Access Management in the Vicinity of Intersections - Technical Summary
Federal Highway Administration
Access Management is:
"Access management" refers to the design, implementation and management of entry and exit points (i.e., driveways, entrances) between roadways and adjacent properties. These entry and exit points can be managed by careful planning regarding their location, the types of turning movements allowed, and if appropriate, medians that provide or prohibit access to the driveways. Developing and implementing effective access management strategies that promote or improve safety requires considering the location of driveways in the context of current and future access needs, current and future intersection operations, and mobility for pedestrians and bicyclists.
This technical summary is designed as a reference for State and local transportation officials, FHWA Division Safety Engineers, and other professionals involved in the design, selection, and implementation of access management near traditional intersections (e.g. signalized, unsignalized and stop-controlled intersections). Its purpose is to provide an overview of safety considerations in the design, implementation, and management of driveways near traditional intersections in urban, suburban, and rural environments where design considerations can vary as a function of land uses, travel speeds, volumes of traffic by mode (e.g. car, pedestrian, or bicycle), and many other variables.
The first part of this presentation explains an overview of general access management factors that should be considered for improving safety near intersections in any setting. Planning, designing, and implementing access management strategies to promote safety near intersections begins with an awareness of several general considerations. These considerations are independent of the environment or setting in which the driveway is located (i.e. urban, suburban, or rural). These factors include roadway functional classification (sometimes referred to as "roadway hierarchy"); the functional area of the intersection; the location and number of driveways and resulting conflict points; the use of medians; and driveway design.
The next section presents access management considerations and strategies to improve safety near traditional intersections in suburban, urban, and rural settings. Urban, suburban, and rural areas each present unique opportunities and challenges with respect to design, selection and implementation of access management strategies that provide the highest level of safety in the vicinity of intersections. The following sections provide an overview of some additional special considerations that apply within each of these environments. Because suburban areas offer the greatest opportunities to improve safety through access management strategies (due to development trends and traffic volumes), this discussion addresses suburban areas first, followed by a discussion of urban and rural environments. This section features a case study of an access management retrofit project in a suburban area.
Consideration 1: Locate Driveways on the Appropriate Roadways
Providing access (i.e. driveways, entrances) onto roadways with the lowest traffic volumes and speeds generally improves safety near intersections. In planning, designing, and managing access, critical consideration must be given to arterial and collector streets as these streets serve both mobility and access functions. To the extent possible, it is best to manage driveways so that access is provided to and from the roadway with the lower functional classification as these roadways typically have lower traffic volumes and speeds. This helps to reduce the frequency of conflicts, which minimizes both the opportunity for crashes and the severity of those crashes should they occur.
Consideration 2: Limiting Driveways in the Functional Area of an Intersection Improves Safety
This slide shows a schematic representation of functional and physical areas of an intersection. The physical area of an intersection is a fixed area that represents the space confined within the corners of the intersection. The functional area of an intersection includes areas upstream and downstream of the intersection. Unlike the physical area of an intersection, the functional area of an intersection is variable. The American Association of State Highway and Transportation Officials' (AASHTO) A Policy On Geometric Design of Highways and Streets defines the upstream functional area of an intersection as a variable distance, influenced by: 1) distance traveled during perception-reaction time, 2) deceleration distance while the driver maneuvers to a stop, and 3) the amount of queuing at the intersection. The upstream functional area is highly dependent on whether or not the traffic in the through lane is required to come to a stop at the intersection. Therefore the functional area should be a consideration in situations where a driveway is near an intersection and where through traffic may be stopped at the intersection due to a traffic signal or stop sign.
The AASHTO Policy on Geometric Design does not define the downstream functional area of the intersection as the criteria used to determine the downstream functional area can vary between jurisdictions. The Access Management Manual, published by the Transportation Research Board, notes that "stopping sight distance is one method for establishing the downstream functional distance of an intersection." When calculating downstream functional area with this method, traffic control at the intersection is not a factor. Additionally, a recent study by the Texas Transportation Institute (TTI), Roadway Safety Design Synthesis discusses the safety effect of driveways in rural areas. The study includes equations to calculate the Accident Modification Factor (AMF) for access control based the number of driveways within 250 feet of a rural intersection.
Limiting or, where possible, eliminating driveways within the functional area of an intersection (upstream and downstream) helps reduce the number of decisions motorists must make while traveling through an intersection and improves safety in the vicinity of an intersection. A recent study evaluating crashes in the vicinity of signalized intersections in suburban areas completed by the Utah Department of Transportation provides one illustration of the correlation between driveways in the functional area of intersections and increased safety risk. The study evaluated right-turn and rear-end crashes at signalized intersections in suburban areas. The study found that the existence of accesses within the upstream functional area of the intersection correlated to increased crashes and crash severity costs. The report identified an even greater increase in total crashes, crash rates, and rear-end crashes as commercial access density increased.
Consideration 3: Reducing Conflict Points May Reduce Crashes
Full-Movement Driveways Without Median
In general, the number and types of conflict points (i.e., the number of locations where the travel paths of two different vehicles may cross) at the intersection of a driveway and a public road influence the safety of motorists. It is desirable to minimize the number of conflict points created with existing and future driveways since more conflict points increase the risk of a crash occurring. For example, a crash due to crossing maneuvers (created by motorists turning across the roadway or making left turns) can lead to more severe crashes then merging or diverging conflicts because of the angle and speed differentials between the vehicles. As the angle and speed differentials increases, crash severity can also increase.
The number and type of conflict points at a driveway can be managed by limiting both the amount of access allowed at the driveway (e.g. full movement, left-in/left-out, right-in/right-out, right-in only or right-out only) and the location of the driveway relative to other driveways in the area. In most cases, property owners prefer to have at least one direct, full-movement driveway from their property onto the major street adjacent to the property (i.e., the street with higher traffic volumes). In many cases it may occur that property owners are requesting direct, full-movement driveways to different properties on both sides of the major street. It is not always possible to align these driveways to minimize the number of conflict points, so another strategy, such as implementation of a raised median, should be considered. (Information on raised medians will be discussed later in the presentation.)
This figure illustrates a scenario like this, in which it is not possible to align the full-movement driveways in a manner that would reduce conflict points.
Consideration 3: Reducing Conflict Points May Reduce CrashesLimited Access Driveways with Raised Median
This figure illustrates how construction of a raised median on the major roadway could reduce the number of conflict points in this situation. The raised median limits the access to Driveways A and B to right-in/right-out movements only. Also, the number of conflicts in the vicinity of Driveways C and D are reduced by relocating Driveway C to the minor road. This solution also limits conflict points by providing a direct, full-movement driveway (i.e. left-in/left-out/right-in/right-out) to the minor road and by constructing a median on the major road and limiting access at Driveway D to right-in/right-out only.
Additional right-in/right-out only driveways are also provided on the minor street (Driveways F and E) to improve the access to the properties adjacent to Driveways B and D. If possible, it is preferable to provide driveways onto the minor street instead of on the major street in order to preserve mobility on the major street. Limiting turn movements to properties adjacent to the roadway can result in circuitous travel to and from a site. For example, a motorist exiting Driveway B is limited to one direction of travel and is required to make a U-turn or use Driveway F to reach other destinations.
Consideration 4: Eliminating Left-Turn Movements Improves Safety
It is most beneficial from a safety perspective to prohibit left-turning movements where restricting turning movements to and from a driveway is possible. Research suggests that approximately 72 percent of the crashes at a driveway involve a left-turning vehicle. As illustrated in the figure on this slide, approximately 34 percent of these crashes are due to an outbound vehicle turning left across through traffic. Twenty-eight percent of crashes are due to an inbound left-turning vehicle conflicting with opposite direction through traffic, and 10 percent are due to outbound left-turning movements incorrectly merging into the same direction through movement. This suggests that reducing or eliminating left turns to or from driveways, combined with efforts to reduce conflict points, enhances safety. When turn movements are restricted at driveways, roadway engineers, planners, and policy makers need to consider the tradeoffs of shifting the turning movement to another location along the roadway.
Consideration 5: Medians Can Reduce Crashes
One method to manage or limit left-turns to and from driveways is with the proper use of medians. Proper use of medians has been found to improve roadway safety significantly relative to undivided roadways. National Cooperative Highway Research Program (NCHRP) Report 420: Impacts of Access Management Techniques, identifies two types of medians typically used: non-traversable medians and continuous two-way left-turn lanes (TWLTL).
Non-traversable medians separate opposing directions of travel, significantly reducing the potential for head-on crashes and physically eliminating or limiting where left-turns and crossing movements across the median can occur. When a non-traversable median of sufficient width is constructed, it can also provide refuge for pedestrians crossing the roadway. Non-traversable medians generally result in an overall crash reduction of approximately 35 percent as compared to undivided roadways.
TWLTLs provide for left-turns in both directions of travel, except near signalized intersections where the center turn lane transitions to a conventional left-turn lane for one direction of travel. TWLTLs generally result in an overall crash reduction of approximately 33 percent as compared to undivided roadways. However, NCHRP 420 states that "Most studies, and the models derived from them, also suggest that safety is improved where physical medians replace TWLTLs." Factors to consider include differing roadway types, traffic volumes, travel speed, number of through lanes, and the number of left-turns and crossing maneuvers.
Consideration 6: Reducing Driveway Density Reduces Crash Rates
Research over the past decades has consistently shown that crash rates increase as driveway density increases on a roadway (i.e., number of driveways per mile). This graph illustrates this trend under a variety of roadway conditions and environments across the U.S. and in Canada. Property access points should be designed, approved, and permitted within the context of the number of driveways on both sides of the street within the vicinity of the proposed access points and should not be considered in isolation. Possible strategies to reduce the number of driveways over time include the use of shared access to serve more than one property, the planning and development of additional roadways to provide connectively and complementary mixed uses to minimize the need for multiple parking areas, and multiple driveways.
Consideration 7: Driveway Design Influences Safety and Mobility
Driveway connections to public roads must be adequately designed to ensure safe and efficient movement of vehicles to and from the roadway, balancing safety with mobility interests. The design of a driveway at any given location is a function of the design vehicle, travel speeds onto and off of the property, traffic volume, pedestrian and bicycle volume, and the type of traffic control (e.g. a signalized driveway should accommodate queues that may conflict with on-site turning movements).
There are many elements to consider in proper driveway design, including upstream and downstream sight distance, the angle at which the driveway intersects the major road, and driveway width and curb radii.
Other elements to consider in proper driveway design include the number of lanes (sufficient for the volume at the site) and the vertical grade of the driveway throat. For motorists leaving a property, the vertical alignment of the driveway should be as close to level as possible where it intersects with the roadway. The driveway should be level for a sufficient distance to allow the motorist to easily stop with an unobstructed view upstream and downstream prior to entering the major roadway.
Consideration 7: Driveway Design Influences Safety and Mobility (Continued)
Another element to consider is the length of the driveway throat. In general, driveways should be of sufficient length to allow motorists to completely pull off the road without interference from on-site parked vehicles, vehicle queues, or pedestrian or vehicle circulation once they enter the property adjacent to the roadway.
The Florida Department of Transportation Driveway Manual provides a thorough overview of the criteria and application methods that a practitioner should consider in the design of a driveway. In addition, jurisdiction-specific design guidelines should be consulted when designing a driveway.
Characteristics of Suburban Intersections
As the distance from the urban core increases, the density of the suburban development decreases. Emphasis on residential land use grows as one moves further from the urban core. Suburban areas tend to be characterized by large-scale and residential, commercial, industrial, or retail development typically separated by larger distances than in the urban core. In developing suburban areas, parcels can be combined to accommodate larger developments such as big box retail and strip malls. Land values often rely on spacious parking lots and convenient access to adjacent roadways.
Traffic characteristics of suburban areas include roadways with speeds that generally range from 35 to 50 mph (and occasionally up to 55 mph); medium to high traffic volumes (30,000 to 50,000 vehicles per day) on mainline roadways and 5,000 to 15,000 vehicles per day on side streets and non-residential driveways.
Physical Characteristics of Suburban Intersections
Signalized intersections on arterials and major collectors are characteristic of suburban areas.
Other physical characteristics include:
Traffic signal spacing in suburban areas is a function of the ability to progress two-way traffic along the mainline roadway. Signalized driveways are often not permitted in private developments but may be allowed if the spacing and timing can meet established standards to ensure adequate progression of traffic on the mainline roadway. Roadway and intersection improvement projects often are required to provide additional capacity for increasing traffic volumes.
Access Management Near Suburban Intersections
Suburban areas offer the greatest opportunity to positively impact safety through access management strategies for several reasons. New development and redevelopment often occurs on large parcels of land, providing planners with more flexibility and options for implementing optimal access management strategies. This can provide the opportunity for access to be considered from a systematic perspective, from the outset of a project, where stakeholders have the opportunity to plan for the appropriate number of driveways and optimum types of access (e.g. right-in/right-out only; right-in/right-out/left-in). For example, access to developments on corner lots may be limited to a side street where traffic volumes and speeds are typically lower. Where access to a major roadway is allowed, agencies with authority over the roadways have opportunities to limit turn movements to and from the driveway with physical treatments such as medians along the major roadway and/or median islands at location where the driveway connects to the major roadway. Further, adjacent land uses, including residential, commercial, and industrial, require significant access management planning and accommodation. Finally, while suburban areas are often lower density than urban areas, their residential and commercial centers are often connected by higher speed arterials (35 to 50 mph and occasionally up to 55 mph) than are found in urban areas, creating safety risks and opportunities through access management planning and implementation.
Treatments to Improve Motorist Safety Near Suburban Intersections
This section describes the following treatments to improve motorist safety in the vicinity of suburban intersections:
Suburban Intersections Treatment 1: Locate Driveways Upstream of Queues
Locate driveways upstream of the vehicle queue caused when the downstream traffic signal is red. This figure illustrates this scenario, in which site driveway "A" is located beyond the limits of the typical queue as shown with the yellow vehicles, with current traffic volumes.
Suburban Intersections Treatment 1: Locate Driveways Upstream of Queues (Continued)
Without changes in capacity, existing traffic queues will grow as traffic volumes increase. If possible, therefore, as development occurs, plan and locate driveways for future estimated traffic volumes. This figure demonstrates potential access issues at driveway "A" if future traffic queuing conditions as illustrated with the blue vehicles are not anticipated.
Suburban Intersections Treatment 2: Prohibit Median Openings
Prohibit median openings to restrict driveway movements to and from the left-turn lane at a major intersection. This figure illustrates the risks of allowing such a median opening. In this example, motorists turning left into the site access may conflict with the left-turning or through traffic.
Suburban Intersections Treatment 3: Driveway Alignment
In cases where there is a traversable median (e.g. TWLTL), aligning driveways to have a positive offset to minimize conflicts between left-turning vehicles is advantageous. The first figure illustrates driveway alignment with a positive offset. With a positive offset, motorists can use the two-way left-turn lane to access either driveway with a reduced likelihood of a crash. The second figure demonstrates a negative offset of driveway. Note here that if two motorists are using the two-way left turn lane at the same time, the drivers' paths would overlap (i.e. a crash may occur) as each driver tries to access the driveways.
Where it is not possible to align driveways with a positive offset, align driveways directly across the street from one another. The figure on the right illustrates this technique, which allows drivers to access either driveway without utilizing the same median area while decelerating prior to turning from the major roadway.
Treatments to Improve Bicyclist and Pedestrian Safety Near Suburban Intersections
The following access management approaches can help to improve pedestrian and bicyclist safety as well as mobility at access points in the vicinity of urban and suburban intersections (both signalized and unsignalized).
Suburban Intersections – Pedestrian and Bicyclist Safety Treatment 1: Construct a Channelized Island at Right-In Right-Out Driveways
Construct a channelized island between the inbound and outbound movements at right-turn-only driveways to provide a pedestrian refuge across the driveway.
Suburban Intersections – Pedestrian and Bicyclist Safety Treatment 2: Include Bike Lanes and Signage
Include bike lanes and signage, as appropriate, to alert bicyclists that motorists may be entering or exiting a driveway and to alert motorists that bicyclists may be crossing the driveway.
Other Treatments to Improve Bicyclist and Pedestrian Safety Near Suburban Intersections
Suburban Case Study: La Grande, Oregon
In June 1996, the Oregon Department of Transportation (ODOT) initiated a study of an unsignalized, full-access driveway (i.e., left and right turns allowed for inbound and outbound vehicles) in a suburban area. The study driveway was located on Oregon 82 (OR 82), approximately 600 feet south of Walton Road in La Grande, Oregon. La Grande has a population of approximately 12,500 and OR 82 is an undivided five-lane road. Adjacent land use development is a mix of big-box retail, commercial, and some industrial uses. The posted speed on this segment of OR 82 is 40 mph. This figure depicts this driveway (Drive "A") and the roadway configuration during the study period, prior to implementation of any access management improvements.
Suburban Case Study: La Grande, Oregon
As you can see in this table, during a 34-month period between November 1994 and August 1997, ODOT crash reports show that 12 crashes occurred at this unsignalized driveway.
During the study period, the average annual daily traffic volumes on OR 82 approached 17,200 in the vicinity of driveway "A." Data also showed that approximately 500 vehicles per day traveled inbound/outbound on the study driveway "A." The average crash rate was 0.66 crashes per million entering vehicles.
This table summarizes the crashes reported during this period. Eleven of the crashes involved vehicles turning left from the driveway onto southbound OR 82. All of these crashes included a collision with a vehicle moving northbound on OR 82, and all of the reported crashes involved left-turning vehicles. One of the crashes involved a motorist turning left out of the driveway onto southbound OR 82 and colliding with a motorist traveling northbound on OR 82 who was turning right into driveway "A."
Suburban Case Study: La Grande, Oregon
After performing a review of the roadway configuration, ODOT staff recommended the following access management improvements, depicted in this figure:
The proposed access management improvements were approved and funded through the State's access management fund. The State implemented the recommended improvements in August 1997. The improvements dramatically increased the safety of motorists. In the ten years following implementation, only two crashes have occurred at the unsignalized driveway on OR 82 and neither involved another vehicle. Given the estimated 18,900 vehicles per day on OR 82 at this time, this equates to a crash rate of approximately 0.06 over the 10-year period after the access management improvements were made.
Characteristics of Urban Intersections
Urban areas (including central business districts) are typically characterized by dense, multi-modal, fully built-out transportation systems. Adjacent land uses are typically high density office, commercial, and retail developments with minimal setbacks from the street. Parking is usually along roadways, in parking structures, and in some cases available via surface parking lots. Older businesses often rely on on-site parking; therefore, eliminating driveways on these properties can significantly impact business operations.
Traffic characteristics include low to medium driveway volumes (500 to 5,000 vehicles per day), medium to high adjacent street traffic volumes (20,000 to 50,000 vehicles per day), and coordinated, fixed signal timing. Pedestrians, bicycles, and buses often are present, and speeds generally are equal to or below 30 mph.
Physical Characteristics of Urban Intersections
Physical characteristics of urban environments include short block lengths (200 to 350 feet); two-way streets with some left-turn lanes; six or more traffic signals per mile; and minimal site setbacks. Where there are driveways to and from the streets, the driveways have small radii and width, and curbs and gutters exist in almost all areas. Intersections are controlled with a mix of signalized or unsignalized intersections while the driveways are generally unsignalized.
Access Management Near Urban Intersections
Note: The photos on this slide show typical urban intersections. The following text explains challenges and strategies for access management in urban areas like these depicted in the photos.
Implementing access management strategies in urban areas can be difficult to achieve because of some of the constraints in urban areas and the amount of time planning and implementation can consume for local jurisdictions. This section describes specific characteristics and design challenges associated with access management near urban intersections and provides a summary of potential access management treatments that can improve the safety of motorists, bicyclists, and pedestrians.
One-way couplets are often found within the urban area and provide access management benefits. One-way streets limit driveways to right-in/right-out only or left-in/left-out only turning maneuvers and reduce the number of crossing conflict points and the spacing required between adjacent driveways. One-way streets also reduce the need for intersection sight distance downstream of a driveway as there is no oncoming vehicular traffic. One-way streets can also be beneficial for pedestrians crossing the street as they only need to look for oncoming traffic in one direction.
Based on a speed of 30 mph, often the upstream functional area of an urban signalized intersection exceeds the length of a typical urban block. For this reason, engineers in urban areas often cannot avoid placing driveways within functional areas of intersections. Furthermore, on-street parking and other sources of friction within an intersection's functional area, including bus pull-outs and areas for truck loading/unloading can diminish the benefits otherwise associated with placing driveways outside of the functional area of an intersection.
Treatments to Improve Motorist Safety Near Urban Intersections
slide notesThe following access management approaches can help to improve motorist safety and mobility at access points implemented in the vicinity of urban area intersections. The techniques apply whether or not the urban intersection is signalized.
Urban Intersections Treatment 1: Parking Near Driveways
Avoid locating on-site parking bays near site driveways. This allows motorists to drive completely onto the property without having to stop for other motorists completing on-site parking maneuvers as illustrated in the figure on the left. Parking maneuvers near the site driveway can also result in delays for inbound motorists, creating queues that extend back into the major roadway. The second figure illustrates an unobstructed driveway, which allows motorists to exit the roadway unimpeded by other motorists maneuvering in the driveway.
Urban Intersections Treatment 2: Place Driveways on Lower Volume Roadways
Place driveways on lower volume roadways (side streets or alleys) wherever possible. This helps to reduce the frequency of conflicts, which minimizes both the opportunity for crashes and the severity of those crashes should they occur.
Other Treatments to Improve Motorist Safety Near Urban Intersections
Other Treatments to Improve Motorist Safety Near Urban Intersections (Continued)
Treatments to Improve Pedestrian and Bicyclist Safety Near Urban Intersections
In addition to the access management treatments identified earlier, the following access management approaches can help to improve pedestrian and bicyclist safety and mobility at access points in the vicinity of urban intersections. The techniques apply whether or not the urban intersection is signalized:
Characteristics of Rural Intersections
Rural areas are characterized by low-density commercial development such as gas stations and small convenience stores, industrial land and farm land, as well as, in some cases, large expanses of private or publicly owned undeveloped property. Large property frontages along rural roadways allow jurisdictions to adequately space driveways, though topographical and environmental constraints (e.g. steep hills, wetlands, or rivers) may impact where driveways can be located.
Traffic characteristics include speeds of 50 mph and higher. Pedestrian and bicycle volumes are typically the lowest on rural roads as compared to urban and suburban roadways. Rural areas may be subject to development in the future.
Physical Characteristics of Rural Intersections
Physical characteristics of rural roads include divided or undivided two-lane and multilane highways; paved and unpaved shoulders; and infrequent full-access, unsignalized, and on occasion, signalized intersections.
The Texas Transportation Institute (TTI) has published research on rural intersections that relates driveway frequency to crash frequency. Researchers estimate that a rural signalized intersection with no driveways within 250 feet "will be associated with 13 percent fewer crashes than an intersection with three driveways (say, two driveways on one major-approach and one on the other approach)". Researchers also estimate that a rural unsignalized intersection with no driveways within 250 feet will be associated with 20 percent fewer crashes than an intersection with three driveways.
Access Management Near Rural Intersections
Rural areas in general have fewer access management needs in the vicinity of rural intersections than urban or suburban areas. Intersections with county roads are generally infrequent and these roads often have fairly low traffic volumes. The majority of driveways in the vicinity of intersections serve low traffic generators such as single family homes and/or farms. Large property frontages adjacent to the roadway allow the regulating jurisdiction to locate a driveway a significant distance from the intersection. However, rural intersections can have intersecting high speed roadways, which can create access management risks. This section describes specific characteristics and access management challenges and opportunities associated with rural areas and intersections, and provides a summary of potential access management treatments that can improve the safety of motorists, bicyclists, and pedestrians.
Treatments to Improve Motorist Safety Near Rural Intersections
The following access management approaches can help to improve motorist safety and mobility at access points implemented in the vicinity of intersections in rural areas:
Treatments to Improve Pedestrian and Bicyclist Safety Near Rural Intersections
In rural areas, where there is no sidewalk, pedestrians and bicyclists benefit from roadway shoulders that are at least 4 feet wide or wider, paved and well maintained. As access and intersection modifications are considered in these environments, to the extent possible shoulder widths should be maintained in order to provide facilities for non-auto travelers. The photo in this slide depicts a shoulder on a rural road suitable for pedestrians or bicyclists.
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