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slide 0 Technical Summary Roundabouts
Federal Highway Administration FHWA-SA-10-006 slide notes Modern roundabouts are a type of intersection characterized by a generally circular shape, yield control on entry, and geometric features that create a low-speed environment. Modern roundabouts have been demonstrated to provide a number of safety, operational, and other benefits when compared to other types of intersections. On projects that construct new or improved intersections, the modern roundabout should be examined as an alternative. This technical summary explores the characteristics of modern roundabouts while reinforcing the need to apply a principles-based approach to design. It provides an overview of the key considerations for planning, analysis, and design of single-lane and multilane roundabouts. Circular intersection forms have been part of the transportation system in the United States for over a century. Their widespread usage decreased after the mid-1950s, as rotary intersections began experiencing problems with congestion and safety. However, the advantages of the modern roundabout, including modified and improved design features, have now been recognized and put to the test in the United States. There are now estimated to be well over a thousand roundabouts in the United States and tens of thousands worldwide, with the number estimated to be increasing in the United States each year. slide 1 Presentation Overview
slide notes: This presentation summarizes a Roundabout Technical Summary produced by FHWA, one of a suite of technical outreach products produced to improve intersection safety by making practitioners and decisionmakers aware of proven and emerging intersection safety solutions. Roundabout intersections are one of nine proven safety countermeasures identified by the FHWA Office of Safety (the memo outlining all nine is available on the FHWA Office of Safety web site). The information presented in this summary outlines the principles described in the FHWA document Roundabouts: An Informational Guide and the forthcoming 2nd Edition of that document (hereafter referred to as the Roundabout Guide), which is in progress at the time of this writing and due to be published in 2010. Specific considerations for mini-roundabouts are summarized in a companion FHWA document titled Technical Summary on Mini-Roundabouts. Figures are from the forthcoming 2nd Edition unless otherwise noted. slide 2 Presentation Outline
slide notes: -None.-slide 3 Key Roundabout Characteristics
slide notes: A modern roundabout has the following distinguishing characteristics and design features:
Modern roundabouts are different from other types of circular intersections in use in some parts of the United States. Roundabouts are typically smaller than the large, high-speed rotaries still in use in some parts of the country, and they are typically larger than most neighborhood traffic calming circles. Further discussion can be found in the Roundabout Guide. (See the Mini-Roundabout Technical Summary for more information about mini-roundabouts). slide 4 Roundabout Category Comparison
slide notes: Roundabouts have been classified into three basic categories according to size and number of lanes to facilitate discussion of specific performance or design issues: mini-roundabouts, single-lane roundabouts, and multilane roundabouts. These are summarized in the tables on this slide and the next. slide 5 Roundabout Category Comparison (Continued)
slide notes: -None- slide 6 Presentation Outline
slide notes: Roundabouts are becoming more popular based on the multiple opportunities to improve safety and operational efficiency, and provide other benefits. Of course, roundabouts are not always feasible and do not always provide the optimal solution for every problem. The benefits of roundabout intersections, and some constraining factors, are described in this section. slide 7 Benefits of Roundabouts
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slide 8 Benefits of Roundabouts (Continued)
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slide 9 Benefits of Roundabouts (Continued)
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slide 10 Presentation Outline
slide notes: The various user types of a roundabout have unique characteristics that should be considered in the planning and design processes. Some of the characteristics of four primary user groups—motorists, pedestrians, bicyclists, and emergency vehicles—are discussed here; a more complete discussion can be found in the Roundabout Guide. slide 11 Motorist and Emergency Vehicle Considerations
slide notes: Research indicates roundabouts address some of the problems drivers experience in dealing with intersections. One of the key design features of a roundabout is the geometric shape of the roundabout that causes all traffic to slow down as it enters the intersection. Roundabouts can enhance the safety of drivers, including older drivers, by:
Attention should be paid to the layout of signs and pavement markings to make them clear, visible, and unambiguous to all users, including older drivers. Trucks and other large vehicles can be accommodated at a roundabout with proper attention to design. Roundabouts provide emergency vehicles the benefit of lower vehicle speeds, which may make roundabouts safer for them to negotiate than signalized crossings. Unlike signalized intersections, emergency vehicle drivers will not encounter through vehicles unexpectedly running the intersection and hitting them at high speed. Emergency services personnel may have some concern about their ability to navigate a roundabout in an emergency vehicle, although this can be readily addressed in design. On emergency response routes, the delay for the relevant movements at a planned roundabout should be compared with alternative intersection types and control. As with conventional intersections, motorists should be educated not to enter a roundabout when an emergency vehicle is approaching on another leg. Once entered, they should clear out of the circulatory roadway if possible, facilitating queue clearance in front of the emergency vehicle. slide 12 Pedestrian and Bicyclist Considerations
slide notes: Pedestrians are accommodated at pedestrian crosswalks around the perimeter of the roundabout. By providing space to pause on the splitter island, pedestrians can consider one direction of conflicting traffic at a time, which simplifies the task of crossing the street. The low vehicular speeds through a roundabout also allow more time for drivers and pedestrians to react to one another and to reduce the consequences of error. As a result, few crashes involving pedestrians have been reported at roundabouts. Pedestrians with vision impairments may have more difficulty crossing roundabouts due to the following key factors:
Bicyclists should be provided similar options to negotiate roundabouts as they have at conventional intersections, where they navigate either as motor vehicles or pedestrians depending on the size of the intersection, traffic volumes, their experience level, and other factors. Bicyclists are often comfortable riding through single-lane roundabouts in low-volume environments in the travel lane with motor vehicles, as speeds are comparable and potential conflicts are low. At larger or busier roundabouts, many cyclists may be more comfortable and safer using ramps connecting to a sidewalk or multi-use path around the perimeter of the roundabout as a pedestrian. slide 13 Presentation Outline
slide notes: -None.- slide 14 Location Considerations
slide notes: In the planning process for a new or improved intersection where a traffic signal or stop control is under consideration, a modern roundabout should likewise receive serious consideration as an alternative. This begins with understanding the site characteristics and determining a preliminary configuration. There are a number of locations where roundabouts are commonly found to be advantageous and a number of situations that may adversely affect their feasibility. As with any decision regarding intersection treatments, care should be taken to understand the particular benefits and trade-offs for each project site. This section outlines some location considerations to help determine whether a roundabout is a feasible intersection alternative. slide 15 Common Site Applications
 Photos: Lee Rodegerdts (used with permission) slide notes:
slide 16 Other Common Site Applications
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slide 17 Potential Site Constraints
slide notes: Certain site-related factors may significantly influence the design requiring that a more detailed investigation of some aspects of the design or operation be carried out. A number of these factors (many of which are valid for any intersection type) are listed below:
slide 18 Methods to Address Site Constraints
slide notes: The existence of one or more of these conditions may or may not preclude the installation of a roundabout. Roundabouts have, in fact, been built at locations that exhibit one or more of the conditions listed above. To address these conditions, additional analysis, design work, and/or coordination with affected parties may be needed to resolve conflicts and help in the decision-making process. In some cases, the conditions identified above cannot be overcome, and another intersection type may be more suitable. slide 19 Presentation Outline
slide notes: -None.- slide 20 Levels of Analysis
slide notes: Roundabouts can be analyzed on both a planning level and an operational level. Planning level analyses are conducted based on the daily volumes to determine the necessary number of lanes. Operational level analyses are performed based on the peak hour volumes to determine the capacity and operational performance. slide 21 Planning Level – Number of Lanes
slide notes: A basic question that needs to be answered at the planning level is how many entering and circulating lanes a roundabout would require to serve the traffic demand. The number of lanes affects not only the capacity of the roundabout, but also the size of the roundabout footprint. This slide presents ranges of average annual daily traffic (AADT) volumes to identify scenarios under which one-lane and two-lane roundabouts may perform adequately. These ranges represent total entering volume thresholds where a one-lane or two-lane roundabout should operate acceptably and ranges of volumes over which more detailed analysis is required. This procedure is offered as a simple, conservative method for estimating roundabout lane requirements. If the volumes fall within the ranges identified in this figure where "additional analysis is needed," a single-lane or double-lane roundabout may still function quite well, but it requires using the procedures described in the following section to obtain a closer look at the actual turning movement volumes during the design hour. Variable-sized roundabouts (e.g., one lane for part of the circle, and two lanes at other parts within the same roundabout), roundabouts with peak-period metering, and three-lane roundabouts have been successful in some locations. slide 22 Capacity of Single-Lane and Multilane Entries
slide notes: The Draft 2010 Highway Capacity Manual (HCM) employs a number of models to reflect the capacity of roundabout entries with up to two lanes. The capacity of each entry lane is calculated based on the conflicting traffic flow in the circulatory roadway, which comprises the various turning movements from other approaches that pass in front of (and thus conflict with) the subject entry. This figure shows the capacity curves for various one- and two-lane roundabout scenarios. The "Single-Lane Capacity" graph can be used to calculate the capacity of a one-lane entry to a one-lane roundabout, or either lane of a two-lane entry conflicted by one circulating lane. For a roundabout with two circulatory lanes, the two curves representing the left and right entry lanes should be used. As an example, for a given circulatory flow rate of 600 pc/h across two lanes, the left lane of a two-lane entry would have a capacity of approximately 720 pc/h, and the right lane of a two-lane entry would have a capacity of approximately 740 pc/h. More detail, including sample calculations of roundabout volumes, conversion of vehicles per hour (veh/h) to passenger cars per hour (pc/h), lane use, capacity, and performance measures, can be found in the 2010 HCM. Different methods of analysis are available and are in common use for a variety of applications, including software programs with specific roundabout analysis procedures and simulation models. These models may be capable of analyzing situations beyond the methodologies presented in the Highway Capacity Manual or Roundabout Guide; refer to these documents for further discussion. Regardless of the analytical tools used, it is critical to understand that each model and analysis method makes certain operational and performance assumptions to approximate reality. Along with an understanding of the inherent imprecision of traffic forecasting, this makes the application of engineering judgment crucial in the analytical process. slide 23 Presentation Outline
slide notes: The geometric design of a roundabout requires the balancing of competing design objectives. Roundabouts operate most safely when their geometry forces traffic to enter and circulate at slow speeds. Poor roundabout geometry has been found to negatively impact roundabout operations by affecting driver lane choice and behavior through the roundabout. Many of the geometric parameters are governed by the maneuvering requirements of the design vehicle and the accommodation of nonmotorized users. Thus, designing a roundabout is a process of determining the optimal balance among safety provisions, operational performance, and accommodation of design users. This design balance is further influenced by physical, environmental, economic, and political constraints and opportunities, which further increases the variability from site to site. For example, a roundabout that is built to its ultimate configuration on opening day may have different design characteristics from one that is initially built in an interim configuration (e.g., a single-lane roundabout converted later to a double-lane roundabout), and the techniques for those conversions can vary (e.g., adding lanes to the outside versus the inside). For these reasons, roundabout design techniques are difficult to standardize, and there is rarely only one correct or even best way to design a roundabout. slide 24 Key Objectives of Roundabout Design
slide notes: Fundamentally, roundabout design involves achieving the following key objectives:
Since roundabouts are applied in many different situations and under differing site specific conditions, each roundabout design requires distinctive design choices. The general nature of the roundabout design process is an iterative one. Minor adjustments in geometric design attributes can result in significant effects on the operational and safety performance of the roundabout. Also, many of the individual design components interact with each other, and therefore considering the roundabout design in whole (the outcome of the design) is more important than focusing on the isolated components. Because of this iterative process, it may be advantageous to prepare initial layout drawings to a "hand-sketch" level of detail and investigate the compatibility of the design principles presented below before further design effort is invested. The optimal position of the roundabout may not be established until geometrics are roughly investigated for various location options. slide 25 Horizontal Design
slide notes: Three of the key considerations that affect horizontal design of roundabouts include design speed, path alignment, and design vehicle. These in turn influence the size of the roundabout and the design of the central island and splitter islands. This section highlights these considerations and design details. slide 26 Consideration 1: Design Speed
slide notes: Achieving appropriate vehicular speeds entering and traveling through the roundabout is a critical design objective as it has profound impacts on safety. A well-designed roundabout reduces vehicle speeds upon entry and achieves consistency in the relative speeds between conflicting traffic streams by requiring vehicles to negotiate the roundabout along a curved path. Generally speaking, although the frequency of crashes is most directly tied to volume, the severity of crashes is most directly tied to speed. Therefore, careful attention to the design speed of a roundabout is fundamental to attaining good safety performance. The recommended design speed of a roundabout is primarily a function of the number of lanes rather than the design speed of the intersecting roadways. The design speed of a roundabout is defined by the theoretical speed that drivers could achieve through the roundabout if taking the fastest path alignment through the roundabout without regard to lane line striping, if present. This figure illustrates the construction of the fastest vehicle path at a multi-lane roundabout. In practice, actual speeds through the roundabout will be less than these theoretical values, as drivers will be decelerating into the roundabout, yielding to other users, and staying within their lanes (for multilane roundabouts). For single-lane roundabouts, typical maximum theoretical entering speeds of 20 to 25 mph are recommended; for multilane roundabouts, typical maximum theoretical entering speeds of 25 to 30 mph are recommended. This design technique ensures that speeds observed in practice will fall within a reasonable range. slide 27 Consideration 1: Design Speed (Continued)
slide notes: The fastest path should be drawn and checked for all approaches of the roundabout. Once the fastest paths are drawn, the above radii are measured and corresponding design speeds are calculated using standard horizontal curve guidelines from AASHTO. Typically, roundabouts are designed with a cross slope of 2 percent toward the outside (i.e., a superelevation of -0.02). This slide contains a graphical representation of the speed-radius relationships (in U.S. Customary units). In addition to achieving an appropriate design speed for the entry movements, another important objective is to achieve consistent speeds for all movements, which are influenced by choices on geometric elements. The key benefits of achieving speed consistency among the movements are safety related. In practice, by keeping the recommended maximum entry design speed below the recommended values, the goal of consistent speeds for all movements can be readily achieved. There are differences of opinion on the importance of tangential versus curved exit geometry for the purpose of controlling exit speeds, particularly at the pedestrian crosswalk. Some designers advocate for a relatively tight exit radius to minimize exit speeds; however, others advocate for a more relaxed exit radius for improved drivability. Theoretical exit speeds can be checked using the above method. However, research has found that observed exit speeds are more commonly limited by circulating speeds and acceleration out of the roundabout than by the radius of the exit path. More information on calculating exit speeds can be found in the Roundabout Guide. It is important to understand the relative trade-offs of design choices, and choices may vary based upon the location context. slide 28 Consideration 2: Path Alignment
slide notes: With multilane roundabouts, the designer should also consider the alignment of vehicles, or the natural path, to ensure the proposed geometry directs vehicles to stay within the proper lanes through the circulatory roadway and exits. Path overlap occurs when the natural paths of vehicles in adjacent lanes overlap or cross one another. The entry design should align vehicles into the appropriate lane within the circulatory roadway, using the technique shown in the figure or others that promote good path alignment. Designing multilane roundabouts with good path alignment while also controlling entry speeds through adequate deflection can be difficult. Strategies that improve path alignment may result in increased fastest path speeds. A good design attempts to balance the entry speed, path alignment needs, and other factors (e.g., design vehicle needs) through design iterations and checks of the various factors. The primary objective of a multilane design technique is to locate the entry curve at the optimal placement so that the projection of the inside entry lane at the entrance line connects tangentially or nearly tangentially to the central island. The design of the exits should also provide sufficiently large exit radii and alignment to allow drivers to intuitively maintain the appropriate lane. Other techniques involve changes to approach alignment, entry curvature, and/or inscribed circle diameter; these are discussed in the Roundabout Guide. Each of these adjustments could create trade-offs; for example, increasing the inscribed circle diameter could result in faster circulatory speeds, greater land impacts, and so on. A good design attempts to balance these factors through design iteration. slide 29 Consideration 2: Path Alignment (Continued)
slide notes: Likewise, problems can also occur when the design allows for too much separation between entries and subsequent exits. Large separations between legs cause entering vehicles to join next to circulating traffic that may be intending to exit at the next leg, rather than crossing the path of the exiting vehicles. This can create conflicts at the exit point between exiting and circulating vehicles. A variety of solutions are possible to address this problem, including changes to lane configurations, changes to inscribed circle diameter, and realignment of the approaches. This figure illustrates one of these possible solutions, which involves changing the alignment of the approach legs from the previous alignment from the previous slide (noted in the figure) to a realignment that has the paths of entering vehicles cross the paths of the circulating traffic (rather than merging) to minimize the conflict. This significantly increases the likelihood that entering drivers making a through movement will yield to both conflicting lanes. slide 30 Consideration 3: Design Vehicle
slide notes: Large trucks, buses, and emergency vehicles often dictate many of the roundabout's dimensions, particularly for single-lane roundabouts. Therefore, the design vehicle is best identified at the start of the project and evaluated early in the design process. A truck apron will often be needed within the central island to accommodate larger design vehicles (including the common WB-62, WB-65, or WB-67 design vehicles) but maintain a relatively narrow circulatory roadway to adequately constrain passenger car speeds. Design details regarding truck aprons are provided in the "Grades" section of the Technical Summary. Appropriate vehicle-turning templates or a CAD-based computer program should be used to determine the swept path of the design vehicle through each of the turning movements. Usually, the left-turn movement is the critical path for determining circulatory roadway width while the right-turn movement is the critical path for entry and exit widths. This figure illustrates an example vehicle path check. Buses should generally be accommodated within the circulatory roadway without tracking over the truck apron, which could cause discomfort to bus occupants. For multilane roundabouts, there are different philosophies regarding the extent to which trucks need to stay in their lane throughout their movement; these are discussed further in the Roundabout Guide. slide 31 Consideration 4: Size
slide notes: The size of a roundabout, measured by its inscribed circle diameter, is determined by a number of design objectives, including design speed, path alignment, and design vehicles as discussed above. Selection of an initial inscribed circle diameter is the first step towards preparing a design. The selected diameter may be somewhat subjective, but its ultimate size is an output of meeting other objectives (e.g., speed control, design vehicle, etc.). Smaller inscribed circle diameters can be used for some local street or collector street intersections where the design vehicle may be a fire truck or single-unit truck. Larger inscribed circle diameters generally provide increased flexibility for the entry design to meet design criteria (e.g., speed, adequate visibility to the left, etc.) while accommodating large design vehicles. This table provides common ranges of inscribed circle diameters for various roundabout categories and typical design vehicles; values outside these ranges are possible but less common. (See the Roundabout Technical Summary for more detail). slide 32 Consideration 5: Central Island
slide notes: The central island of a roundabout is the raised, mainly non-traversable area surrounded by the circulatory roadway. It may also include a traversable truck apron. The island is typically landscaped for aesthetic reasons and to enhance driver recognition of the roundabout upon approach. A circular central island is preferred because the constant-radius circulatory roadway helps promote constant speeds around the central island. Oval or irregular shapes may be necessary at irregularly shaped intersections or intersections with more than four legs. Raindrop-shaped islands are sometimes used in areas where certain movements do not exist, such as interchanges, or at locations where certain turning movements cannot be safely accommodated, such as roundabouts with one approach on a relatively steep grade. The size of the central island plays a key role in determining the amount of deflection imposed on the through vehicle's path. However, its diameter is dependent upon the inscribed circle diameter and the required circulatory roadway width. The central island may include enhancements (e.g., landscaping, sculptures, fountains) serving both an aesthetic purpose and providing conspicuity of the intersection for approaching motorists. These treatments should not attract pedestrians to the central island, as they should never cross the circulating roadway. Furthermore, care is needed when including any fixed objects within the central island in environments where the speeds on the approaching roadways are higher. slide 33 Consideration 6: Splitter Island
slide notes: Splitter islands should be provided on all roundabouts, and these islands should be raised on all but those with small diameters. Their purpose is to provide refuge for pedestrians, assist in controlling speeds, guide traffic into the roundabout, physically separate entering and exiting traffic streams, and deter wrong-way movements. Additionally, splitter islands can be used as a place for mounting signs. There are benefits to providing longer and larger splitter islands. An increase in the splitter island width results in greater separation between the entering and exiting traffic streams of the same leg and increases the time for approaching drivers to distinguish between exiting and circulating vehicles. In this way, larger splitter islands can help reduce confusion for entering motorists. However, increasing the width of the splitter islands generally requires increasing the inscribed circle diameter to maintain speed control on the approach. Thus, these safety benefits may be offset by higher construction cost and greater land impacts. Standard AASHTO guidelines for island design should be followed for the splitter island. This includes using larger nose radii at approach corners to maximize island visibility and offsetting curb lines at the approach ends to create a funneling effect. The funneling treatment also aids in reducing speeds as vehicles approach the roundabout. Additional details can be found in the Roundabout Guide. slide 34 Pedestrian Design Treatments
slide notes: Wherever possible, sidewalks at roundabouts should be set back from the edge of the circulatory roadway by a landscape buffer. The buffer discourages pedestrians from crossing to the central island or cutting across the circulatory roadway of the roundabout, and it helps guide pedestrians with vision impairments to the designated crosswalks. A buffer width of 5 ft (1.5 m) (minimum 2 ft [0.6 m]) or greater is recommended, and it is best to plant low shrubs or grass in the area between the sidewalk and curb to maintain sight distance needs. This figure shows this technique. Crosswalks should be located in vehicle-length increments away from edge of the circulatory roadway. A typical (and minimum) crosswalk setback of 20 ft (6 m) is recommended. The raised splitter island width should be a minimum of 6 ft (1.8 m) at the crosswalk to adequately provide shelter for persons pushing a stroller or walking a bicycle. At some roundabouts, it may be desirable to place the crosswalk two or three car lengths (45 ft [13.5 m] or 70 ft [21.5 m]) back from the edge of the circulatory roadway. This longer setback is typically used in situations with relatively high volumes of pedestrian crossings that may cause queues on the exit roadway to frequently extend into the circulatory roadway. Other treatments for the accommodation of pedestrians, including signalization, are discussed in the Roundabout Guide. slide 35 Bicycle Design Treatments
slide notes: Bicycle lanes are not recommended within the circulatory roadway of roundabouts, as it has been demonstrated internationally to have adverse safety effects (see the Roundabout Guide). Where bicycle lanes or shoulders are used on approach roadways, they should be terminated in advance of roundabouts. Bicyclists may choose to merge with traffic and travel like other vehicles, or they may choose to exit the roadway onto the sidewalk (or shared use path) and travel as pedestrians. The full width bicycle lane should normally end at least 100 feet before the edge of the circulatory roadway. An appropriate taper (a rate of 7:1 is recommended) should be provided to narrow the combined travel lane and bike lane width down to the appropriate width necessary to achieve desired motor vehicle speeds on the roundabout approach. Because some bicyclists may not feel comfortable traversing some roundabouts in the same manner as other vehicles, bicycle ramps can be provided to allow access to the sidewalk or a shared use path at the roundabout. This figure displays a possible layout of bicycle treatments. To minimize confusion between bicycle ramps and pedestrian ramps, the detectable warning surfaces are placed at the top of the bicycle ramps rather than at the bottom as is the practice with pedestrian ramps. In general, bicycle ramps should only be used where the roundabout complexity or design speed may result in less comfort for some bicyclists. Ramps may not be needed at urban one-lane roundabouts, as the low-speed and lower-volume environment will typically allow cyclists to navigate comfortably as vehicles. slide 36 Sight Distance and Visibility
slide notes: Adequate sight distance and visibility is needed for a roundabout to operate safely. These factors can be contradictory: sight distance at the roundabout can be increased in some cases at the expense of the visibility of the roundabout from a distance. Evaluation of sight distance at roundabouts includes both intersection sight distance and stopping sight distance. The fundamental principles of both forms of sight distance are the same at roundabouts as for other types of intersections and roadways. Intersection sight distance is evaluated at each entry to ensure a driver can see and safely react to potentially conflicting vehicles. Providing intersection sight distance ensures drivers can safely enter the circulatory roadway without impeding the flow of traffic within the circulatory roadway. This figure illustrates the measurement of intersection sight distance. slide 37 Sight Distance and Visibility (Continued)
slide notes: Stopping sight distance should be provided at every point within a roundabout and on each entering and exiting approach. The required distance is based on speed, as determined from the fastest path speed checks, and can be calculated using AASHTO guidelines. This figure illustrates the stopping sight distance related to approaching speed. Sight distance needs may limit the height of landscaping and objects around the outer edge of the central island. In general, it is recommended to provide no more than the minimum required intersection sight distance on each approach. Excessive intersection sight distance can lead to higher vehicle speeds that may reduce the safety of the intersection. slide 38 Vertical Design
slide notes: As a general practice, a cross slope of 2 percent away from the central island should be used for the circulatory roadway on single-lane roundabouts. This technique of sloping outward is recommended because it promotes safety by raising the height of the central island and improving its visibility; promotes lower circulating speeds; minimizes breaks in the cross slopes of the entrance and exit lanes; and drains surface water to the outside of the roundabout. Where truck aprons are used, the slope of the apron should generally be 1 to 2 percent; greater slopes may increase the likelihood of loss-of-load incidents. Examples of traversable curb details can be found in the Roundabout Guide. There are a variety of possible methods for the vertical design of a circulatory roadway within a multilane roundabout, and many are the byproduct of fitting a roundabout to its topography. The two common methods include the following:
It is generally not desirable to locate roundabouts in locations where grades through the intersection are greater than 4 percent, although roundabouts have been installed on grades of 10 percent or more. Care is needed when designing roundabouts on steep grades. On approach roadways with grades steeper than -4 percent, it is more difficult for entering drivers to slow or stop on the approach (as with any intersection). slide 39 Pavement Markings and Signs
slide notes: At roundabouts, pavement markings and signs work together to create a comprehensive system to guide and regulate road users. In order for pavement markings at roundabouts to provide appropriate guidance, the following general principles should be considered:
The Federal Highway Administration has published the 2009 Manual on Uniform Traffic Control Devices, which includes major revisions and additions related to signage and markings at roundabouts. Typical pavement markings for roundabouts delineate the entries, exits, and the circulatory roadway, providing guidance for pedestrians and vehicle operators. This figure shows example pavement markings for a multilane roundabout. The overall concept for roundabout signing is similar to general intersection signing. Proper regulatory control, advance warning, and directional guidance are required to avoid driver expectancy-related problems. Figure source: Adapted from the Roundabout Guide. slide 40 Lighting and Landscaping
slide notes: Roundabouts, including their pedestrian crossing areas and bicycle design features, should be conspicuous and visible to approaching drivers. The overall illumination of the roundabout should be based on local and national guidelines for street lighting. The Design Guide for Roundabout Lighting, published by the Illuminating Engineering Society (IES), is the primary resource that should be consulted in completing a lighting plan for all roundabout types. Local illumination standards should also be considered when establishing the illumination at the roundabout to ensure that the lighting is consistent. The Roundabout Guide provides a more detailed summary of lighting principles and guidelines. Landscaping of roundabouts plays an important role in improving the aesthetics of an area. However, landscaping has a number of functional purposes:
Any landscaping that is provided should be designed to minimize roadside hazards, particularly in higher speed environments, and to maintain adequate stopping and intersection sight distance throughout the roundabout. slide 41 Other Design Details and Applications
slide notes: More design details and applications of roundabouts exist than can be covered in this technical summary; however, some of the more notable considerations are described below:
slide 42 Presentation Outline
slide notes: -None.- slide 43 Costs
slide notes: Construction costs for roundabouts vary widely, from tens of thousands of dollars for minor retrofits of small intersections using existing curb lines, existing pavement, and no landscaping to millions of dollars for major reconstruction of large intersections with significant earthwork, structures, and landscaping. Right-of-way costs also vary widely depending on impact area and land uses. As a result, a case-by-case evaluation of construction costs is needed for a reasonable assessment. A benefit-cost analysis may be useful in alternatives analysis, as it recognizes that not all of the benefits and costs of an alternative can be quantified by pure construction costs. The safety, operational, and environmental benefits of roundabouts can be quantified and compared to the initial construction and ongoing maintenance cost over the life cycle of the roundabout. While initial construction costs might be higher for a roundabout in a retrofit situation (they are often comparable in new installations), the roundabout's ongoing maintenance is often cheaper than for signalized intersections, as there is typically no signal hardware to power, maintain, and keep current in terms of signal timing. Finally, while many factors influence the potential service life of a roundabout (types of construction materials, weather conditions, traffic conditions, growth in the area, etc.), roundabouts can often serve for longer periods of time between major upgrades (repaving, reconstruction, etc.) than comparable signalized intersections. More detail on estimating lifecycle benefits and costs can be found in the Roundabout Guide. slide 44 Conclusion
slide notes: -None.- slide 45 References
slide notes: -None.- slide 46 References (Continued)
slide notes: -None.- slide 47 For More Information
slide notes: -None.- |
Program ContactWhat’s NewSouth Carolina Case Study: Systematic Intersection Improvements Roundabout Outreach and Education Toolbox Stop-Controlled Intersection Safety: Through Route Activated Warning Systems Roundabouts: An Informational Guide, Second Edition (NCHRP Report 672) Roundabouts Peer-to-Peer Assistance How to drive a roundabout (WSDOT) Modern Roundabouts: A Safer Choice HighlightsFHWA's Intersection Resources Library CD-ROM Mini-Roundabouts Technical Summary Access Management in the Vicinity of Intersections Technical Summary Intersection Safety Case Studies Intersection Safety Technologies Presentation: Intersection Safety Example Intersection Safety Implementation Plan |
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