to reach their properties. Where practical, this strategy should be implemented as part of a comprehensive corridor access management plan. Cost: Moderate (1 on a scale of 1 to 4) Timeframe: Medium(1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Effective access management is key to improving safety at and adjacent to unsignalized intersections. Highway agencies are increasingly using access management techniques on urban and suburban arterials. A key element of access management is closure or relocation of driveways adjacent to intersections. Access points within 250 feet upstream and downstream of an intersection are generally undesirable. Strategies for mitigating safety problems that may arise from a driveway located too close to an unsignalized intersection are to close the driveway (if other access to the adjacent property already exists) or to relocate the driveway (if no other appropriate access is available). It is desirable to relocate access points from the major road to the minor road (away from the intersection), or (where practical) to another street or frontage road. Where there is access from the minor road, a side street, or a frontage road, relocating the driveway to the major road farther from the intersection may be considered. KEY TO SUCCESS Agencies should work with owners of adjacent properties to assure them that some restriction of access to their properties will improve safety and will not affect their ability (or, in the case of a retail business, their customers' ability) to reach their properties. Where practical, these strategies should be implemented as part of a comprehensive corridor access management plan. ISSUES Access restrictions could cause some owners of retail businesses to lose (or to think they will lose) customers. This is highly dependent on the type of business and the nature of the access restriction. Such impacts need to be carefully considered by highway agencies before implementing this strategy. It is advisable to involve stakeholders at the early stages of planning for these improvements. TIME FRAME: Medium Implementation of driveway closures and relocations can require 3 months to 3 years. While an extensive project development process usually is not required, discussions with affected property owners must be carried out to reach agreement on access provisions. Essential aspects of such an agreement may include driveway permits, easements, and driveway-sharing agreements. Where agreement cannot be reached, the highway agency may choose to initiate legal proceedings to modify access rights; such contested solutions are undesirable and require considerable time to resolve. COSTS: Moderate Costs are highly variable. These costs mostly involve acquiring access or constructing replacement access. EFFECTIVENESS TRIED: The strategy of closing or relocating driveways adjacent to intersections is considered effective and has been addressed in published literature, but there is no consensus on quantitative estimates of its effectiveness. The safety effectiveness of this strategy is highly site dependent and will vary with the driveway location relative to the intersection before and after the project, the traffic volume using the driveway, the traffic volume and speed on the relevant intersection approaches, and the type of development served by the driveway. Some of the states that have implemented access management policies include: Iowa, Minnesota, and Florida. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections and, in particular, those strategies discussed in A2 (Driveway Turn Restrictions). SUPPLEMENTAL INFORMATION Highway agencies should establish formal access management policies to guide the planning and permitting process and to provide a basis for remedial treatments at existing locations where driveway-related safety problems occur. For more information on access management, visit: www.accessmanagement.gov. [Home] Slide 6Implement Driveway Turn RestrictionsNCHRP Report 500 • Vol. 5 • Strategy A2
Where to use:Driveways located near unsignalized intersections that experience high crash frequencies but that cannot practically be closed or relocated. Key to success:Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS When a driveway on a high-volume street adjacent to an unsignalized intersection cannot be closed or relocated, it may be appropriate to restrict turning maneuvers at the driveway. For example, left turns at the driveway can be restricted and driveway movements limited to right turns in and right turns out. In other cases, turning movements into a property may be permitted at a particular driveway, but turning movements out of the property may be diverted to a different driveway. Furthermore, driveway usage may be restricted at particularly critical times of the day. Such restrictions can be implemented by signing, channelizing islands, redesigning the internal circulation patterns within a property, installing a median on the major street, or a combination of these approaches. KEY TO SUCCESS Agencies should work with owners of adjacent properties to assure them that some restriction of access to their properties will improve safety and will not affect their ability (or, in the case of a retail business, their customers' ability) to reach their properties. Where practical, these strategies should be implemented as part of a comprehensive corridor access management plan. ISSUES Access restrictions could cause some owners of retail businesses to lose (or think they will lose) customers. This is highly dependent upon the type of business and the nature of the access restriction. Such impacts need to be carefully considered by highway agencies in implementing projects. It is advisable to involve stakeholders at the early stages of planning for these improvements. TIME FRAME: Short The time to implement this strategy can range from 3 months to 4 years. Turn restrictions implemented by signing alone can be implemented very quickly when the adjacent property owner is agreeable. Where changes in driveway channelization or internal circulation patterns are involved or where the property owner does not agree with the proposed change, additional time may be required. Where a median is to be installed on the major street, particularly if right-of-way acquisition is required, up to 4 years may be required for the project development process and construction of the improvement. COSTS: Low Costs may be highly variable. Note that compensation is generally not owed to property owners for loss of direct left-turn access. EFFECTIVENESS TRIED: Further evaluations are needed to quantify the safety effectiveness of this strategy. Some of the states that have implemented access management policies include: Iowa, Minnesota, and Florida. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections and, in particular, those strategies discussed in A1 (Driveway Closures/Relocations). SUPPLEMENTAL INFORMATION Highway agencies should establish formal access management policies to guide the planning and permitting process and to provide a basis for remedial treatments at existing locations where driveway-related safety problems occur. For more information on access management, visit: www.accessmanagement.gov. [Home] Slide 7Provide Left-Turn Lanes at IntersectionsNCHRP Report 500 • Vol. 5 • Strategy B1
Where to use:Unsignalized intersections with a high frequency of crashes resulting from the conflict between (1) vehicles turning left and following vehicles and (2) vehicles turning left and opposing through vehicles. Key to success:Make sure that any left-turn lane considered is operationally warranted based on traffic volumes or justified on the basis of an existing pattern of left-turn collisions. Cost: Moderate (2 on a scale of 1 to 4) Timeframe: Medium (2 on a scale of 1 to 3) Effectiveness: Proven Slide Notes DETAILS Many crashes at unsignalized intersections are related to left-turn maneuvers. Left-turn lanes remove vehicles waiting to turn left from the through-traffic stream, thus reducing the potential for rear-end crashes. Because they provide a sheltered location for drivers to wait for a gap in opposing traffic, left-turn lanes may encourage drivers to be more selective in choosing a gap to complete the left turn maneuver. This may reduce the potential for crashes between left-turn and opposing through vehicles. KEY TO SUCCESS Make sure that any left-turn lane considered is operationally warranted based on traffic volumes or justified on the basis of an existing pattern of left-turn collisions. ISSUES Sight Distance. In providing left-turn lanes, vehicles in opposing left-turn lanes may block their respective driver's view of approaching vehicles in the through lanes. This potential problem can be resolved by offsetting the left-turn lanes (see Strategy B3 fact sheet). Roadway Design. If the shoulder and through lanes are restriped to make provision for a left-turn lane, part of the safety benefits may be lost due to the loss of shoulder, the greater proximity of traffic to roadside objects, and, possibly, a reduction in intersection sight distance. Turn Restrictions. Provision of a left-turn lane on an intersection approach may involve restricting left turns in and out of driveways on that intersection approach. Such restrictions may be implemented by signing or provision of a median adjacent to the left-turn lane. Pedestrians. When installation of left-turn lanes increases the overall width of the intersection, the additional width may cause problems for pedestrians crossing the intersection. One possible solution to this problem is to provide a pedestrian refuge island in the median. TIME FRAME: Medium Implementation time may vary from 3 months to 4 years. At some locations, left-turn lanes can be quickly installed simply by restriping the roadway. At other locations, widening the roadway, installing a median, or acquiring additional right-of-way may be needed. Where right-of-way is required or where the environmental process requires analysis and documentation, project development and implementation may require as long as 4 years. COSTS: Moderate Costs are highly variable. Where restriping within an existing roadway is possible, the costs are nominal. Where widening and/or reconstruction are necessary, costs will be significantly higher. EFFECTIVENESS PROVEN: Research has determined that installation of a single left-turn lane on a major road approach would be expected to reduce total intersection crashes at rural unsignalized intersections by 28% for four-legged intersections and by 44 % for three-legged intersections. At urban unsignalized intersections, installation of a left-turn lane on one approach would be expected to reduce total crashes by 27 % for four-legged intersections and by 33 % for three-legged intersections. Installation of left-turn lanes on both major road approaches to a four-legged intersection would be expected to increase, but not quite double, the resulting effectiveness measures for total intersection crashes. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections. SUPPLEMENTAL INFORMATION Optimal operation and safety of left-turn lanes require appropriate design. This includes sufficient length of lane and taper (see Strategy B2 fact sheet). [Home] Slide 8Provide Longer Left-Turn Lanes at IntersectionsNCHRP Report 500 • Vol. 5 • Strategy B2
Where to use:Unsignalized intersections with existing left-turn lanes that are not long enough to store all left turning vehicles and have a high frequency of rear-end crashes resulting from the conflict between vehicles waiting to turn left and following vehicles. Key to success:Make sure that a longer left-turn lane is warranted or justified on the basis of left-turn volumes or an existing pattern of left-turn related rear-end crashes. Cost: Moderate (2 on a scale of 1 to 4) Timeframe: Medium (2 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS The length of a left-turn lane is among its most important design elements. Left-turn lanes should be designed to accommodate vehicle deceleration and storage. In particular, the left-turn lane length should allow for the removal of slow or decelerating vehicles from through-traffic, thus reducing the potential for rear-end collisions. The length of a left-turn lane consists of three components: (1) entering taper, (2) deceleration length, and (3) storage length. Design criteria for selecting an appropriate left-turn lane length are presented in the AASHTO Policy on Geometric Design for Highways and Streets and in the policies of individual highway agencies. KEY TO SUCCESS Make sure that a longer left-turn lane is warranted or justified on the basis of left-turn volumes or an existing pattern of left-turn related rear-end crashes. ISSUES If a left-turn lane is excessively long, drivers proceeding through the intersection may enter the lane by mistake without realizing that it is a left-turn lane. This difficulty may be remedied by effective signing, marking, and/or median geometrics at the upstream end of the left-turn lane. Also, if a decision is made to provide a longer left-turn lane by restriping a shoulder and through lane, part of the safety benefits from the improvement may be lost because of the loss of shoulder and the greater proximity of through or right-turning traffic to roadside objects, and possibly because of a reduction in intersection sight distance. Lengthening of a left-turn lane on an intersection approach may involve restricting left turns in and out of driveways on that intersection approach. Such restrictions may be implemented by signing or by provision of a raised median adjacent to the left-turn lane (see Strategy A2 fact sheet). TIME FRAME: Medium Implementation time may vary from 3 months to 4 years. At some locations, left-turn lanes can be lengthened simply by restriping the roadway. Others may require widening the roadway, cutting further into a median, or acquiring additional right-of-way. Such projects require a substantial time for development and construction. Where right-of-way is required or where the environmental process requires analysis and documentation, the time will be longer. COSTS: Moderate Costs are highly variable. Where restriping within an existing roadway is possible, the costs are nominal. Where widening and/or reconstruction are necessary, costs over $100,000 per intersection approach may be incurred. EFFECTIVENESS TRIED: This strategy will reduce rear-end collisions resulting from conflicts between vehicles waiting to turn left and following vehicles during periods when the left-turn demand exceeds the existing storage capacity of the left-turn lane. When a queue of vehicles overflows the left-turn lane and extends into the through lanes of the intersection approach, rear-end collisions are likely. Such overflows may also result in operational delays to through or right-turning vehicles. Lengthening of left-turn lanes may also reduce the potential for rear-end collisions between left-turning vehicles by providing longer entering taper and deceleration lengths. There is no consensus on a quantitative estimate of the safety effectiveness of lengthening left-turn lanes. This effectiveness is likely to depend on the existing length of the left-turn lane, the proportion of time during which the storage capacity of the left-turn lane is exceeded, the volume and speed of traffic on the intersection approach, and the available sight distance upstream of the left-turn queue. Further research to quantify the safety effectiveness of lengthening left-turn lanes is needed. COMPATIBILITY This strategy can be used in conjunction with other strategies for improving safety at unsignalized intersections. SUPPLEMENTAL INFORMATION Optimal operation and safety of left-turn lanes require appropriate design. This includes sufficient length of lane and taper. [Home] Slide 9Provide Offset Left-Turn Lanes at IntersectionsNCHRP Report 500 • Vol. 5 • Strategy B3
Where to use:Unsignalized intersections with a high frequency of crashes between vehicles turning left and opposing through vehicles, as well as rear-end crashes between through vehicles on the opposing approach. Also at intersections on divided highways with medians wide enough to provide the appropriate offset but can be implemented on approaches without medians if sufficient width exists. Key to success:Identify candidate locations where opposing left-turn vehicles block drivers' views of approaching traffic. This can be determined by measuring the amount of offset (or lack of offset) present at existing intersections. Any intersection with a pattern of crashes between left-turning vehicles and opposing through vehicles that has existing left-turn lanes (or where installation of left-turn lanes is being considered) should be checked to determine the amount of available offset. Cost: Moderate to High (3 on a scale of 1 to 4) Timeframe: Medium (2 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS A potential problem in installing left-turn lanes at intersections is that vehicles in opposing turn lanes on the major road may block drivers' views of approaching traffic. This can lead to crashes between vehicles turning left from the major road and through vehicles on the opposing major road approach. To reduce the potential for crashes of this type, the left-turn lanes can be offset by moving them laterally so that vehicles in opposing lanes no longer obstruct the opposing driver. Two treatments for offsetting turn lanes are parallel and tapered offset left-turn lanes. These treatments have been evaluated in research and are addressed in the AASHTO Policy on Geometric Design of Highways and Streets. While offset left-turn lanes have been used most extensively at signalized intersections, they are suitable for use at unsignalized intersections. KEY TO SUCCESS Identify candidate locations where opposing left-turn vehicles block drivers' views of approaching traffic. This can be determined by measuring the amount of offset (or lack of offset) present at existing intersections. Any intersection with a pattern of crashes between left-turning vehicles and opposing through vehicles that has existing left-turn lanes (or where installation of left-turn lanes is being considered) should be checked to determine the amount of available offset. ISSUES A potential pitfall of installing offset left-turn lanes is that drivers initially may be confused by the change in traffic patterns, particularly in areas where offset left-turn lanes are not prevalent. This can be minimized by effective use of advance guide signing and pavement markings. Research has verified that, in areas where drivers have become familiar with offset left-turn lanes, they operate effectively. When installation of offset left-turn lanes increases the overall width of the intersection, the additional width may cause potential problems for pedestrians crossing the intersection. One possible solution to this problem is to provide a refuge island in the median for pedestrians. TIME FRAME: Medium The implementation period for provision of offset left-turn lanes is 2 to 4 years. Intersections at which offset left-turn lanes can be provided simply by restriping the roadway are relatively rare. Therefore, time for project development and construction is required. Where a wide median is available, offset left-turn lanes can usually be provided without purchasing additional right-of-way; in such cases, implementation in 2 years may be possible. If the median must be widened, additional right-of-way may be needed and there may be substantial social and environmental impacts that need to be evaluated; in such cases, the implementation may take up to 4 years. The implementation period can be reduced when an agency adopts this design by policy and implements it on projects in preliminary or final design. COSTS: Moderate to High Costs may be highly variable and depend largely on the existing median width. EFFECTIVENESS TRIED: Research has verified that offset left-turn lanes operate safely, but there are no reliable estimates of their safety effectiveness. Safety effectiveness is likely to depend upon the traffic volumes of the conflicting turning and through movements and the amount of offset between the left-turn lanes at the intersection. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections. [Home] Slide 10Provide Bypass Lanes on Shoulders at T-IntersectionsNCHRP Report 500 • Vol. 5 • Strategy B4
Where to use:At three-legged unsignalized intersections on two-lane highways with moderate through and turning volumes, especially intersections that have a pattern of rear-end collisions involving vehicles waiting to turn left from the highway. Key to success:Provide a shoulder area for the bypass lane that has sufficient structural strength to withstand repeated usage, even by trucks. Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS At three-legged intersections on two-lane highways, shoulder bypass lanes can provide an effective substitute for a left-turn lane on the major road where provision of a left-turn lane is economically infeasible. Instead of providing a left-turn lane for drivers turning left from the major road, part of the shoulder may be marked as a travel lane to encourage following through drivers to use this shoulder lane to bypass vehicles waiting to turn left. This treatment involves substantially less cost than providing a conventional left-turn lane, and, at low-volume intersections, it may be just as effective. KEY TO SUCCESS Provide a shoulder area for the bypass lane that has sufficient structural strength to withstand repeated usage, even by trucks. ISSUES There may be an upper limit of traffic volumes above which shoulder bypass lanes should not be used. No such limit has been quantified, but highway agencies should still carefully consider the appropriateness of shoulder bypass lanes on high-volume two-lane roads. Shoulder bypass lanes should not be viewed as a substitute for conventional left-turn lanes as part of a reconstruction or major redesign project where right-of-way is available and construction is feasible. TIME FRAME: Short This strategy can be implemented within 3 months at locations with an existing paved shoulder. Some locations may need only pavement marking and signing changes. Paving an unpaved shoulder or strengthening a paved shoulder may take longer. In rare cases where acquisition of right-of-way is needed, a project development process of up to 4 years may be required. COSTS: Low Costs should be relatively low since little to no additional right-of-way is necessary for this strategy. Construction involves paving and marking a portion of the existing shoulder. EFFECTIVENESS TRIED: Minnesota evaluated the operational and safety effects of using bypass lanes at rural intersections by comparing the operational and safety characteristics of rural intersections without turning lanes, with bypass lanes, and with left-turn lanes. Based upon a comparative crash analysis and a before-after evaluation, Minnesota was unable to conclude that the use of a bypass lane provides a greater degree of safety when compared to intersections without a bypass lane or a left-turn lane. However, Nebraska has reported a marked decrease in rear-end collisions at shoulder bypass lanes, and other states have reported relatively few crashes occurring at shoulder bypass lane installations. A Florida study concluded that left-turn injury crashes were reduced up to 36% and rear-end injury crashes were reduced 24%. Property damage only crashes were also reduced up to 28% and 53% for left-turn and rear-end crashes, respectively. COMPATIBILITY This strategy can be used in conjunction with most others for improving safety at unsignalized intersections. It is, however, an alternative to providing a left-turn lane. [Home] Slide 11Provide Left-Turn Acceleration Lanes at Divided Highway IntersectionsNCHRP Report 500 • Vol. 5 • Strategy B5
Where to use:Unsignalized intersections on divided highways that experience a high proportion of rear-end crashes related to the speed differential caused by vehicles turning left onto the highway. Also where intersection sight distance is inadequate or where there are high volumes of trucks or recreational vehicles entering the divided highway. Key to success:Make sure that the acceleration lanes are operationally warranted by relatively high left-turn volumes, justified on the basis of an existing pattern of rear-end or sideswipe crashes related to left-turn maneuvers, or justified by having a high percentage of vehicles requiring more time to accelerate. Another key is appropriate design of the median opening area to minimize conflicts between vehicles entering the left-turn acceleration lane and other vehicles using the median opening. Cost: Moderate (2 on a scale of 1 to 4) Timeframe: Medium (2 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Drivers turning onto a highway accelerate until the desired highway speed is reached. When acceleration by entering traffic takes place directly on the traveled way, it may disrupt the flow of through-traffic. To minimize this operational problem due to left-turning traffic at divided highway intersections, median acceleration lanes may be used. An acceleration lane is an auxiliary or speed change lane that allows vehicles to accelerate to highway speeds before entering the through-traffic lanes of a highway. Acceleration lanes should be of sufficient length to permit adjustments in speeds of both through and entering vehicles so that the driver of the entering vehicle can safely maneuver into a gap before reaching the end of the acceleration lane. KEY TO SUCCESS Make sure that the acceleration lanes are operationally warranted by relatively high left-turn volumes, justified on the basis of an existing pattern of rear-end or sideswipe crashes related to left-turn maneuvers, or justified by having a high percentage of vehicles requiring more time to accelerate, such as trucks or recreational vehicles. Another key to success is appropriate design of the median opening area to minimize conflicts between vehicles entering the left-turn acceleration lane and other through and turning vehicles using the median opening. ISSUES If a left-turn acceleration lane is excessively long or poorly marked, through drivers may mistake it for an additional through lane. There is little guidance available on the best geometric design for median acceleration lanes. Both parallel and tapered acceleration-lane designs have been used. The AASHTO Policy on Geometric Design for Highways and Streets provides guidance on the design of acceleration lanes for freeway entrance ramps, but there is no specific design guidance for acceleration lanes at divided highway intersections. When installation of left-turn acceleration lanes increases the overall width of the intersection, the additional width may cause potential problems for pedestrians crossing the intersection. One possible solution to this problem is to provide a pedestrian refuge island in the median. TIME FRAME: Medium Implementation time of left-turn acceleration lanes at divided highway intersections may vary from 3 months to 4 years. At some locations, left-turn acceleration lanes can be constructed simply by restriping the roadway. At other locations, widening the roadway, cutting further into a median, or acquiring additional right-of-way may be needed. Such projects may require a substantial time for development and construction. COSTS: Moderate Costs are highly variable. Where sufficient median width to provide a left-turn acceleration lane is available, it may be possible to provide a median acceleration lane at moderate cost. Where additional right-of-way must be acquired, higher costs are likely. EFFECTIVENESS TRIED: By removing the slower accelerating left-turning vehicles from the through lanes, this strategy is expected to reduce rear-end and sideswipe crashes resulting from conflicts between vehicles turning left onto the highway and through vehicles on the highway. Research has shown that left turn acceleration lanes at divided highway intersections function effectively and do not create safety problems. However, no quantitative estimates of the safety effectiveness of left-turn acceleration lanes at divided highway intersections are available. COMPATIBILITY This strategy can be used in conjunction with other strategies for improving safety at unsignalized intersections. [Home] Slide 12Provide Right-Turn Lanes at IntersectionsNCHRP Report 500 • Vol. 5 • Strategy B6
Where to use:Unsignalized intersections with a high frequency of rear-end crashes resulting from conflicts between (1) vehicles turning right and following vehicles and (2) vehicles turning right and through vehicles coming from the left on the cross street. Key to success:Make sure that any right-turn lane considered is operationally justified on the basis of right-turning volumes or an existing pattern of right-turn related crashes. At some locations, it may be desirable to create a right-turn roadway by a channelizing island on the intersection approach. This allows the turning radius to be increased without introducing a large unused pavement area that might lead to operational problems. The right-turn lane may be controlled by a yield sign where the roadway enters the intersecting street or may operate as a free-flow roadway. Cost: Moderate (2 on a scale of 1 to 4) Timeframe: Medium (2 on a scale of 1 to 3) Effectiveness: Proven Slide Notes DETAILS Many crashes at unsignalized intersections are related to right-turn maneuvers. Right-turn lanes remove slow vehicles that are decelerating to turn right from the through-traffic stream, thus reducing the potential for rear-end crashes. KEY TO SUCCESS Make sure that any right-turn lane considered is operationally justified on the basis of right-turning volumes or an existing pattern of right-turn related crashes. At some locations, it may be desirable to create a right-turn roadway by a channelizing island on the intersection approach. This allows the turning radius to be increased without introducing a large unused pavement area that might lead to operational problems. The right-turn lane may be controlled by a yield sign where the roadway enters the intersecting street or may operate as a free-flow roadway where a right-turn acceleration lane is provided on the intersecting street (see Strategy B9). ISSUES One of the potential problems with installing a right-turn lane may occur in the design stage of this strategy. If, for example, a decision is made to restripe a shoulder and through lane to provide a right-turn lane, part of the safety benefits may be lost due to the loss of shoulder and the greater proximity of traffic to roadside objects. Vehicles using a major road right-turn lane may obstruct the sight lines of drivers on the minor road approach. Similarly, addition of the right-turn lane may be accompanied by shifting of the minor road stop bar. Care should be taken to ensure that the sight triangle remains clear of obstructions on the stopped approach. When installation of right-turn lanes increases the overall width of the intersection, the additional width may cause potential problems for pedestrians crossing the intersection. One possible solution to this problem is to provide a pedestrian refuge island in the median. If the right-turn lane is designed as a free-flow lane, it may cause safety problems for pedestrians and bicyclists. TIME FRAME: Medium Implementing this strategy may take from 3 months to 4 years. At some locations, right-turn lanes can be quickly and simply installed by restriping the roadway. At other locations, widening of the roadway or acquisition of additional right-of-way may be needed. Such projects require a substantial time for development and construction. Where right-of-way is required or where the environmental process requires analysis and documentation, project development and implementation may require as long as 4 years. COSTS: Moderate Costs are highly variable. Where restriping within an existing roadway is possible, the costs are nominal. Where widening and/or reconstruction are necessary, costs may be significantly higher. EFFECTIVENESS Research found that added right-turn lanes are effective in improving safety at rural unsignalized intersections. Installation of a single right-turn lane on a rural major road approach would be expected to reduce total intersection crashes by 14%. Right-turn lane installation reduced crashes on individual approaches to four-legged rural unsignalized intersections by 27%. COMPATIBILITY This strategy can be used in conjunction with other strategies for improving safety at unsignalized intersections. [Home] Slide 13Provide Longer Right-Turn Lanes at IntersectionsNCHRP Report 500 • Vol. 5 • Strategy B7
Where to use:Unsignalized intersections with an existing right-turn lane that is not long enough to store all right-turning vehicles and that are experiencing a high frequency of rear-end crashes resulting from the conflict between vehicles waiting to turn right and following vehicles. Key to success:Make sure that a longer right-turn lane is warranted or justified on the basis of right-turn volumes or an existing pattern of right-turn crashes. If access to adjacent properties will potentially be affected, it will be important to include the stakeholders early in the planning process. Cost: Moderate (2 on a scale of 1 to 4) Timeframe: Medium (2 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS The provision of exclusive right-turn lanes minimizes crashes related to right-turn maneuvers, particularly on high-volume and high-speed major roads. However, if the length of a right-turn lane is inadequate, vehicles waiting to turn may be doing so from the through-traffic lane, thus increasing the potential for rear-end crashes. If long enough, right-turn lanes provide sheltered locations for drivers decelerating or waiting to make a right-turn maneuver. Design criteria for selecting an appropriate right-turn lane length are presented in the AASHTO Policy on Geometric Design for Highways and Streets and in the policies of individual highway agencies. KEY TO SUCCESS Make sure that a longer right-turn lane is warranted or justified on the basis of right-turn volumes or an existing pattern of right-turn crashes. If access to adjacent properties will potentially be affected, it will be important to include the stakeholders early in the planning process. ISSUES If a right-turn lane is excessively long, through drivers may enter the lane by mistake without realizing it is a right-turn lane. Effective signing and marking of the upstream end of the right-turn lane may remedy this difficulty. Also, a decision may be made to provide a longer right-turn lane by restriping a shoulder and through lane. In such cases, part of the safety benefits from the improvement may be lost due to the loss of shoulder, the greater proximity of through or right-turning traffic to roadside objects, and possible a reduction in intersection sight distance, as well. Lengthening of a right-turn lane on an intersection approach may involve restricting right turns in and out of driveways on that intersection approach. Such restrictions may be implemented by signing or by provision of a median. Approaches to dealing with such issues are discussed in connection with Strategy A2. TIME FRAME: Medium Implementation may require from 3 months to 4 years. At some locations, right-turn lanes can be lengthened simply by restriping the roadway. Others may require widening the roadway, cutting further into a median, or acquiring additional right-of-way. Such projects require a substantial time for development and construction. COSTS: Moderate Costs are highly variable. Where restriping within an existing roadway is possible, the costs are nominal. Where widening and/or reconstruction are necessary, costs may be significantly higher. EFFECTIVENESS TRIED: This strategy should reduce rear-end crashes resulting from the conflict between vehicles waiting to turn right and following vehicles during the period when demand exceeds the storage capacity of the lane. When a queue of vehicles overflows the right-turn lane and extends into the through lanes of the intersection approach, rear-end crashes are likely. Lengthening of right-turn lanes may also reduce the potential for rear-end collisions between right-turning vehicles by providing longer entering taper and deceleration lengths. While there is no consensus on a quantitative estimate of the safety effectiveness of lengthening right turn lanes, one study indicated that crashes could be reduced up to 15%. This effectiveness is likely to depend on the existing length of the right-turn lane, the proportion of time during which the storage capacity of the lane is exceeded, the volume and speed of traffic on the intersection approach, and the available sight distance to the rear of the right-turn queue. COMPATIBILITY This strategy can be used in conjunction with other strategies for improving safety at unsignalized intersections. SUPPLEMENTAL INFORMATION Optimal operation and safety of right-turn lanes require appropriate design. This includes sufficient length of lane and taper. [Home] Slide 14Provide Offset Right-Turn Lanes at IntersectionsNCHRP Report 500 • Vol. 5 • Strategy B8
Where to use:Unsignalized intersections with a high frequency of crashes between vehicles on the minor road that are turning left, turning right, or proceeding straight through, and vehicles on the major road. Key to success:Identify candidate locations where right-turn vehicles block drivers' views of approaching traffic. Any intersection with a pattern of crashes between minor-road vehicles and major-road vehicles with existing right-turn lanes (or where installation of right-turn lanes is being considered) should be checked to determine the amount of available offset. Cost: Moderate to High (3 on a scale of 1 to 4) Timeframe: Medium (2 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS A potential problem in installing right-turn lanes at intersections is that vehicles in the right-turn lane on the major road may block the minor-road drivers' views of traffic approaching on the major road. This can lead to crashes between vehicles turning left, turning right, or crossing from the minor road and through vehicles on the major road. To reduce the potential for crashes of this type, the right turn lanes can be offset by moving them laterally so that vehicles in the right-turn lanes no longer obstruct the view of the minor road driver. KEY TO SUCCESS Identify candidate locations where right-turn vehicles block drivers' views of approaching traffic. Any intersection with a pattern of crashes between minor-road vehicles and major-road vehicles with existing right-turn lanes (or where installation of right-turn lanes is being considered) should be checked to determine the amount of available offset. ISSUES A potential pitfall of installing offset right-turn lanes is that drivers initially may be confused by the change in traffic patterns, particularly in areas where offset right-turn lanes have not been used previously. This can be minimized by effective use of advance guide signing and pavement markings. Installation of offset right-turn lanes increases the overall width of the intersection. Therefore, the additional width may cause potential problems for pedestrians crossing the intersection. A possible solution to this problem would be to provide a pedestrian refuge island between the offset right turn lane and through lanes. However, it is not advisable to use raised islands on high speed approaches. TIME FRAME: Medium Intersections where offset right-turn lanes can be provided simply by restriping the roadway are relatively rare. Therefore, time for project development and construction is required. Where an existing right-turn lane and wide shoulder are present, offset right-turn lanes can usually be provided without purchasing additional right-of-way; in such cases, implementation in 2 years may be possible. If additional right-of-way is needed and substantial social and environmental impacts need to be evaluated, the implementation may take up to 4 years. The implementation period can be reduced when an agency adopts this design by policy and implements it on projects in preliminary or final design. COSTS: Moderate to High Costs may be highly variable and depend on right-of-way needs. EFFECTIVENESS TRIED: No research has been conducted on offset right-turn lanes to determine their safety effectiveness. Safety effectiveness is likely to depend upon the traffic volumes of the conflicting turning and through movements and the amount of offset between the right-turn lanes at the intersection. COMPATIBILITY This strategy can be used in conjunction with other strategies for improving safety at unsignalized intersections. SUPPLEMENTAL INFORMATION There is no formal policy on the use of offset right-turn lanes in the AASHTO Policy on Geometric Design of Highways and Streets. Highway agencies may consider incorporating a policy in their own design manuals and guides. [Home] Slide 15Provide Right-Turn Acceleration Lanes at IntersectionsNCHRP Report 500 • Vol. 5 • Strategy B9
Where to use:Unsignalized intersections that experience a high proportion of rear-end and/or sideswipe crashes related to the speed differential caused by vehicles making a right-turn maneuver onto the highway. Key to success:Make sure that right-turn acceleration lanes are operationally warranted by relatively high right-turn volumes or justified on the basis of an existing pattern of rear-end or sideswipe crashes related to right-turn maneuvers. Cost: Moderate (2 on a scale of 1 to 4) Timeframe: Medium (2 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Drivers turning onto an uncongested highway accelerate until the desired open-road speed is reached. When acceleration by entering traffic takes place directly on the traveled way, it may disrupt the flow of through-traffic. To minimize this operational problem due to right-turning traffic at divided highway intersections, right-turn acceleration lanes may be used. An acceleration lane is an auxiliary or speed-change lane that allows vehicles to accelerate to highway speeds before entering the through-traffic lanes of a highway. Acceleration lanes should be of sufficient length to permit adjustments in speeds of both through and entering vehicles so that the driver of the entering vehicle can position the vehicle opposite a gap in the through-traffic stream and maneuver into that gap before reaching the end of the acceleration lane. KEY TO SUCCESS Make sure that right-turn acceleration lanes are operationally warranted by relatively high right-turn volumes or justified on the basis of an existing pattern of rear-end or sideswipe crashes related to right-turn maneuvers. ISSUES If a right-turn acceleration lane is excessively long or poorly marked, through drivers may mistake it for an additional through lane. There is little guidance available on the best geometric design for right-turn acceleration lanes. Both parallel and tapered acceleration-lane designs have been used. The AASHTO Policy on Geometric Design for Highways and Streets provides guidance on the design of acceleration lanes for freeway entrance ramps, but there is no specific design guidance for acceleration lanes at intersections. Installation of right-turn acceleration lanes increases the overall width of the intersection. Therefore, the additional width may cause potential problems for pedestrians crossing the intersection. One possible solution to this problem is to provide a pedestrian refuge island in the median. TIME FRAME: Medium Time for implementation of right-turn acceleration lanes at intersections may vary from 3 months to 4 years. At some locations, right-turn acceleration lanes can be constructed simply by restriping the roadway. At other locations, widening the roadway, cutting further into a shoulder, or acquiring additional right-of-way may be needed. Such projects may require a substantial time for development and construction. Where additional right-of-way is required or where the environmental process requires analysis and documentation, project implementation may take up to 4 years. COSTS: Moderate Costs are highly variable. Where sufficient roadway or shoulder width to provide a right-turn acceleration lane is available, it may be possible to provide a right-turn acceleration lane at moderate cost. Where additional right-of-way must be acquired, higher costs are likely. EFFECTIVENESS TRIED: By removing the slower right-turning vehicles from the through lanes, this strategy is expected to reduce rear-end and sideswipe crashes resulting from conflicts between vehicles making a right turn maneuver onto the highway and through vehicles on the highway. Research has shown that right-turn acceleration lanes at intersections function effectively and do not create safety problems. However, no quantitative estimates of the safety effectiveness of right-turn acceleration lanes at intersections are available. COMPATIBILITY This strategy can be used in conjunction with other strategies for improving safety at unsignalized intersections. [Home] Slide 16Provide Full-Width Paved Shoulders in Intersection AreasNCHRP Report 500 • Vol. 5 • Strategy B10
Where to use:Unsignalized intersections on divided highways with no shoulder or shoulder widths less than 8 feet that experience a high proportion of run-off-road crashes as a result of avoidance maneuvers or a high proportion of rear-end crashes that could have been avoided had a full-width paved shoulder been provided. Key to success:Make sure that full-width paved shoulders are operationally justified on the basis of an existing crash pattern. Cost: Moderate (2 on a scale of 1 to 4) Timeframe: Medium (2 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Well-designed and properly maintained shoulders in intersection areas provide
Furthermore, the sense of openness created by shoulders of adequate width contributes to driving ease and freedom from strain. Finally, full-width shoulders can be used for temporary storage of snow that is plowed from the road during times of heavy snowfall, allowing the full width of the lanes to be available for moving traffic and minimizing the potential sight obstruction of plowed snow. KEY TO SUCCESS Make sure that full-width paved shoulders are operationally justified on the basis of an existing crash pattern. ISSUES There are three potential difficulties associated with this strategy. The first difficulty concerns recognizing a crash pattern for which this strategy is applicable. This may require reviewing police crash reports to determine why a vehicle ran off the road or whether a rear-end crash could have been avoided had a shoulder been present. Second, vehicles turning right may use a full-width shoulder as a pseudo right-turn lane, which may or may not be desirable. Third, when full-width paved shoulders increase the overall width of the intersection, the additional width may cause potential problems for pedestrians crossing the intersection. One possible solution to this third issue is to provide a pedestrian refuge island in the median. TIME FRAME: Medium Implementing this strategy may take from 3 months to 4 years. At some locations, full-width shoulders are already provided and simply need to be paved. At other locations, acquisition of additional right-of-way may be needed. Where right-of-way is required or where the environmental process requires analysis and documentation, project design and implementation periods can become lengthy. COSTS: Moderate Costs are highly variable. Where paving an existing full-width shoulder is possible, the costs are relatively low. Costs may be moderate where both grading and paving are needed. Higher costs will be incurred where right-of-way must be acquired. EFFECTIVENESS TRIED: The published literature on the safety effectiveness of shoulder widening and paving deals primarily with shoulders with roadway segments rather than shoulders at intersections. One research project concluded that the expected reduction in run-off-road and opposite-direction crashes from shoulder-widening projects ranged from 6 to 21%, depending upon the amount of widening. Other past research has concluded that shoulder widening on higher-volume, two-lane roadways reduces total crashes by 2.8% per foot of additional shoulder width. It was also concluded that there is a small safety benefit to paving existing unpaved shoulders. The magnitude of this benefit increases with increasing shoulder width. The results of these studies are not directly applicable to quantify the safety effectiveness of providing full-width paved shoulders at intersections. However, the results do provide an indication that providing full-width paved shoulders at intersections may improve safety. COMPATIBILITY This strategy can be used in conjunction with other strategies for improving safety at unsignalized intersections. [Home] Slide 17Restrict or Eliminate Turning Maneuvers by SigningNCHRP Report 500 • Vol. 5 • Strategy B11
Where to use:Unsignalized intersections with patterns of crashes related to particular turning maneuvers where it is impractical to reduce that pattern of crashes by improving sight distance or providing a left turn or shoulder bypass lane. Key to success:Anticipate the destinations of traffic making the affected turning maneuver and ensure the availability of alternatives that can safely accommodate that traffic. It is also important that the turn restriction or prohibition be clearly signed so that motorists become aware of the restriction or prohibition and do not make illegal turns. Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Safety at some unsignalized intersections can be enhanced by restricting turning maneuvers, particularly left turns, during certain periods of the day (such as peak traffic periods) or by prohibiting particular turning movements altogether. Turn restrictions and prohibitions can be implemented by signing. KEY TO SUCCESS Anticipate the destinations of traffic making the affected turning maneuver and ensure the availability of alternatives that can safely accommodate that traffic. It is also important that the turn restriction or prohibition be clearly signed so that motorists become aware of the restriction or prohibition and do not make illegal turns. ISSUES A potential pitfall of a turn restriction or prohibition is that suitable alternatives may not be available, resulting in drivers continuing to make illegal turning maneuvers or taking unanticipated alternatives through private property or minor local streets. Another potential pitfall occurs where commercial properties are affected and business owners resist the action because of fears of losing business due to restricted access. Finally, experience demonstrates that the effectiveness of turn restrictions is maximized when they are accompanied by physical barriers. Where no such barriers exist and police do not regularly enforce the turning restrictions, violations of turn restrictions may be expected and, hence, the safety effectiveness degraded. TIME FRAME: Short Since turn prohibitions are normally implemented by signing, they can be implemented quickly, often within 3 months or less. COSTS: Low Since this strategy is implemented through signing, its cost is low. EFFECTIVENESS TRIED: Turn restrictions or prohibitions should reduce crashes related to the affected turning maneuver by nearly 100% during the period that the restriction or prohibition is in effect. However, a complete assessment of the effect of a turn restriction or prohibition on safety requires consideration of the alternatives to which the traffic that desires to make the affected turn is diverted, as well as the potential effect of that traffic on the safety performance of that alternative route. The net effect on safety of turn prohibitions and restrictions is highly site specific and difficult to quantify. One Florida study indicated prohibiting left turns can reduce crashes by up to 45% (and left-turn crashes 90%). A Virginia study concluded that the installation of turn prohibition signs at urban intersections could result in a 62% decrease in crashes. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections. It is intended as an alternative to provisions of left-turn lanes or shoulder bypass lanes, so it is not appropriate for use in conjunction with those strategies. A traffic law enforcement program in coordination with the restrictions, especially following their introduction, is also desirable. SUPPLEMENTAL INFORMATION Signing in conformance with the Manual on Uniform Traffic Control Devices should be provided. [Home] Slide 18Restrict or Eliminate Turning Maneuvers by Providing Channelization or Closing Median OpeningsNCHRP Report 500 • Vol. 5 • Strategy B12
Where to use:Unsignalized intersections with patterns of crashes related to particular turning maneuvers where it is impractical to reduce that pattern of crashes by improving sight distance or providing a left turn or shoulder bypass lane. Also, at locations where it is possible to restrict or eliminate turning maneuvers by providing channelization or by closing the median opening. Key to success:Anticipate the destinations of traffic making the affected turning maneuver and ensure that alternatives that can safely accommodate that traffic are available. It is also important that the turn restriction or prohibition be clearly signed so that motorists become aware of the restriction or prohibition and do not make illegal turns. Furthermore, it will be important to include all stakeholders in the early planning stages, especially business property owners whose access may be made less convenient for customers. Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Safety at some unsignalized intersections can be enhanced by prohibiting particular turning movements altogether with the use of channelization or by closing median openings. KEY TO SUCCESS Anticipate the destinations of traffic making the affected turning maneuver and ensure that alternatives that can safely accommodate that traffic are available. It is also important that the turn restriction or prohibition be clearly signed so that motorists become aware of the restriction or prohibition and do not make illegal turns. Furthermore, it will be important to include all stakeholders in the early planning stages, especially business property owners whose access may be made less convenient for customers. ISSUES A potential pitfall of a turn restriction or prohibition is that suitable alternatives may not be available, resulting in drivers taking unanticipated alternative routes through private property or minor local streets. Another potential pitfall occurs where commercial properties are affected and business owners resist the action because of fears of losing business. A difficulty with this strategy is that it commits the agency to prohibition of turning movements 100% of the time (i.e., this strategy should not be employed to treat temporal or short-lived problems). TIME FRAME: Short Turn prohibitions that are implemented by closing a median opening can be implemented quickly, often within 3 months or less. Turn prohibitions requiring the installation of channelization may take from 3 months to 1 year to implement. COSTS: Low The cost of this strategy will depend on the treatment. Closing a median opening is considerably less costly than installing channelization. EFFECTIVENESS TRIED: Turn restrictions or prohibitions should reduce crashes related to the affected turning maneuver by nearly 100% at the locations where the restriction or prohibition is in effect. However, a complete assessment of the effect of a turn restriction or prohibition on safety requires consideration of the alternatives to which the traffic that desires to make the affected turn is diverted, as where as the potential effect of that traffic on the safety performance of that alternative route. Adequate evaluations of this type are not known. One study estimated a 51% decrease in crashes where directional medians were installed. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections. It is intended as an alternative to providing left-turn lanes or shoulder bypass lanes, so it is not appropriate for use in conjunction with those strategies. [Home] Slide 19Close or Relocate "High-Risk" IntersectionsNCHRP Report 500 • Vol. 5 • Strategy B13
Where to use:Unsignalized intersections with high levels of intersection-related crashes that other strategies have not been successful in reducing or for which other strategies are not considered appropriate. Also at locations where a particular strategy such as installing a turn lane or increasing sight distance is impractical at the current location, but could be applied if the intersection were moved. Key to success:Involve the affected neighborhood early in the decision-making process to develop and maintain support for the project. Cost: High (4 on a scale of 1 to 4) Timeframe: Long (3 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS For some unsignalized intersections with crash histories, the best method of improving safety may be to close or relocate the intersection. This is a radical approach to safety improvement that should generally be considered only when less restrictive measures have been tried and have failed. Intersection relocation can be accomplished by realigning the minor-road approaches so that they intersect the major road at a different location or a different angle. Intersection closure can be accomplished by closing and abandoning the minor street or by converting the minor street approaches so that they dead-end before reaching their former intersection with the major street. KEY TO SUCCESS Involve the affected neighborhood early in the decision-making process to develop and maintain support for the project. ISSUES Diverted traffic may contribute to safety problems at adjacent intersections or on alternative routes, resulting in no net benefit. Owners of properties where access would be reduced, especially those with commercial operations, may oppose this strategy. Temporary unsafe conditions may occur immediately after the change due to erratic maneuvers by drivers whose expectancy has been violated. Care should be taken during the transition period, both before and after the change is made, to alert drivers to the changes as they approach the section involved. TIME FRAME: Long This strategy requires an implementation time of 1 to 4 years. At least 1 year is necessary to work out the details of street relocation or closure and to communicate the plan to affected business owners and residents. Where relocation requires right-of-way acquisition and/or demolition of existing structures, an extensive project development process up to 4 years long may be required. COSTS: High Costs to implement this strategy are highly variable. Where mere closure of an existing intersection is all that is needed, costs are low. In other cases, construction of a new intersection or diversion of traffic to a different existing intersection may require substantially higher expenditures. EFFECTIVENESS TRIED: Closure of an intersection should eliminate crashes at that location. Consideration must be given to the adjacent intersections, to alternative routes onto which traffic would be diverted, and to the potential impact of safety on those routes. COMPATIBILITY Closure of an intersection is an alternative to other strategies for improving safety and is not typically used in conjunction with other strategies. Relocation of an intersection is nearly always used in conjunction with most other strategies for improving safety. Indeed, in many cases, the purpose of relocating an intersection may be to make those other strategies feasible. SUPPLEMENTAL INFORMATION This strategy is primarily appropriate for urban and suburban intersections where reasonable alternative access or routes are readily available. [Home] Slide 20Convert Four-Legged Intersections to Two T-IntersectionsNCHRP Report 500 • Vol. 5 • Strategy B14
Where to use:Unsignalized four-legged intersections with very low through volumes on the cross street. Key to success:Depends upon the through volume of the cross street. If through volumes are high, the intersection may be safer if left as a conventional four-legged intersection. Converting it to two T-intersections would only create excessive turning movements at each of the T-intersections. Cost: High (4 on a scale of 1 to 4) Timeframe: Medium (2 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS For some unsignalized four-legged intersections with very low through volumes on the cross street, the best method of improving safety may be to convert the intersection to two T-intersections. This conversion to two T-intersections can be accomplished by separating the two cross-street approaches an appreciable distance along the major road, thus creating two separate T-intersections that operate independently of one another. KEY TO SUCCESS Depends upon the through volume of the cross street. If through volumes are high, the intersection may be safer if left as a conventional four-legged intersection. Converting it to two T-intersections would only create excessive turning movements at each of the T-intersections. ISSUES A potential difficulty with this strategy is the spacing between the two T-intersections. If the two intersections are not spaced far enough apart, two problems can occur. First, there may not be enough storage length for the left-turning vehicles between the two intersections. Second, the operations of the two intersections may interfere with one another. Another difficulty may occur in providing safe access to the properties adjacent to the existing four-legged intersection. TIME FRAME: Medium This strategy requires an implementation time of 1 to 4 years. At least 1 year is necessary to work out the details of intersection approach relocation and to communicate the plan to affected business owners and residents. Where relocation requires right-of-way acquisition and/or demolition of existing structures, an extensive project development process up to 4 years in duration may be required. COSTS: High Converting a conventional four-legged intersection to two T-intersections involves the realignment of at least one intersection approach. The cost of this type of construction project is usually high. Furthermore, additional right-of-way will generally need to be acquired. EFFECTIVENESS TRIED: In one study conducted, offset intersections had crash rates that were approximately 43% of the crash rate at comparable four-legged intersections. Thus, it is expected that this strategy would reduce the crash experience of targeted four-legged intersections. Research completed in the U.K. indicates that total crashes may be reduced by 25-33% when the ratio of major street to minor street traffic is <85% / >15%. However, when the ratio is >85% / <15%, crashes tend to increase. COMPATIBILITY The conversion of a conventional four-legged intersection to two T-intersections may be used in conjunction with most other strategies for improving safety. In many cases, the relocation of an intersection approach may be done to make those other strategies feasible. [Home] Slide 21Convert Offset T-Intersections to Four-Legged IntersectionsNCHRP Report 500 • Vol. 5 • Strategy B15
Where to use:Unsignalized offset T-intersections where through volumes on the cross street are very high. Key to success:Depends upon the through volume of the cross street. If through volumes are low, the intersection may be safer if left as two offset T-intersections. Two offset T-intersections with low cross-street through volumes are generally safer than a four-legged intersection. Cost: High (4 on a scale of 1 to 4) Timeframe: Medium (2 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS For some unsignalized offset T-intersections with very high through volumes on the cross street, the best method of improving safety may be to convert the intersection to a single four-legged intersection. This conversion to a four-legged intersection can be accomplished by realigning the two cross-street approaches to meet at a single point along the major road, thus creating one four-legged intersection. KEY TO SUCCESS Depends upon the through volume of the cross street. If through volumes are low, the intersection may be safer if left as two offset T-intersections. Two offset T-intersections with low cross-street through volumes are generally safer than a four-legged intersection. ISSUES There should be no potential difficulties with this strategy, as long as the resulting four-legged intersection is properly designed and traffic control devices are properly used. TIME FRAME: Medium This strategy requires an implementation time of 1 to 4 years. At least 1 year is necessary to work out the details of intersection approach relocation and to communicate the plan to affected businesses and residents. Where relocation requires right-of-way acquisition and/or demolition of existing structures, an extensive project development process up to 4 years long may be required. COSTS: High Converting two offset T-intersections to a conventional four-legged intersection involves the realignment of at least one intersection approach. The cost of this type of construction project is usually high. Furthermore, additional right-of-way will generally need to be acquired. EFFECTIVENESS TRIED: It is expected that this strategy would reduce crashes involving left-turning traffic from the major road onto the cross street at each of the two T-intersections. It can reduce or eliminate safety problems associated with insufficient spacing between existing offset T-intersections. COMPATIBILITY The conversion of two offset T-intersections to a conventional four-legged intersection may be used in conjunction with most other strategies for improving safety. In many cases, the purpose of relocating an intersection approach may be to make those other strategies feasible. [Home] Slide 22Realign Intersection Approaches to Reduce or Eliminate Intersection SkewNCHRP Report 500 • Vol. 5 • Strategy B16
Where to use:Unsignalized intersections with a high frequency of crashes resulting from insufficient intersection sight distance and awkward sight lines at a skewed intersection. Key to success:Identify candidate locations where there exist crash patterns related to the intersection angle. Any intersection with a pattern of right-angle or turning crashes should be checked to determine whether the skew angle of the intersection is contributing to these crashes. Cost: High (4 on a scale of 1 to 4) Timeframe: Medium (2 on a scale of 1 to 3) Effectiveness: Proven Slide Notes DETAILS When roadways intersect at skewed angles, the intersections may experience one or more of the following problems:
Realignment of intersection approaches to reduce or eliminate intersection skew may be desirable to improve safety at a skewed intersection. KEY TO SUCCESS Identify candidate locations where there exist crash patterns related to the intersection angle. Any intersection with a pattern of right-angle or turning crashes should be checked to determine whether the skew angle of the intersection is contributing to these crashes. ISSUES When realigning a skewed intersection approach, it is possible to create such a sharp horizontal curve that the curve itself becomes a safety concern. Thus, the designer should be alert to avoid trading one safety concern for another. Realignment may negatively affect adjacent properties. TIME FRAME: Medium This strategy requires an implementation time of 1 to 4 years. At least 1 year is necessary to work out the details of intersection approach realignment and to communicate the plan to affected business owners and residents. Where relocation requires right-of-way acquisition and/or demolition of existing structures, an extensive project development process up to 4 years long may be required. COSTS: High Reducing or eliminating the skew angle of an intersection involves the realignment of at least one intersection approach. The cost of this type of construction project is usually high. Furthermore, additional right-of-way will generally need to be acquired. EFFECTIVENESS PROVEN: A recent study concluded from a review of the literature that realigning intersection approaches to reduce or eliminate intersection skew improves safety at unsignalized intersections. The study concluded the safety effectiveness of realignment to be as follows: AMF = e0.0040xSKEW for three-legged intersections and AMF = e0.0054xSKEW for four-legged intersections Where: AMF = Accident modification factor SKEW = Intersection skew angle (degrees), expressed as the absolute value of the difference between 90 degrees and the actual intersection angle. Example: Three-leg intersection with a 15 degree skew on the approach. AMF = e0.004x15 = 1.06 (6% more crashes than an approach with no skew) COMPATIBILITY Reducing or eliminating the skew angle of an intersection may be done in conjunction with most other strategies for improving safety. Indeed, in many cases, the purpose of realigning an intersection approach may be to make those other strategies feasible. [Home] Slide 23Use Indirect Left-Turn Treatments to Minimize Conflicts at Divided Highway IntersectionsNCHRP Report 500 • Vol. 5 • Strategy B17
Where to use:Unsignalized intersections with operational and safety problems that can be traced to difficulties of accommodating left-turn demand. Key to success:Make sure that this strategy is justified on the basis of high left-turn demand or an existing pattern of left-turn collisions. Also, involve the affected owners of adjacent property and residents in the decision-making process to develop and maintain support for the project. Cost: Moderate (2 on a scale of 1 to 4) Timeframe: Medium (2 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Many operational and safety problems at two-lane and divided highways can be traced to difficulties of accommodating left-turn demand. Such difficulties involve both demand volume and the frequency of demand along a corridor. Furthermore, vehicles that slow down or stop to turn left in a lane used primarily by through traffic increase the potential for rear-end crashes. One way to address the impacts of such left-turn movements is the use of indirect left-turn treatments. Indirect left-turn treatments include the use of jug-handle roadways before the crossroad, loop roadways beyond the crossroad, and directional median crossovers beyond the crossroad. Indirect left-turn treatments enable drivers to make left turns efficiently on divided highways, including highways with relatively narrow medians. KEY TO SUCCESS Make sure that this strategy is justified on the basis of high left-turn demand or an existing pattern of left-turn collisions. Also, involve the affected owners of adjacent property and residents in the decision-making process to develop and maintain support for the project. ISSUES Diverted traffic may contribute to safety problems at adjacent intersections or on alternative routes, resulting in no net benefit. Owners of properties where access may be reduced, especially those with commercial operations, may oppose this strategy. Thus, careful evaluation of the potential impacts of proposed improvements is needed to avoid or minimize such problems. A temporary hazard may exist during the transition period after the changed condition is opened to traffic. Advance notification of drivers is important, both in terms of notification prior to instituting the change and in signing that provides the appropriate notice of a change. TIME FRAME: Medium Implementation time can vary from 3 months to 4 years. At some locations, indirect left turns can be implemented simply by appropriate signing. Other locations may require major reconstruction. Such projects require a substantial time for development and construction. Where right-of-way is required or where the environmental process requires analysis and documentation, the time will be longer. COSTS: Moderate Costs are highly variable. Where an improvement can be implemented by signing an existing roadway, the costs are nominal. Where reconstruction is necessary, costs over $100,000 per intersection approach may be incurred. EFFECTIVENESS TRIED: It is expected that this strategy will reduce (1) rear-end crashes resulting from the conflict between vehicles waiting to turn left and following vehicles, and (2) right-angle crashes resulting from the conflict between vehicles turning left and oncoming through vehicles. One study concluded that installing indirect left-turn treatments may result in an 18-26% decrease in all crashes depending on the number of lanes. COMPATIBILITY This strategy can be used in conjunction with other strategies for improving safety at unsignalized intersections. SUPPLEMENTAL INFORMATION Optimal operation and safety of indirect left turns requires appropriate design and signing. [Home] Slide 24Improve Pedestrian and Bicycle Facilities to Reduce Conflicts Between Motorists and NonmotoristsNCHRP Report 500 • Vol. 5 • Strategy B18
Where to use:Unsignalized intersections that experience crashes involving pedestrians and/or bicyclists with motor vehicles or that have the potential for such crashes. Key to success:Get the appropriate agencies to look at pedestrian and bicycle facilities from a more systematic point of view. That is, rather than making improvements where problems occur, the needs of pedestrians and bicyclists should be anticipated during the design of other intersection improvements, and appropriate improvements should be incorporated in the design before such problems occur. It is desirable to involve groups representing pedestrians and bicyclists in the early stages of a program's development. Cost: Moderate (2 on a scale of 1 to 4) Timeframe: Medium (2 on a scale of 1 to 3) Effectiveness: Varies Slide Notes DETAILS Nearly one-third of all pedestrian-related crashes occur at or within 50 feet of an intersection. Of these, 30% involve a turning vehicle. Another 22% of pedestrian crashes involve a pedestrian either running across the intersection or darting out in front of a vehicle whose view was blocked just prior to the impact. Finally, 16% of these intersection-related crashes occur because of a driver violation (e.g., failure to yield right-of-way). Improvements to pedestrian facilities (short of grade separation) that may reduce conflicts between motorists and nonmotorists include:
Some of the problems that bicyclists face at intersections include high traffic volumes and speeds and lack of space for bicyclists. Possible improvement projects include:
Further details may be found in the implementation guide (NCHRP Report 500, Volume 10) for addressing pedestrian crashes. FHWA maintains a site that provides detailed information on pedestrian crash countermeasures at intersections. (safety.fhwa.dot.gov) KEY TO SUCCESS Get the appropriate agencies to look at pedestrian and bicycle facilities from a more systematic point of view. That is, rather than making improvements where problems occur, the needs of pedestrians and bicyclists should be anticipated during the design of other intersection improvements, and appropriate improvements should be incorporated in the design before such problems occur. It is desirable to involve groups representing pedestrians and bicyclists in the early stages of a program's development. ISSUES Improving pedestrian and bicycle facilities is not a one-time process. The facilities also need to be properly maintained. For example, some issues are often overlooked—a missing or broken section of sidewalk or a construction zone that forces pedestrians to walk in a traffic lane. TIME FRAME: Medium The implementation time for improvements to pedestrian and bicycle facilities is highly variable. COSTS: Moderate The cost of improvements to pedestrian and bicycle facilities is highly variable. EFFECTIVENESS VARIES: It is expected that improvements to pedestrian and bicycle facilities at unsignalized intersections will reduce the number of crashes between motorists and nonmotorists. Quantitative estimates of effectiveness may exist for some of the countermeasures that may be employed, but not for others. See http://safety.fhwa.dot.gov/safer journey/Library/matrix.htm for further details. One study concluded that installing pedestrian crossings at rural locations can reduce pedestrian crashes by 60%. Another study indicated that bicycle crashes can be reduced by up to 36% by providing bicycle lanes. COMPATIBILITY Strategies to reduce pedestrian and bicycle crashes are compatible with most other strategies for improving safety at unsignalized intersections. SUPPLEMENTAL INFORMATION State and local highway agencies and other local agencies should ensure that policies for new roadway construction include pedestrian and bicycle considerations (e.g., provision of sidewalks or shoulders). There are well-organized pedestrian and bicycling organizations that should be considered partners in any planning effort, such as the National Center for Bicycling and Walking (www.walkinginfo.org) and the Pedestrian and Bicycle Information Center (www.bikewalk.org). [Home] Slide 25 Clear Sight Triangles on Stop- or Yield-Controlled Approaches to IntersectionsNCHRP Report 500 • Vol. 5 • Strategy C1
Where to use:Unsignalized intersections with restricted sight distance and patterns of crashes related to lack of sight distance, where sight distance can be improved by clearing roadside obstructions without major construction. Key to success:Effectively diagnose whether a specific crash pattern observed at an intersection is, in fact, related to restricted sight distance. Currently this is a judgment made by an experienced safety analyst. Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Adequate sight distance for drivers at stop- or yield-controlled approaches to intersections has long been recognized as among the most important factors contributing to overall safety at unsignalized intersections. Recent research has established design requirements for Intersection Sight Distance (ISD) based upon driver and vehicle functional requirements. NCHRP Report 383: Intersection Sight Distance provides design guidelines that have been implemented in the current edition of the AASHTO Policy on Geometric Design of Highways and Streets. NCHRP Report 383 provides a gap-acceptance based approach to sight-distance requirements based upon actual driver behavior at intersections. Since, at least at high-speed intersections, the recommended sight distances are shorter than those in previous policies, they are more practical to achieve in the real world. Sight distance improvements can often be achieved at relatively low cost by clearing sight triangles to restore sight distance obstructed by vegetation, roadside appurtenances, or other natural or artificial objects. Intersection sight-distance-related crashes include angle- and turning-related crashes. KEY TO SUCCESS Effectively diagnose whether a specific crash pattern observed at an intersection is, in fact, related to restricted sight distance. Currently this is a judgment made by an experienced safety analyst. ISSUES The most difficult aspect of this strategy is the removal of sight restrictions located on private property. The legal authority of highway agencies to deal with such sight obstructions varies widely. TIME FRAME: Short Projects involving clearing sight obstructions on the highway right-of-way can typically be accomplished in 3 months or less, assuming the objects are readily moveable. Clearing sight obstructions on private property requires more time for discussions with the property owner. COSTS: Low Costs will generally be low, assuming that in most cases the objects to be removed are within the right-of-way. EFFECTIVENESS TRIED: There is no research that adequately quantifies the effectiveness of improving sight distance at unsignalized intersections. Based on existing literature, it has been estimated that if the available sight distance in any quadrant of an intersection is less than or equal to the design sight distance for a speed of 12 mph less than the actual 85th-percentile speed of the approach, then the frequency of related crashes at the intersection would be increased by a little under 5%. Thus, a project may be 0 to approximately 17% effective in reducing related crashes, depending upon the severity of the existing sight restriction and the number of intersection quadrants affected. COMPATIBILITY TRIED: There is no research that adequately quantifies the effectiveness of improving sight distance at unsignalized intersections. Based on existing literature, it has been estimated that if the available sight distance in any quadrant of an intersection is less than or equal to the design sight distance for a speed of 12 mph less than the actual 85th-percentile speed of the approach, then the frequency of related crashes at the intersection would be increased by a little under 5%. Thus, a project may be 0 to approximately 17% effective in reducing related crashes, depending upon the severity of the existing sight restriction and the number of intersection quadrants affected. SUPPLEMENTAL INFORMATION This strategy should be incorporated in highway design policies, highway maintenance manuals, and educational materials for the public. Unsignalized intersections with sight distance restrictions in one or more quadrants are common. Since highway maintenance operations are often independent of safety operations in a highway agency, it is important that both groups be apprised of the need to protect sight triangles and that there be coordination between them. [Home] Slide 26Clear Sight Triangles in the Medians of Divided Highways near IntersectionsNCHRP Report 500 • Vol. 5 • Strategy C2
Where to use:Unsignalized intersections on divided highways with (a) fixed sight obstructions in the median near the intersection and (b) patterns of crashes related to the lack of sight distance. Key to success:Effectively diagnose whether a specific crash pattern observed at an intersection is, in fact, related to restricted sight distance. Currently this is a judgment made by an experienced safety analyst. Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Adequate sight distance for drivers at stopped approaches to intersections has long been recognized as among the most important factors contributing to overall safety at unsignalized intersections. A particular concern at divided highway intersections is sight obstructions located in the highway median. Such obstructions can restrict sight distance for drivers of vehicles crossing the median, including through vehicles on the crossroad and vehicles making left turns onto and off of the divided highway. Sight obstructions can include vegetation, roadside appurtenances, or other natural and artificial objects. Since sight obstructions located in the highway median are, almost by definition, located in the highway right-of-way, highway agencies should have direct authority to remove them. If the objects are mature trees or plantings, then environmental issues may arise. Intersection sight distance (ISD) related crashes include angle- and turning-related crashes. KEY TO SUCCESS Effectively diagnose whether a specific crash pattern observed at an intersection is, in fact, related to restricted sight distance. Currently this is a judgment made by an experienced safety analyst. ISSUES The difficulties with this strategy primarily relate to public acceptance. From a process and engineering perspective, implementation is relatively straightforward since, by definition, all work is well within the right-of way. However, most plantings located in medians were deliberately placed there for aesthetic reasons, and the public will often object to their removal, particularly where no site-specific safety problem is evident. TIME FRAME: Short Projects involving clearing sight obstructions on the highway right-of-way can typically be accomplished in 3 months or less, assuming that the objects are readily moveable and their removal is not controversial. COSTS: Low Costs will generally be low, assuming that in most cases the objects to be removed are within the right-of-way. EFFECTIVENESS TRIED: There is no research that adequately quantifies the effectiveness of improving sight distance at unsignalized intersections. Based on existing literature, it has been estimated that if the available sight distance in any quadrant of an intersection is less than or equal to the design sight distance for a speed of 12 mph less than the actual 85th-percentile speed of the approach, then the frequency of related crashes at the intersection would be increased by 5%. Although this assessment was made for intersections on rural two-lane highways, it appears appropriate to extend it to intersections on divided highway intersections, as well. Since the median affects two quadrants on the approach to each side of the divided highway from the median roadway, it is estimated that a project to remove sight obstructions in the median may be 0 to 20% effective in reducing related crashes, depending upon the severity of the existing sight restriction and the number of intersection quadrants affected. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections. SUPPLEMENTAL INFORMATION This strategy should be incorporated in highway design policies and in highway maintenance manuals. Since highway maintenance operations are often independent of safety operations in a highway agency, it is important that both groups be apprised of the need to protect sight triangles and that there be coordination between them. [Home] Slide 27Change Horizontal and/or Vertical Alignment of Approaches to Provide More Sight DistanceNCHRP Report 500 • Vol. 5 • Strategy C3
Where to use:Unsignalized intersections with restricted sight distance due to horizontal and/or vertical geometry and with patterns of crashes related to that lack of sight distance that cannot be ameliorated by less expensive methods. Key to success:Effectively diagnose whether a specific crash pattern observed at an intersection is, in fact, related to restricted sight distance. Currently this is a judgment made by an experienced safety analyst. Because adjacent properties may be affected by the redesign, all the stakeholders should be involved early in the planning process. Cost: High (4 on a scale of 1 to 4) Timeframe: Long (34 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Adequate sight distance for drivers at stopped approaches to intersections has long been recognized as among the most important factors contributing to overall intersection safety. Recent research has established design requirements for intersection sight distance based on driver and vehicle functional requirements. NCHRP Report 383: Intersection Sight Distance provides design guidelines that have been incorporated in the current edition of the AASHTO Policy on Geometric Design of Highways and Streets (2001). NCHRP Report 383 provides a gap-acceptance-based approach to sight distance requirements based on actual driver behavior at intersections. Previous strategies addressed sight distance improvements that can be achieved at relatively low cost by clearing sight triangles. This strategy addresses more costly geometric improvements that involve changing the horizontal or vertical alignment of the intersecting roadways. Such strategies should generally be considered only at intersections with a persistent crash pattern that cannot be ameliorated by less expensive methods. KEY TO SUCCESS Effectively diagnose whether a specific crash pattern observed at an intersection is, in fact, related to restricted sight distance. Currently this is a judgment made by an experienced safety analyst. Because adjacent properties may be affected by the redesign, all the stakeholders should be involved early in the planning process. ISSUES The most difficult aspect of this strategy is the potential impact on adjacent property of making improvements to the horizontal or vertical geometry. Because of the potential impacts and the relatively high costs involved, this strategy should generally be considered only when less expensive strategies involving clearing of specific sight obstructions or modifying traffic control devices have been tried and have failed to ameliorate the crash patterns. If additional right-of-way is required, there may be significant environmental issues, as well. TIME FRAME: Long Projects involving changing the horizontal and/or vertical alignment to provide more sight distance are quite extensive and usually take from 1 to 3 years to accomplish. If additional right-of-way is required, these projects will also involve discussions with owners of adjacent properties, which may require a substantial period of time. COSTS: High Projects involving changing the horizontal and/or vertical alignment are generally high cost, especially if additional right-of-way is required. EFFECTIVENESS TRIED: There is no research that adequately quantifies the effectiveness of improving sight distance at unsignalized intersections. Based on existing literature, it has been estimated that if the available sight distance in any quadrant of an intersection is less than or equal to the design sight distance for a speed of 12 mph less than the actual 85th-percentile speed of the approach, then the frequency of related crashes at the intersection would be increased by approximately 5%. Each additional quadrant accounts for an approximate 4% decrease in crashes. Thus, a project may be 5 to 17% effective in reducing related crashes, depending upon the severity of the existing sight restriction and the number of intersection quadrants affected. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections. SUPPLEMENTAL INFORMATION This strategy should be incorporated in highway design policies and highway maintenance manuals. [Home] Slide 28Eliminate Parking That Restricts Sight DistanceNCHRP Report 500 • Vol. 5 • Strategy C4
Where to use:Unsignalized intersections with restricted sight distance due to parking. Key to success:Effectively diagnose whether a specific crash pattern observed at an intersection is, in fact, related to restricted sight distance due to parking. Currently this is a judgment made by an experienced safety analyst. It may often require detailed study of individual crash reports for the intersection, as well as field visits and measurements. Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Adequate sight distance for drivers at stop-controlled approaches to intersections has long been recognized as among the most important factors contributing to overall intersection safety. Although geometrically an intersection might have adequate sight distance, parking within the sight triangle might restrict it and should, therefore, be taken into consideration. Increased enforcement of existing parking prohibitions may be needed to ensure the successful implementation of this strategy. Intersection sight distance (ISD) related crashes include angle- and turning-related crashes. KEY TO SUCCESS Effectively diagnose whether a specific crash pattern observed at an intersection is, in fact, related to restricted sight distance due to parking. Currently this is a judgment made by an experienced safety analyst. It may often require detailed study of individual crash reports for the intersection, as well as field visits and measurements. ISSUES The most difficult aspect of this strategy is the reaction of owners of adjacent properties and users who may be negatively impacted by the removal of nearby parking spaces. Public compliance with parking restrictions may present a problem. TIME FRAME: Short Projects involving eliminating parking can typically be accomplished in 3 months or less, assuming that the removal of the parking is not controversial. COSTS: Low Costs will generally be low and will include signing and enforcement costs. Some targeted enforcement may be required, but this may usually be accomplished within the normal patrol activities of the agency(ies) within whose jurisdiction the intersection is located. EFFECTIVENESS TRIED: There is no research that adequately quantifies the effectiveness of improving sight distance at unsignalized intersections due to elimination of parking. Based on existing literature, it has been estimated that if the available sight distance in any quadrant of an intersection is less than or equal to the design sight distance for a speed of 12 mph less than the actual 85th-percentile speed of the approach, then the frequency of related crashes at the intersection would be increased by approximately 5%. Each additional quadrant accounts for an approximate 4% decrease in crashes. Thus, a project may be 5 to 17% effective in reducing related crashes, depending upon the severity of the existing sight restriction and the number of intersection quadrants affected. Estimates of the safety effectiveness of eliminating parking that restricts sight distance have not been developed yet. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections. SUPPLEMENTAL INFORMATION This strategy should be incorporated in highway design policies and highway maintenance manuals. The involvement of law enforcement agencies with jurisdiction for the intersection will be important. This should be sought at the earliest possible point in the process. [Home] Slide 29Provide an Automated Real-Time System to Inform Drivers of the Suitability of Available Gaps for Making Turning and Crossing ManeuversNCHRP Report 500 • Vol. 5 • Strategy D1
Where to use:Unsignalized intersections with a high frequency of right-angle collisions due to restricted sight distance. Key to success:Eliminate as many sight obstructions as practical or possible before implementing an automated system. Signage on the highway that does not stop should be used to alert motorists of the approaching intersection. The system must be maintained in excellent working condition. Cost: Moderate (2 on a scale of 1 to 4) Timeframe: Medium (2 on a scale of 1 to 3) Effectiveness: Experimental Slide Notes DETAILS The lack of adequate sight distance at unsignalized intersections may reduce the ability of drivers to see an approaching vehicle and/or judge the suitable available gap for making turning and crossing maneuvers. Even where sight distance is adequate, drivers may ignore traffic control devices such as stop or yield signs and may misjudge available gaps in traffic. Thus, intersection crashes may occur because drivers are unable to judge adequately the distance to an approaching vehicle. Automated systems can be used to assist drivers in judging the adequacy of available gaps in traffic for entering the major road from a stop- or yield-controlled approach. Such systems can range from simple pavement loop detectors and flashing lights with a simple control algorithm to more complex realtime, computer-controlled systems. KEY TO SUCCESS Eliminate as many sight obstructions as practical or possible before implementing an automated system. Signage on the highway that does not stop should be used to alert motorists of the approaching intersection. The system must be maintained in excellent working condition. ISSUES Care must be taken to keep the system operating properly. If the system fails (e.g., light burns out, detector malfunctions), a driver may assume that there is no traffic approaching and proceed with little or no caution. TIME FRAME: Medium Time frame for implementation can generally be short to medium if equipment and right-of-way is available. COSTS: Moderate Costs are generally low to moderate for a simple automated system but will increase for more complex systems. EFFECTIVENESS EXPERIMENTAL: This strategy has been implemented in a few locations, but there are no conclusive results on safety effectiveness to date. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections. [Home] Slide 30Provide Innovative Signs and Markings to Assist Drivers in Judging the Suitability of Available Gaps for Making Turning and Crossing ManeuversNCHRP Report 500 • Vol. 5 • Strategy D2
Where to use:Unsignalized intersections where crash data shows a high occurrence of crashes where vehicles on secondary roadways intersecting at grade misjudge the gap between approaching vehicles. Key to success:It is very important that a driver on the secondary road, while stopped to make the decision whether to enter the intersection, can clearly view the "Look Left-Right-Left Before Pulling Out" warning sign. If the warning sign is not easily viewed from the decision point on the secondary road, it should be shifted to a more visible location. Cost: Low (1 on a scale of 1 to 4) Timeframe: Medium (2 on a scale of 1 to 3) Effectiveness: Experimental Slide Notes DETAILS The lack of adequate sight distance at unsignalized intersections may reduce the ability of drivers to see an approaching vehicle and/or judge the suitable available gap for making turning and crossing maneuvers. Even where sight distance is adequate, drivers may ignore traffic control devices such as stop or yield signs and may misjudge available gaps in traffic. Thus, intersection crashes may occur because drivers are unable to judge adequately the distance and time to an approaching vehicle. This strategy involves the use of innovative signing and passive markings to assist drivers in deciding when to accept a gap. The markings could take the form of pavement markings placed in the field of view of a driver observing the approaching traffic stream. Drivers would need to be told, by signing or through a public education campaign, not to proceed when an approaching vehicle is closer to the intersection than the pavement marker. In the illustration above, the entire treatment consists of the following components:
KEY TO SUCCESS It is very important that a driver on the secondary road, while stopped to make the decision whether to enter the intersection, can clearly view the "Look Left-Right-Left Before Pulling Out" warning sign. If the warning sign is not easily viewed from the decision point on the secondary road, it should be shifted to a more visible location. ISSUES This strategy is considered experimental. If an agency desires to pursue its application, it is recommended that the agency proceed with caution, conducting pilot tests in conjunction with a carefully planned evaluation. TIME FRAME: Medium Time frame for implementation can generally be short if right-of-way is available. COSTS: Low Costs are generally low for a simple system but will increase for more complex systems. EFFECTIVENESS EXPERIMENTAL: This strategy has been experimented with in few locations with no conclusive results. Pennsylvania has experimented with a similar type of countermeasure. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections. SUPPLEMENTAL INFORMATION The information in this fact sheet differs from that presented in the NCHRP Report 500, Volume 5. The countermeasure discussed in the report was found to not increase safety and therefore is not recommended. [Home] Slide 31Retime Adjacent Signals to Create Gaps at Stop-Controlled IntersectionsNCHRP Report 500 • Vol. 5 • Strategy D3
Where to use:Unsignalized intersections (between signalized intersections) with a high frequency of right-angle or turning-related crashes due to a lack of sufficient gaps in through traffic on the major road. Key to success:Identify signal timing for operation of the signalized intersections that results in suitable gaps in traffic at downstream unsignalized intersections. Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Drivers have difficulty making turning maneuvers at some unsignalized intersections because of the lack of sufficiently large gaps in through traffic. The lack of gaps can lead some impatient drivers to accept gaps shorter than needed for safe turning maneuvers, thus leading to turn-related crashes. Such crashes could be minimized if longer gaps could be made available. One method to provide longer gaps is to retime traffic signals at nearby intersections to create more gaps in traffic for turning maneuvers at the unsignalized intersection. The process of retiming signals may also involve rephasing. KEY TO SUCCESS Identify signal timing for operation of the signalized intersections that results in suitable gaps in traffic at downstream unsignalized intersections. ISSUES A potential pitfall can occur when signal timing changes significantly reduce the level of service and/or progression on the through street or elsewhere in the system. Furthermore, the distribution of gaps at other unsignalized intersections may be negatively affected. Care must be taken to check for system effects of a timing change. This pitfall can theoretically extend to conflicts with other programs. For example, arterial and major intersection signal-timing projects are often justified by, and funded through, special congestion mitigation and air quality improvement programs. Suggestions to alter the signal timing in a corridor to achieve safety improvements could result in unintended consequences to previous engineering decisions focusing on other issues. TIME FRAME: Short This strategy requires only changes to signal timing or hardware, so it can be implemented very quickly. The strategy can be implemented in 1 month or less if only reprogramming of signal hardware is required. Where signal hardware must be upgraded to implement this strategy, a lead time of 6 months to 1 year is needed. COSTS: Low Unless new hardware is required, costs to retime signals are nominal; the greatest costs will be associated with conducting the necessary traffic field studies to verify the problem and develop an effective solution. EFFECTIVENESS TRIED: The strategy is presumed to be effective in reducing right-angle and turn-related crashes, but its actual effectiveness has not been quantified. COMPATIBILITY The strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections. This strategy may be an alternative to closing or restricting turning movements associated with existing crash patterns. SUPPLEMENTAL INFORMATION This strategy often requires close cooperation among multiple agencies. In many cases, particularly in urban and suburban areas, the through roadway and signalization is the responsibility of one agency (e.g., a state highway agency), and the intersecting roadway is the responsibility of a local community. [Home] Slide 32Improve Visibility of Intersections by Providing Enhanced Signing and DelineationNCHRP Report 500 • Vol. 5 • Strategy E1
Where to use:Unsignalized intersections that are not clearly visible to approaching motorists, particularly approaching motorists on the major road. The strategy is particularly appropriate for intersections with patterns of rear-end, right-angle, or turning crashes related to lack of driver awareness of the presence of the intersection. Key to success:Select a combination of signing and delineation techniques appropriate to conditions on particular unsignalized intersection approaches. This engineering assessment should, where possible, be accompanied by a human factors assessment of signing and delineation needs. Also, the ability and commitment of the highway agency to adequately maintain the signing or delineation is important. Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Many unsignalized intersections are not readily visible to approaching drivers, particularly drivers on major-road approaches that are not controlled by stop or yield signs. Thus, intersection crashes may occur because approaching drivers may be unaware of the presence of the intersection. The visibility of intersections and, thus, the ability of approaching drivers to perceive them can be enhanced by signing and delineation. Improvements may include advance guide signs, advance street name signs, warning signs, pavement markings, post-mounted delineators, and supplemental beacons on advance signs. The FHWA Older Driver Highway Design Handbook encourages such improvements to contribute to a better driving environment for older drivers. In particular, the handbook addresses advance guide signs and letter height on guide signs as key issues for older drivers. Advance warning signs, such as the standard intersection warning sign, can also alert drivers to the presence of an intersection. Providing a break in pavement markings—including centerlines, lane lines, and edge lines—at intersections also helps to alert drivers to the presence of an intersection. KEY TO SUCCESS Select a combination of signing and delineation techniques appropriate to conditions on particular unsignalized intersection approaches. This engineering assessment should, where possible, be accompanied by a human factors assessment of signing and delineation needs. Also, the ability and commitment of the highway agency to adequately maintain the signing or delineation is important. ISSUES Care should be taken not to overuse traffic signing, which could result in drivers not perceiving the presence of intersections. TIME FRAME: Short This strategy does not require a long development process. Signing and delineation improvements can typically be implemented in 3 months or less. COSTS: Low Costs to implement signing and delineation are relatively low. An agency's maintenance costs may increase. EFFECTIVENESS TRIED: Making drivers aware that they are approaching an intersection, through the use of enhanced signing and delineation, should improve safety at the intersection because drivers will be more alert to potential vehicles on the cross streets. This heightened awareness will quicken drivers' reaction times when conflicts occur. One study concluded that installing double stop signs can reduce all crashes up to 11% and right-angle crashes up to 55%. The same study concluded that installing advance warning signs can reduce all crashes up to 30% at urban locations and 40% at rural locations. Another analysis indicated a crash reduction of 70% when flashing beacons were installed on advance of 3-leg intersections and up to 39% at 4-leg intersections. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections. SUPPLEMENTAL INFORMATION Signing in conformance with the Manual on Uniform Traffic Control Devices should be provided. [Home] Slide 33Improve Visibility of the Intersection by Providing LightingNCHRP Report 500 • Vol. 5 • Strategy E2
Where to use:Unsignalized, unlit intersections with substantial patterns of nighttime crashes. In particular, patterns of rear-end, right-angle, or turning crashes on the major-road approaches to an unsignalized intersection may indicate that approaching drivers are unaware of the presence of the intersection. Key to success:Identifying sites where a lack of lighting is truly a significant factor in the nighttime crash experience. Also, develop an appropriate lighting system following AASHTO and the Illuminating Engineering Society of North America (IESNA) criteria. Cost: Moderate to High (3 on a scale of 1 to 4) Timeframe: Medium (2 on a scale of 1 to 3) Effectiveness: Proven Slide Notes DETAILS Providing lighting at the intersection itself, or both at the intersection and on its approaches, can make drivers aware of the presence of the intersection and reduce nighttime crashes. KEY TO SUCCESS Identifying sites where a lack of lighting is truly a significant factor in the nighttime crash experience. Also, develop an appropriate lighting system following AASHTO and the Illuminating Engineering Society of North America (IESNA) criteria. ISSUES Lighting is feasible only where an appropriate supply of electrical power is available. This is not usually a problem in urban and suburban areas, but some rural intersections where lighting would be desirable may be isolated from power sources. TIME FRAME: Medium A lighting project generally requires at least 1 year to implement because the lighting system must be designed and the provision of electrical power must be arranged. COSTS: Moderate to High The provision of lighting involves both a fixed cost for lighting installation and an ongoing maintenance and power cost. EFFECTIVENESS PROVEN: Minnesota evaluated the effectiveness of installing streetlights at rural intersections. As part of the evaluation, Minnesota conducted a literature review and found that previously published research reported 25 to 50% reductions in the nighttime crash/total crash ratio due to the installation of intersection lighting. Based upon a comparative crash analysis and a before-after evaluation, Minnesota concluded that the installation of streetlights reduced nighttime crashes at rural intersections and would be more effective in reducing nighttime crashes than either rumble strips or overhead flashing beacons. From an economic standpoint, Minnesota indicated that the benefits associated with the installation of streetlights at rural intersections outweigh the costs by a margin of 15 to 1. Based upon the Minnesota study and previous studies, providing lighting at an intersection improves the safety of an intersection during nighttime conditions by (1) making drivers more aware of the intersection, which improves drivers' perception-reaction times, (2) enhancing drivers' available sight distances, and (3) improving the visibility of nonmotorists. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections. In particular, this strategy may be compatible with Strategy E11 (Install Flashing Beacons), a strategy that also requires an electrical power source. [Home] Slide 34Install Splitter Islands on the Minor Road Approach to an IntersectionNCHRP Report 500 • Vol. 5 • Strategy E3
Where to use:Minor road approaches to unsignalized intersections where the presence of the intersection or the stop sign is not readily visible to approaching motorists. The strategy is particularly appropriate for intersections where the speeds on the minor road are high. Key to success:Designing the island in accordance with the principles of channelization presented in the AASHTO Policy on Geometric Design of Highways and Streets and NCHRP Report 279: Intersection Channelization Design Guide. The visibility of the splitter island will, in part, depend on its placement relative to the profile of the major road. Cost: Moderate (2 on a scale of 1 to 4) Timeframe: Medium (2 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Many unsignalized intersections are not visible to approaching drivers. Thus, intersection crashes may occur because one or more drivers may be unaware of the intersection. "Splitter" islands can be installed on minor road approaches to call attention to the presence of the intersection and to guide traffic through the intersection. A splitter island refers to a channelizing island that separates traffic in opposing directions of travel, as opposed to islands that separate merging or diverging traffic in the same direction of travel. Splitter islands are particularly appropriate on approaches to skewed intersections. KEY TO SUCCESS Designing the island in accordance with the principles of channelization presented in the AASHTO Policy on Geometric Design of Highways and Streets and NCHRP Report 279: Intersection Channelization Design Guide. The visibility of the splitter island will, in part, depend on its placement relative to the profile of the major road. ISSUES There is a potential for the safety effectiveness of splitter islands to be negated if the shoulder is used in place of widening the roadbed to accomplish the channelization. Raised islands on a minor street could also become a safety hazard along a high-speed major roadway if not designed and delineated properly. TIME FRAME: Medium Intersection improvements involving splitter islands generally take approximately 1 to 2 years to design and construct. Significant channelization may require minor right-of-way acquisition, which could further increase implementation time. COSTS: Moderate Costs involved in implementing splitter islands are moderate, unless acquisition of additional right-of-way is required, in which case costs may be higher. EFFECTIVENESS TRIED: Splitter islands are generally perceived to be effective in defining the presence of an intersection. When properly applied, they may reduce traffic speeds and intersection crashes, but there is no consensus on their effectiveness. An Australian study concluded that installing splitter islands on minor road approaches can reduce injury crashes by 35% at rural locations and by 40% at urban locations. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections. [Home] Slide 35Provide a Stop Bar (or Provide a Wider Stop Bar) on Minor Road ApproachesNCHRP Report 500 • Vol. 5 • Strategy E4
Where to use:Approaches to unsignalized intersections having traffic control devices that are not currently being recognized by some approaching motorists. Locations should be identified by patterns of crashes related to lack of driver recognition of the traffic control device (e.g., right-angle crashes related to stop sign violations). Key to success:Identify appropriate intersection approaches that would benefit from its use. The strategy is expected to be especially effective when applied on approaches where conditions allow the stop bar to be seen by an approaching driver at a significant distance from the intersection. This strategy is appropriate for locations with a pattern of angle crashes associated with stop sign violations where approaching drivers may not realize that an intersection is present until it is too late to stop. Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Providing visible stop bars on minor road approaches to unsignalized intersections can help direct the attention of drivers to the presence of the intersection. Where a stop bar is already in place, provision of a wider stop bar may be considered. KEY TO SUCCESS Identify appropriate intersection approaches that would benefit from its use. The strategy is expected to be especially effective when applied on approaches where conditions allow the stop bar to be seen by an approaching driver at a significant distance from the intersection. This strategy is appropriate for locations with a pattern of angle crashes associated with stop sign violations where approaching drivers may not realize that an intersection is present until it is too late to stop. ISSUES None identified. TIME FRAME: Short This strategy can be implemented quickly, typically in less than 3 months. COSTS: Low Costs for implementing this strategy are nominal. An agency's maintenance costs may increase. EFFECTIVENESS TRIED: One limited study has indicated that installing stop bars (or wider stop bars) on minor road approaches may reduce crashes by up to 19% and reduce right-angle crashes by up to 47%. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections. SUPPLEMENTAL INFORMATION Signing in conformance with the Manual on Uniform Traffic Control Devices should be provided. [Home] Slide 36Install Larger Regulatory and Warning Signs at IntersectionsNCHRP Report 500 • Vol. 5 • Strategy E5
Where to use:Approaches to unsignalized intersections with patterns of rear-end, right-angle, or turning collisions related to lack of driver awareness of the presence of the intersection. Key to success:Select a combination of regulatory and warning sign techniques appropriate to conditions on particular approaches to unsignalized intersections. This engineering judgment should, where possible, be accompanied by a human factors assessment of the need for regulatory and warning signs. Another key is the ability and commitment of the highway agency to adequately maintain the signs. Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS The visibility of intersections and, thus, the ability of approaching drivers to perceive them can be enhanced by installing larger regulatory and warning signs at intersections. Such improvements may include stop signs, intersection warning signs, stop ahead signs, pavement markings, and post-mounted delineators. The FHWA Older Driver Highway Design Handbook encourages such improvements to contribute to a better driving environment for older drivers. KEY TO SUCCESS Select a combination of regulatory and warning sign techniques appropriate to conditions on particular approaches to unsignalized intersections. This engineering judgment should, where possible, be accompanied by a human factors assessment of the need for regulatory and warning signs. Another key is the ability and commitment of the highway agency to adequately maintain the signs. ISSUES Care should be taken not to overuse traffic signing, as it is likely that drivers will become accustomed to their presence and fail to respond as desired or intended. Agencies should strive to use special signing only where a specific problem or circumstance indicates the need. TIME FRAME: Short This strategy does not require a long development process. Signing improvements can typically be implemented in 3 months or less. COSTS: Low Costs for implementing this strategy are nominal. An agency's maintenance costs may increase. EFFECTIVENESS TRIED: One limited study has indicated that installing larger stop signs may decrease all collisions by up to 19%. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections. SUPPLEMENTAL INFORMATION Signing in conformance with the Manual on Uniform Traffic Control Devices should be provided. [Home] Slide 37Call Attention to the Intersection by Installing Rumble Strips on Intersection ApproachesNCHRP Report 500 • Vol. 5 • Strategy E6
Where to use:Approaches to unsignalized intersections with traffic control devices that are not currently being recognized by some approaching motorists. Locations should be identified by patterns of crashes related to lack of driver recognition of the traffic control device (e.g., right-angle crashes related to stop sign violations). Rumble strips should be considered only after an adequate trial of less intrusive treatments. Key to success:Use rumble strips sparingly so that they retain their surprise value in gaining the driver's attention. Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Rumble strips can be installed on intersection approaches to call attention to the presence of the intersection and to the traffic control in use at the intersection. Rumble strips should be used sparingly. Their effectiveness is dependent on being unusual. Rumble strips are normally applied when less intrusive measures—such as pavement markings like "STOP AHEAD" signs, markings, or flashers—have been tried and have failed to correct the crash pattern. Rumble strips can be used to supplement such traffic control devices. For example, a rumble strip can be located so that when the driver crosses the rumble strip, a key traffic control device such as a "STOP AHEAD" sign is directly in view. Rumble strips in the traveled way can also be used on a temporary basis to call attention to changes in traffic control devices, such as installation of a stop sign where none was present before. NCHRP Synthesis of Highway Practice 191 reviews the state of the art of rumble strip usage. KEY TO SUCCESS Use rumble strips sparingly so that they retain their surprise value in gaining the driver's attention. ISSUES Rumble strips in the traveled way have several potential pitfalls that should be considered carefully in any decision to implement them. They include (1) noise that may disturb nearby residents; (2) potential loss-of-control problems for motorcyclists and bicyclists; (3) difficulties created for snowplow operations; and (4) inappropriate driver responses, such as using the opposing travel lanes to drive around the rumble strips. TIME FRAME: Short Rumble strips typically can be implemented in 3 months or less. COSTS: Low Costs to implement rumble strips would normally be nominal. EFFECTIVENESS TRIED: Rumble strips are generally perceived to be effective in reducing intersection crashes when used appropriately, but there is no consensus on their effectiveness. One study concluded that transverse rumble strips may decrease overall crashes by up to 28% and rear-end crashes by up to 90%. Another study indicated that rumble strips installed in rural locations can decrease overall crashes up to 35%. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections, except that it would not be compatible with strategies involving removal or relocation of an intersection. [Home] Slide 38Provide Dashed Markings (Extended Left Edgelines) for Major Road Continuity Across the Median Opening at Divided Highway IntersectionsNCHRP Report 500 • Vol. 5 • Strategy E7
Where to use:Unsignalized intersections on divided highways. The strategy is particularly appropriate for intersections with patterns of rear-end, right-angle, or turning crashes related to lack of awareness by the driver on the minor road to the presence of the intersection. Key to success:Select a combination of marking techniques appropriate to conditions on particular unsignalized intersection approaches on divided highways. This engineering judgment should, where possible, be accompanied by a human-factors assessment of marking needs. Another key is the ability and commitment of the highway agency to maintain the markings adequately. Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Providing dashed markings (extended left edgelines) to define median roadway area at divided highway intersections can help distinguish the median area from the through roadway and, thus, enhance the ability of approaching drivers to be more aware of the presence of the intersection. KEY TO SUCCESS Select a combination of marking techniques appropriate to conditions on particular unsignalized intersection approaches on divided highways. This engineering judgment should, where possible, be accompanied by a human-factors assessment of marking needs. Another key is the ability and commitment of the highway agency to maintain the markings adequately. ISSUES None identified. TIME FRAME: Short This strategy does not require a long development process and can typically be implemented in 3 months or less. COSTS: Low Costs to implement this strategy are nominal. An agency's maintenance costs may increase. EFFECTIVENESS TRIED: The effectiveness of this strategy in reducing crashes has not been satisfactorily quantified. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections. [Home] Slide 39Provide Supplementary Stop Signs Mounted Over the RoadwayNCHRP Report 500 • Vol. 5 • Strategy E8
Where to use:Unsignalized intersections with patterns of right-angle crashes related to lack of driver awareness of the presence of the intersection. In particular, it might be appropriate to use this strategy at the first stop-controlled approach (possibly of a series) located on a long stretch of highway without any required stops, or at an intersection located after a sharp horizontal curve. Key to success:Locate the supplementary overhead sign (or signs) in the direct line of sight of approaching drivers. Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Many stop signs at stop-controlled intersections are not readily visible to approaching drivers due to geometric conditions, presence of vegetation, or other objects (such as tall vehicles) that can limit the view of the regular stop signs. Thus, intersection crashes may occur because approaching drivers may be unaware of the presence of the stop sign at the intersection. The visibility of stop signs and, thus, the ability of approaching drivers to perceive them, can be enhanced by providing supplementary stop signs suspended over the roadway. The target for this strategy should be stop signs at intersections that are not clearly visible to approaching motorists, particularly approaching motorists on the minor road. The strategy is particularly appropriate for intersections with patterns of rear-end, right-angle, or turning collisions related to lack of driver awareness of the presence of the intersection or stop sign. KEY TO SUCCESS Locate the supplementary overhead sign (or signs) in the direct line of sight of approaching drivers. ISSUES Unless the signs are mounted on existing overhead structures (mast arms), additional hardware will have to be placed on the roadside, which could become an additional object that a vehicle may strike if it leaves the roadway. TIME FRAME: Short This strategy does not require a long development process and can typically be implemented in 3 months or less. COSTS: Low The costs involved in providing supplementary overhead stop signs are minimal when the signs are mounted on existing structures. The additional cost of providing a mast arm is moderate. Agencies may experience additional maintenance costs. EFFECTIVENESS TRIED: The safety effectiveness of providing supplementary stop signs mounted over the roadway has not been quantified. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections. SUPPLEMENTAL INFORMATION Supplementary signs should be in accordance with the MUTCD. [Home] Slide 40 Provide Pavement Markings With Supplementary MessagesNCHRP Report 500 • Vol. 5 • Strategy E9
Where to use:Unsignalized intersections with patterns of rear-end, right-angle, or turning crashes related to lack of driver awareness of the presence of the intersection. Key to success:Select a combination of marking techniques appropriate to conditions on particular unsignalized intersection approaches. Another key is the ability and commitment of the highway agency to maintain the markings adequately. Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Providing pavement markings with supplementary messages (such as "STOP AHEAD") can help alert drivers and thus enhance the ability of approaching drivers to be more aware of the presence of the intersection. These markings should follow the Manual on Uniform Traffic Control Devices. KEY TO SUCCESS Select a combination of marking techniques appropriate to conditions on particular unsignalized intersection approaches. Another key is the ability and commitment of the highway agency to maintain the markings adequately. ISSUES Potential difficulties may be encountered in the winter, when these markings may not be as visible to the driver. The pavement markings may also have a lower coefficient of friction compared to the rest of the approach, especially during wet conditions. TIME FRAME: Short This strategy does not require a long development process and can typically be implemented in 3 months or less. COSTS: Low Costs to implement this strategy are nominal. An agency's maintenance costs may increase. EFFECTIVENESS TRIED: Limited studies have suggested that installing supplementary pavement messages may decrease overall crashes by 6% and right-angle crashes at urban locations by 30%. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections. SUPPLEMENTAL INFORMATION Supplementary pavement markings should follow the MUTCD, which drivers should understand with no need for special public education campaigns. [Home] Slide 41 Provide Improved Maintenance of Stop SignsNCHRP Report 500 • Vol. 5 • Strategy E10
Where to use:All stop-controlled intersections. Key to success:Determine an effective maintenance schedule that may be adequately sustained by highway agencies. Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Maintenance of stop signs must be at a high standard to ensure that the effectiveness of the signs is retained. According to the Manual on Uniform Traffic Control Devices, stop signs must be kept in proper position, clean, and legible at all times (both day and night). Damaged signs should be replaced without undue delay. To ensure adequate maintenance, a suitable schedule for inspection, cleaning, and replacement of stop signs should be established. Employees of highway agencies, police, and other governmental employees whose duties require that they travel on the highways should be encouraged to report any damaged or obscured signs at the first opportunity. Special attention and necessary action should be taken to see that trees, shrubbery, and construction materials do not obscure stop signs and that the stop signs present proper reflectorization. KEY TO SUCCESS Determine an effective maintenance schedule that may be adequately sustained by highway agencies. ISSUES None identified TIME FRAME: Short This strategy does not require a long development process. A maintenance schedule can typically be developed in 3 months or less. COSTS: Low Costs for maintenance of stop signs are relatively low. An agency's maintenance costs may increase. EFFECTIVENESS TRIED: The effectiveness of this strategy has not been satisfactorily quantified. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections. [Home] Slide 42Install Flashing Beacons at Stop-Controlled IntersectionsNCHRP Report 500 • Vol. 5 • Strategy E11
Where to use:Unsignalized intersections with patterns of right-angle crashes related to lack of driver awareness of the intersection on an uncontrolled approach and lack of driver awareness of the stop sign on a stop-controlled approach. Key to success:Select intersections with crash patterns appropriate to mitigation by flashing beacons. Otherwise, the use of a flashing beacon may provide no safety benefit (or a negative safety benefit). Crash types mitigated by flashing beacons may include right-angle, rear-end, and turning crashes. Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Overhead flashing beacons can be used at stop-controlled intersections to supplement and call driver attention to stop signs. Flashing beacons are intended to reinforce driver awareness of the stop sign and to help mitigate patterns of right-angle crashes related to stop sign violations. At two-way stop-controlled intersections, flashing beacons are used with red flashers facing the stop-controlled approaches and yellow flashers facing the unstopped approaches. At all-way stop controlled intersections, red flashers face all approaches. Use of overhead flashing beacons can increase the visibility of intersections for approaching drivers, and can be used in conjunction with the signing, delineation, and flashing improvements discussed in Strategy E1. KEY TO SUCCESS Select intersections with crash patterns appropriate to mitigation by flashing beacons. Otherwise, the use of a flashing beacon may provide no safety benefit (or a negative safety benefit). Crash types mitigated by flashing beacons may include right-angle, rear-end, and turning crashes. ISSUES If the flashing beacons are not properly placed where they are clearly visible to approaching drivers, they may not be effective. Flashing beacons also should not be overused. Their effectiveness is attributed in part to their relative uniqueness (i.e., they are not typically found at every stop-controlled intersection). Some agencies have reported crashes at red/amber flashers where a driver facing a red flasher assumed that the intersecting approach also had a red flasher. Flashing beacons are generally well understood by drivers. At times, drivers on minor streets may be confused regarding the nature of control on the major street. TIME FRAME: Short Use of flashing beacons does not require extensive development; flashing beacons can be installed within 3 to 6 months. The major implementation problem is providing power to the site. COSTS: Low Costs of installing flashing beacons are generally nominal, with the greatest cost being the provision of power to the site. EFFECTIVENESS TRIED: Several studies have evaluated the safety effectiveness of flashing beacons at stop-controlled intersections. Ohio compared the safety at rural, low-volume intersections controlled by stop signs and controlled by flashing beacons in conjunction with stop signs. Ohio found that flashing beacons generally reduced vehicular speeds on the major road, particularly at intersections with sight distance restrictions, but the flashing beacons were not necessarily effective in reducing stop sign violations or crashes. Similarly, California found that overhead yellow-red flashing beacons did not significantly reduce the number of fatal crashes at stop-controlled intersections. Therefore, additional research may be desirable to further evaluate the safety effectiveness of this strategy. Florida estimated that overall crashes may be reduced up to 26% and injury crashes may be reduced up to 50% after installing flashing yellow-red signal indications. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections. [Home] Slide 43Avoid Signalizing Through RoadsNCHRP Report 500 • Vol. 5 • Strategy F1
Where to use:Medium- to high-volume unsignalized intersections where installation of signals is being considered. Before a decision to install a signal is made, adequate consideration should be given to less restrictive forms of traffic control. Key to success:Identify an appropriate alternative design or traffic control method that will operate more safely than a signalized intersection. Some intersections serve traffic volumes that are so high that signalization may not be avoided. Cost: High (4 on a scale of 1 to 4) Timeframe: Long (3 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Signalization of unsignalized intersections often leads to an increased frequency of crashes on major roadways. Signals associated with new developments introduce congestion and increase crashes on through roadways that previously operated relatively safely and smoothly. Thus, the key to crash reduction is to avoid installing signal control whenever possible. Alternatives to signal control include all-way stop control; roundabouts; turn prohibitions (e.g., limiting movements to right-turn in and right-turn out); indirect left-turn movements (e.g., jug handles, loops, and median crossovers); and provision of flyovers and other grade separations. KEY TO SUCCESS Identify an appropriate alternative design or traffic control method that will operate more safely than a signalized intersection. Some intersections serve traffic volumes that are so high that signalization may not be avoided. ISSUES A potential difficulty with this strategy is that the selected intersection control strategy may operate less efficiently than a signal (i.e., may involve more delay to motorists or produce out-of-direction travel), or the costs and feasibility of alternatives to signals are much greater. The project development process should include an explicit review of the traffic operational performance of the alternatives considered (e.g., roundabouts – see Fact Sheet F3). Whenever implementing intersection control treatments, the safety of pedestrians, bicyclists and other users should always be considered. TIME FRAME: Long Simple changes in intersection traffic control, such as all-way stop control, can be made in 3 months or less. Projects involving more extensive construction, such as provision of roundabouts or construction of grade-separated interchanges, may involve a project development process up to 4 years or more in duration. COSTS: High Most construction alternatives, such as jug handles, grade separations, interchanges, and roundabouts, would require significant investment. In many cases, right-of-way acquisition would be a part of this. EFFECTIVENESS TRIED: The strategies that can be used as alternatives to signals are known to be effective, but their safety effects are highly site specific. It is known that traffic signals generally increase crash frequency when installed. However, there are no established quantitative measures of the effects of traffic signals in increasing crashes or the effects of the alternative strategies in mitigating those effects. The effect of these strategies on crash severity distributions also has not been quantified. Some of the alternative strategies (e.g., indirect left turns) have been used by some highway agencies for many years, but there is no consensus on the strategies' quantitative safety effects. Other strategies (e.g., roundabouts) have only recently come into more widespread use. COMPATIBILITY This strategy can be used in conjunction with most other strategies to improve safety at unsignalized intersections. SUPPLEMENTAL INFORMATION There is a potential need for public information and education about the strategies selected, particularly when unfamiliar techniques such as roundabouts are used in an area for the first time. If not already in place, a set of warrants and guidelines for the alternative controls or treatments should be developed as part of the agency's policies. For example, warrants and guidelines may be needed for determining the appropriate conditions under which roundabouts are to be used in rural areas. Some highway agencies have adopted policies wherein construction of multilane divided arterials in rural areas will exclude provision of signalized intersections in favor of interchanges. [Home] Slide 44Provide All-Way Stop Control at Appropriate IntersectionsNCHRP Report 500 • Vol. 5 • Strategy F2
Where to use:Unsignalized intersections with patterns of right-angle and turning crashes and moderate and relatively balanced volumes on the intersection approaches. Key to success:Identify moderate volume situations where all-way stop control will operate efficiently without substantially more delay than a signalized intersection. It is important that the driving public be alerted to the change of control during a transition period. Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Proven Slide Notes DETAILS All-way stop control can reduce right-angle and turning collisions at unsignalized intersections by providing more orderly movement at an intersection, reducing through and turning speeds, and minimizing the safety effect of any sight distance restrictions that may be present. However, all-way stop control is suitable only at intersections with moderate and relatively balanced volume levels on the intersection approaches. Under other conditions, the use of all-way stop control may create unnecessary delays and aggressive driver behavior (e.g., deliberate ignoring of the stop control). KEY TO SUCCESS Identify moderate volume situations where all-way stop control will operate efficiently without substantially more delay than a signalized intersection. It is important that the driving public be alerted to the change of control during a transition period. ISSUES Not every two-way stop-controlled intersection should be considered as a candidate for all-stop control. This strategy should be used selectively, recognizing traffic volumes and patterns and potentially adverse reaction by the driving population to being stopped for no apparent reason. If drivers encounter substantial delays, they may become impatient and act irrationally, which can lead to crash patterns of the type that the strategy is intended to correct. TIME FRAME: Short All-way stop control can normally be implemented with just a change in signing at the intersection or on intersection approaches, typically in 3 months or less. COSTS: Low The costs involved in converting to all-way stop control are relatively low. However, an agency's maintenance costs may increase. EFFECTIVENESS PROVEN: A recent review of the effectiveness of various strategies in reducing crashes concluded that conversion from two-way to all-way stop control could reduce total intersection crashes by 53%. Another study determined that converting to an all-way stop from a two-way stop may reduce overall crashes at urban locations by up to 71%. Similarly, reductions were seen for left-turn crashes (20%), right-angle crashes (72%), rear-end crashes (13%), and pedestrian crashes (39%). COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections. SUPPLEMENTAL INFORMATION Drivers understand all-way stop control with no need for special public education campaigns. However, public information should be distributed about any forthcoming change in traffic control. [Home] Slide 45Provide Roundabouts at Appropriate LocationsNCHRP Report 500 • Vol. 5 • Strategy F3
Where to use:Unsignalized intersections that are experiencing right-angle, rear-end, and turning crashes. Roundabouts are appropriate at most intersections, and at intersections with large traffic delays roundabouts are oftentimes a superior alternative to signalization. Roundabouts can also be very effective at intersections with complex geometry and intersections with frequent left-turn movements. Key to success:One key to success is designing the roundabout and its approaches in accordance with accepted geometric design and traffic control criteria. Designs that accommodate the needs of non-motorized users and include proper signing, pavement marking, and intersection lighting are keys to the success of a roundabout. Because public understanding of roundabouts is somewhat limited in many parts of the United States, educating the general public and local units of government about the effectiveness of roundabouts in reducing crashes is advisable. Cost: High (4 on a scale of 1 to 4) Timeframe: Long (3 on a scale of 1 to 3) Effectiveness: Proven Slide Notes DETAILS The modern roundabout is a circular intersection with design features that promote safe and efficient traffic flow whereby vehicles travel counterclockwise around a raised center island, with entering traffic yielding the right-of-way to circulating traffic. Drivers approaching a roundabout must reduce speed, be prepared to stop for pedestrians and bicyclists, and look for potential conflicts with vehicles already in the circulatory roadway. Appropriate geometric curvature on the roundabout entry controls speeds. Once in the roundabout, vehicles maintain low speeds due to deflection of traffic around the center island as drivers proceed to the appropriate exit following the guidance provided by traffic signs and pavement markings. Modern roundabouts have key differences that distinguish them from older traffic circles and rotaries. Design guidance for roundabouts is provided in Roundabouts: An Informational Guide (www.tfhrc.gov/safety/00068.htm). KEY TO SUCCESS One key to success is designing the roundabout and its approaches in accordance with accepted geometric design and traffic control criteria. Designs that accommodate the needs of non-motorized users and include proper signing, pavement marking, and intersection lighting are keys to the success of a roundabout. Because public understanding of roundabouts is somewhat limited in many parts of the United States, educating the general public and local units of government about the effectiveness of roundabouts in reducing crashes is advisable. ISSUES A major impediment is the negative perception held by some drivers and elected officials. Drivers may be skeptical, or even opposed, to roundabouts when they are proposed. However, opinions quickly change when drivers become familiar with roundabouts. A study by the Insurance Institute for Highway Safety in communities where roundabouts replaced stop signs or signal-controlled intersections found 31% of drivers supported the roundabouts before construction, compared with 63% shortly after. Follow-up surveys conducted in these communities after roundabouts had been in place for more than one year found the level of public support increased to about 70% on average. Roundabouts are not appropriate everywhere. Intersections that may not be good candidates include those with topographic or site constraints that limit the ability to provide appropriate geometry, those with highly unbalanced traffic flows, and isolated intersections in a network of traffic signals. Roundabouts provide several advantages for pedestrians over traditional intersections. The splitter islands at the roundabout approaches provide a pedestrian refuge area, and pedestrians cross only one direction of traffic at a time. In addition, crossing distances are relatively short, and traffic speeds are lower than at traditional intersections. The observational studies performed for NCHRP Report 572 – Roundabouts in the United States showed that the overwhelming majority of roundabouts observed showed very few problems for crossing pedestrians and traversing bicyclists. However, two-lane legs are more difficult for pedestrians to cross than one-lane legs primarily because of the non-yielding behaviors of motorists. Accessibility of roundabouts for pedestrians with visual impairments is a very important issue. The U.S. Access Board is active in proposing guidelines for accessible rights-of-way, and particular attention to accessibility issues is needed when considering the design of pedestrian facilities at roundabouts. TIME FRAME: Long Provision of a roundabout requires substantial project development. The need to acquire right-of-way will vary from site to site and depends upon the geometric design. These activities may require 4 years or longer to implement. Mini-roundabouts may be able to be built more expediently with signs and markings. COSTS: High Costs are variable, but construction of a roundabout to replace an existing intersection could run from several hundred thousand dollars to over $1 million, based on the project location and constraints. EFFECTIVENESS PROVEN: Provision of modern roundabouts is a relatively new strategy in the United States, although roundabouts have been used overseas for many years. Recent research has estimated the effectiveness of installing a modern roundabout at previously unsignalized locations at a 38% reduction in total crashes, a 76% reduction in injury crashes, and a 90% reduction in fatal and incapacitating-injury crashes. NCHRP Report 572 presents the results of the conversion of 36 two-way stop intersections to modern roundabouts, which resulted in an overall 44% percent reduction in total crashes and an 82% reduction in injury crashes. COMPATIBILITY Provision of a modern roundabout is typically an alternative to other intersection traffic control and intersection geometric improvements and is not typically used in conjunction with other strategies. SUPPLEMENTAL INFORMATION Although the precise number of roundabouts is unknown, estimates are that approximately 1000 have been built in the United States. By comparison, there are about 20,000 roundabouts in France, 15,000 in Australia, and 10,000 in the United Kingdom. Although many states have established design policies and guidelines for roundabouts, they are still relatively new to many highway agencies, and their use is not yet a widespread intersection strategy in some areas. Several states have implemented formal roundabout preference policies including New York, Wisconsin, and Virginia. [Home] Slide 46Provide Targeted Enforcement to Reduce Stop Sign ViolationsNCHRP Report 500 • Vol. 5 • Strategy G1
Where to use:Unsignalized intersections where stop sign violations and patterns of crashes related to stop sign violations have been observed. Crash types potentially related to stop sign violations include right-angle and turning collisions. Key to success:Identify the intersections that can potentially benefit from increased enforcement. Such intersections should have a combination of high stop sign violation rates and related crash patterns. It is important that both the highway agency and the law enforcement agency(ies) in the jurisdiction be involved jointly in planning and operating the program. The success of any enforcement program depends substantially on the performance of the officer in the field. Cost: Moderate (2 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Enforcement options are a potential countermeasure to unsafe and illegal motorist behavior at intersections. Studies report the reduction of traffic law violations when enforcement is used. Traffic law enforcement agencies will often select locations for targeted enforcement when crash, citation, or other sources of information suggest that the site is unusually hazardous due to illegal driving practices. Traffic law enforcement methods vary depending upon the type of program being implemented. KEY TO SUCCESS Identify the intersections that can potentially benefit from increased enforcement. Such intersections should have a combination of high stop sign violation rates and related crash patterns. In some cases, public input or observations by law enforcement personnel may suggest that a location should be targeted with enforcement. It is important that both the highway agency and the law enforcement agency(ies) in the jurisdiction be involved jointly in planning and operating the program. The success of any enforcement program depends substantially on the performance of the officer in the field. It is important that all officers involved be told of the objectives and expected benefits of the program and that they be given regular feedback on their effectiveness. It is also important to have interaction with the court systems operating in the jurisdiction so that the judiciary understands the objectives. It may also be possible in some cases to involve the judiciary in planning and implementing the program. ISSUES The major potential difficulty with a program of increased enforcement is the potential for diverting police officers from more productive work if the locations for stop sign enforcement are not selected carefully. In addition, care must be taken to identify appropriate and safe locations to stop violators and issue citations. Finally, if the court system does not adequately convict and apply sufficiently strong sanctions to the cited offenders, the program will lose its effectiveness. TIME FRAME: Short Targeted enforcement can be implemented in a short period of time. Identified problems can be addressed almost immediately if enforcement manpower is available. COSTS: Moderate There is almost no capital cost involved in increased enforcement, but staff hours and vehicle operating costs may be substantial. EFFECTIVENESS TRIED: This strategy is known to be effective in reducing traffic law violations. Programs within the United States have been found to result in decreases in violations of between 23 and 83%. However, the safety effectiveness of such decreases in violation rates has not been quantified. Enforcement agencies have generally found that the effectiveness of increased enforcement at specific locations has a relatively short duration of effectiveness—measured in days or weeks, rather than months or years. COMPATIBILITY This strategy can be used in conjunction with most other strategies to improve safety at unsignalized intersections. SUPPLEMENTAL INFORMATION There is a potential need for public information and education on the reasons for the strategies selected, particularly when targeted enforcement techniques are used in an area for the first time. A special informational campaign may be needed for the court system. Also refer to Countermeasures That Work from the National Highway Traffic Safety Administration. [Home] Slide 47Provide Targeted Public Information and Education on Safety Problems at Specific IntersectionsNCHRP Report 500 • Vol. 5 • Strategy G2
Where to use:Jurisdictions that have experienced a large number of safety problems at unsignalized intersections. Key to success:Reach as much of the targeted audience as possible, whether it is through television, radio, distribution of flyers, driver education classes, or other methods. Targeted drivers need to be defined in terms of both the location of the hazardous intersection(s) and the attributes of the drivers who may have been identified as over-represented in the population involved in crashes. Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Providing targeted public information and education on safety problems at specific intersections is a preventive measure that can help improve driver compliance with traffic control devices and traffic laws at intersections. Public information and education programs often add effectiveness to targeted enforcement programs, as well. However, this strategy stresses a separate use of the method. KEY TO SUCCESS Reach as much of the targeted audience as possible, whether it is through television, radio, distribution of flyers, driver education classes, or other methods. Targeted drivers need to be defined in terms of both the location of the hazardous intersection(s) and the attributes of the drivers who may have been identified as over-represented in the population involved in crashes. ISSUES A potential difficulty with this strategy is that the public information and education campaign may not reach many members of the targeted audience. It is often difficult to identify and focus upon a subset of the driving population using a specific intersection. Therefore, an area-wide program is often the preferred approach. TIME FRAME: Short Targeted public information and education campaigns should be well planned before implementation. The more time invested in the planning process, the greater the likelihood of the strategy reaching the appropriate audience and being effective. This strategy can be implemented in a relatively short period of time, typically from 6 months to a year. COSTS: Low The costs involved in a public-information and education campaign vary by the type of distribution (e.g., television, radio, newspaper, etc.), but are generally less expensive than many other intersection safety improvement strategies. EFFECTIVENESS TRIED: There are no established quantitative measures of the safety effectiveness of providing targeted public information and education on safety problems at specific intersections. COMPATIBILITY This strategy can be used in conjunction with most other strategies to improve safety at unsignalized intersections. SUPPLEMENTAL INFORMATION There is a potential need for cooperation among various media agencies to effectively implement the selected strategy. A media specialist should be involved from the initial stage of project planning. Also refer to Countermeasures That Work from the National Highway Traffic Safety Administration. [Home] Slide 48Provide Targeted Speed EnforcementNCHRP Report 500 • Vol. 5 • Strategy H1
Where to use:Unsignalized intersections where speed violations and patterns of crashes related to speed violations are observed. Crash types potentially related to speed violations include right-angle, rear-end, and turning crashes. Key to success:Plan the enforcement and prioritize the intersections that need it. Such intersections should have a combination of high speed-violation rates and related crash patterns. It is important that both the highway agency and the law enforcement agency(ies) in the jurisdiction be involved jointly in planning and operating the program. The success of any enforcement program depends substantially on the performance of the officer in the field. It is important that all officers involved be told of the objectives and expected benefits of the program and that they be given regular feedback on their effectiveness. Cost: Moderate (2 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Proven Slide Notes DETAILS Law enforcement is considered an important contributor for maintaining traffic safety. However, limited resources, such as staff and funds, constrain the efforts of police in providing targeted speed enforcement. Studies have shown that speed enforcement helps reduce the mean speed and, consequently, the number of injury, fatal, and property-damage-only crashes in which unsafe speed is the primary collision factor. Traffic law enforcement agencies will often select locations for targeted enforcement when crash, citation, or other sources of information suggest that the site is unusually hazardous due to illegal driving practices. Traffic law enforcement methods vary depending upon the type of program being implemented. KEY TO SUCCESS Plan the enforcement and prioritize the intersections that need it (see TRB Special Report 254). Such intersections should have a combination of high speed-violation rates and related crash patterns. In some cases, public input or observations by law enforcement personnel may suggest that a location should be targeted with enforcement. It is important that both the highway agency and the law enforcement agency(ies) in the jurisdiction be involved jointly in planning and operating the program. The success of any enforcement program depends substantially on the performance of the officer in the field. It is important that all officers involved be told of the objectives and expected benefits of the program and that they be given regular feedback on their effectiveness. It is also important to interact with the court systems operating in the jurisdiction so that the judiciary understands the objectives. It may also be possible in some cases to involve the judiciary in planning and implementing the program. ISSUES The major potential difficulty with a program of targeted speed enforcement is the potential for diverting police officers from more productive work if the locations for speed enforcement are not selected carefully. In addition, care must be taken to identify appropriate and safe locations to stop violators and issue citations. Finally, if the court system does not adequately convict and apply sufficiently strong sanctions to the cited offenders, the program will lose its effectiveness. TIME FRAME: Short Targeted speed enforcement can be implemented in a short period of time. Identified problems can be addressed almost immediately if enforcement manpower is available. COSTS: Moderate There are almost no capital costs involved in speed enforcement, but staff hours and vehicle operating costs may be substantial. EFFECTIVENESS PROVEN: The effectiveness of this strategy has been established by numerous studies. The most effective enforcement is the stopping and ticketing of offenders, as opposed to automated enforcement where fines are mailed on the basis of the vehicle's license plate number. Enforcement agencies have generally found that the effectiveness of increased enforcement at specific locations has a relatively short duration of effectiveness—measured in days or weeks, rather than months or years. One study concluded that pedestrian crashes may be significantly reduced when speed enforcement is increased. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at intersections. SUPPLEMENTAL INFORMATION There is a potential need for public information and education on the reasons for the targeted enforcement, particularly when targeted enforcement techniques are used in an area for the first time. A special informational campaign may be needed for the court system. [Home] Slide 49Provide Traffic Calming on Intersection Approaches Through a Combination of Geometrics and Traffic Control DevicesNCHRP Report 500 • Vol. 5 • Strategy H2
Where to use:Specific approaches to unsignalized intersections that are experiencing crash types potentially related to speed violations, specifically right-angle, rear-end, and turning collisions. Key to success:Carefully plan and determine the type of traffic calming measure viable for the specific intersection approach. Such intersections should have a combination of high-speed violation rates and related crash patterns. Cost: Moderate (2 on a scale of 1 to 4) Timeframe: Medium (2 on a scale of 1 to 3) Effectiveness: Proven Slide Notes DETAILS The goals of traffic calming are typically to reduce vehicle speeds, traffic volume, or both. Volume control measures limit traffic by restricting vehicle access. They include full street closures, half closures, diagonal diverters, median barriers, and forced-turn islands. Speed control measures can be divided into three types: vertical, horizontal, and narrowing. Vertical speed controls include speed humps, which are parabolic, circular, or sinusoidal mounds placed across a roadway. Speed tables are basically flat-topped speed humps. Horizontal speed controls slow traffic by requiring vehicles to shift direction in order to maneuver around them. The most common is the traffic circle. Narrowing roadways controls speed by reducing the amount of lateral space in which vehicles can maneuver. Design is one factor in the ultimate success or failure of a traffic calming measure. Equally important are (1) careful planning to determine whether the measure is a viable means of improving overall safety and mobility, (2) determining what impact the measure may have on street maintenance and emergency vehicles, (3) determining whether the measure will be self-enforcing (that is, not require additional policing), and (4) estimating how the measure will affect surrounding streets and neighborhoods. All of these issues need to be addressed before implementation. The early and continuous involvement of adjacent property owners, neighborhood groups, and relevant city agencies is crucial. KEY TO SUCCESS Carefully plan and determine the type of traffic calming measure viable for the specific intersection approach. Such intersections should have a combination of high-speed violation rates and related crash patterns. It is also critical to involve residents of the neighborhood when implementing traffic calming measures in such areas. ISSUES Traffic calming measures can oftentimes be controversial, especially when used to divert traffic from one road or street to another. TIME FRAME: Medium The implementation time for traffic calming measures will depend upon the type of measure used. Some types of traffic calming improvements may take 3 months or less (e.g., introducing speed humps) while others, especially when geometric improvements are required (e.g., traffic circles), may take 1 year or more. COSTS: Moderate The capital costs and maintenance costs involved in traffic calming measures vary depending on the type of traffic calming measure used. Some may be low cost (e.g., speed humps) while others that require geometric design improvements and/or acquisition of right-of-way may be moderate cost. To the extent required by law, individual property owners may be required to share in the cost of providing traffic calming measures in their area. EFFECTIVENESS PROVEN: Most traffic calming is implemented on local residential roadways where relatively few crashes occur compared to arterials and highways. Thus, the safety effectiveness data is very limited. Safety issues are oftentimes more of a perception problem on the part of the public. Lacking robust crash data, speed is oftentimes used as a surrogate measure of safety. Results from one study showed that the impacts on mean speed at single sites varied from a 3 mph increase to a 12 mph decrease. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at intersections. SUPPLEMENTAL INFORMATION Appropriate public information and education is fundamental for the effectiveness of this strategy, particularly when traffic calming techniques are used in an area for the first time. For additional sources of information on traffic calming, see http://www.fhwa.dot.gov/environment/calm and http://www.ite.org/traffic/. [Home] Slide 50 Post Appropriate Speed Limit on Intersection ApproachesNCHRP Report 500 • Vol. 5 • Strategy H3
Where to use:Unsignalized intersections experiencing a high frequency of speed related crashes. Key to success:Determine the appropriate speed limit for intersection approaches (based upon the functional class of the roadways, average operating speeds, traffic volume, geographical area, and roadside characteristics) and determine whether the speed limit should be reduced in the vicinity of the intersection. Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Speed is often cited as one of the major contributing factors to crashes. It is not necessarily the rate of speed that a vehicle is traveling that causes a crash, but the speed variance between vehicles. In a review of speed and crash probability, one study indicates that there is extensive evidence that speed variance increases crash probability. Due to the number of speed-related crashes, it is important for agencies to post appropriate speed limits on intersection approaches to convey consistent messages to drivers. Posting an appropriate speed limit on an approach may involve reducing the speed limit in the vicinity of the intersection or posting an advisory speed. KEY TO SUCCESS Determine the appropriate speed limit for intersection approaches (based upon the functional class of the roadways, average operating speeds, traffic volume, geographical area, and roadside characteristics) and determine whether the speed limit should be reduced in the vicinity of the intersection. ISSUES Several potential difficulties exist. First, the posted speed limit on an approach may be appropriate, but some studies have shown that this does not guarantee that speeds will change. Second, when it is determined that the current posted speed limit is inappropriate and should be changed, significant variances in speed may occur in the transition period after the new speed limit is posted until drivers become accustomed to the new posted speed. TIME FRAME: Short The implementation time for posting appropriate speed limits should take 3 months or less. COSTS: Low The costs involved in posting appropriate speed limits on intersection approaches are minimal. The costs involve conducting the necessary speed studies and replacing the signs. EFFECTIVENESS TRIED: The safety effectiveness of posting appropriate speed limits on intersection approaches has not been quantified. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at intersections, especially targeted speed enforcement, so that drivers obey the posted speed limit. SUPPLEMENTAL INFORMATION Legislated speed limits by road classification are determined by state legislatures and city councils for state and local roads, respectively. There may be a need to revise existing laws. [Home] Slide 51Provide Turn Path MarkingsNCHRP Report 500 • Vol. 5 • Strategy I1
Where to use:Complex unsignalized intersections with a high frequency of crashes related to turning vehicle positioning (e.g., sideswipe crashes). Key to success:Determine which maneuvers drivers are having trouble performing, and define and mark the appropriate turning paths. This may require extensive review of individual crash reports, as well as observations and measurements at a site. Proper maintenance of the markings will also be important to the success of this strategy. Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS At most intersections, pavement markings are provided on the intersection approaches, but the pavement markings end near the stop line. Rarely are pavement markings extended into or continued through intersections. At complex intersections, however, it may be beneficial to provide motorists with additional information to help with vehicle positioning through the intersections. In particular, it may be desirable to extend pavement markings through intersections that have offset approaches, are skewed, have multiple turn lanes, or are located at unsignalized ramp terminals. This approach is especially useful for delineating vehicle turning paths through an intersection. The MUTCD provides guidance on extending pavement markings through intersections. KEY TO SUCCESS Determine which maneuvers drivers are having trouble performing, and define and mark the appropriate turning paths. This may require extensive review of individual crash reports, as well as observations and measurements at a site. Proper maintenance of the markings will also be important to the success of this strategy. ISSUES If too many markings are extended through the intersection, the intersection could become very confusing for drivers. In cases where snow and ice collect on the road, the effectiveness of the markings may be reduced. TIME FRAME: Short The implementation time for providing turn path markings could be 3 months or less. COSTS: Low The costs involved in providing turn path markings are minimal. Agencies may experience additional maintenance costs. EFFECTIVENESS TRIED: The safety effectiveness of extending pavement markings through intersections has not been evaluated. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at intersections. SUPPLEMENTAL INFORMATION Highway agencies may need to adopt a policy for extending pavement markings through intersections. Guidance is provided in the MUTCD. [Home] Slide 52Provide a Double Yellow Centerline on the Median Opening of a Divided Highway at IntersectionsNCHRP Report 500 • Vol. 5 • Strategy I2
Where to use:Unsignalized intersections on divided highways that are experiencing a high degree of crashes caused by side-by-side queuing and angle stopping within the median area. Key to success:Ensure that the median is of sufficient width (at least 100 feet) so that vehicles can follow a desired path. Proper maintenance of the striping will be important to the strategy's success. Presence of snow or ice on the roadway area may significantly reduce the strategy's effectiveness at critical times. Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Undesirable driving behaviors often occur on the section of roadway at the opening of divided highways at intersections. Common types of undesirable driving behaviors include the following:
This strategy is designed to minimize the occurrence of the first two maneuvers (side-by-side queuing and angle stopping). Side-by-side queuing occurs when one vehicle is waiting on the median roadway for an opportunity to cross or enter the far roadway of a divided highway, and a second vehicle arrives and stops beside rather than behind the first vehicle. Side-by-side queuing can lead to driver confusion about which of the two vehicles is to proceed first and, thus, can lead to potential conflicts. Angle stopping occurs when a vehicle stops on the median roadway at some angle other than perpendicular to the through lanes of the divided highway. Stopping at an unusual angle is undesirable because the vehicle may be hit by another vehicle from any of several directions and because other drivers may be confused about the intended path of that vehicle. Providing a double yellow centerline on the median roadway at the opening can be helpful to define the vehicle paths at divided highway intersections. A double yellow centerline on the median roadway provides visual continuity with the centerline of the crossroad approaches and helps to define a desired path for drivers. The presence of a double yellow centerline on the median roadway should minimize the temptation for drivers to queue side-by-side or to cut over to the left side of the median roadway and stop at an angle when making a left turn. KEY TO SUCCESS Ensure that the median is of sufficient width (at least 100 feet) so that vehicles can follow a desired path. Proper maintenance of the striping will be important to the strategy's success. Presence of snow or ice on the roadway area may significantly reduce the strategy's effectiveness at critical times. ISSUES If the median roadway is narrow and a double yellow centerline is provided, it is possible that as vehicles queue one behind the other in the median, portions of vehicles will stick out (overhang) into the through roadway. TIME FRAME: Short The implementation time for this strategy is 3 months or less. COSTS: Low The costs involved in providing double yellow centerlines on median roadways are minimal. Agencies may experience additional maintenance costs. EFFECTIVENESS TRIED: The safety effectiveness of providing a double yellow centerline on the median opening of a divided highway has not been quantified. However, the presence of a double yellow centerline should minimize side-by-side queuing and angle stopping and, thus, reduce driver confusion near the intersection. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at intersections. SUPPLEMENTAL INFORMATION A highway agency may need to adopt a policy to determine when double yellow centerlines on median roadways of divided highways are warranted and appropriate. Guidance is provided in NCHRP Report 375. [Home] Slide 53Provide Lane Assignment Signing or Marking at Complex IntersectionsNCHRP Report 500 • Vol. 5 • Strategy I3
Where to use:Unsignalized intersections with a high frequency of crashes caused by driver indecision in lane assignment. Key to success:Ensure that lane assignment signs and/or markings are visible to drivers. Overhead signs are preferred to post-mounted signs (placed on the shoulder) because the overhead signs can be placed directly over the lanes to which they apply. In addition, the lane assignment signing/marking should be placed far enough in advance of the intersection so that vehicles can maneuver to the appropriate lane. Cost: Low (1 on a scale of 1 to 4) Timeframe: Short (1 on a scale of 1 to 3) Effectiveness: Tried Slide Notes DETAILS Sometimes, as drivers approach a complex intersection, they have difficulty determining the appropriate lane from which to perform a certain maneuver. This can cause indecision among drivers and result in maneuvers being made from certain lanes that are unexpected. These maneuvers could potentially lead to crashes. Crash patterns that are characteristic of driver indecision related to lane assignment include rear-end and sideswipe crashes on intersection approaches and potential angle crashes when a driver performs an unexpected maneuver from an inappropriate lane (e.g., a vehicle makes a left turn from a through lane). Providing lane assignment signs (or markings) to guide motorists through complex intersections can alleviate this confusion and lead to safer driving conditions. Pavement markings are often used to supplement lane assignment signs. KEY TO SUCCESS Ensure that lane assignment signs and/or markings are visible to drivers. Overhead signs are preferred to post-mounted signs (placed on the shoulder) because the overhead signs can be placed directly over the lanes to which they apply. In addition, the lane assignment signing/marking should be placed far enough in advance of the intersection so that vehicles can maneuver to the appropriate lane. Proper maintenance of the markings will be important to the strategy's success. Presence of snow or ice on the roadway area may significantly reduce the strategy's effectiveness at critical times. ISSUES Unless the lane assignment signs are mounted on existing posts, additional hardware will have to be placed on the roadside. This hardware becomes an additional object that a vehicle may strike if it leaves the roadway. TIME FRAME: Short The implementation time for post-mounted lane assignment signs should be 3 months or less. It may take up to a year to provide overhead signing. COSTS: Low The costs involved in providing lane assignment signs are minimal when post-mounted signs and pavement markings are used. The cost of overhead signing is moderate. Agencies may experience additional maintenance costs. EFFECTIVENESS TRIED: The safety effectiveness of providing lane assignment signing or marking has not been quantified. However, the presence of lane assignment signs and/or markings near the intersection should reduce driver confusion concerning proper lane assignment and minimize the number of unexpected maneuvers from designated lane groups. COMPATIBILITY This strategy can be used in conjunction with most other strategies for improving safety at intersections. |
Program ContactEd Rice What's NewPresentation: Intersection Safety Example Intersection Safety Implementation Plan Intersection Safety Implementation Plan Workshop Example Data Analysis Package and Straw Man Outline Intersection Safety Implementation Plan Process Report on the National Agenda for Intersection Safety Intersection Safety Needs Identification Report HighlightsFHWA Guidance Memo Contains Provisions to Improve Intersection Safety Presentation: Safety at Unsignalized Intersections Presentation: Safety Aspects of Roundabouts Presentation: Unsignalized Intersection Safety Strategies (NCHRP Report 500, Vol. 5) Presentation: Signalized Intersection Safety Strategies (NCHRP Report 500, Vol. 12) Presentation: Safety at Signalized Intersections |
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