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Chapter 2/Roundabout Considerations Page 2-1 Roundabouts: An Informational Guide CHAPTER 2 ROUNDABOUT CONSIDERATIONS CONTENTS 2.1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3 2.2 GENERAL CHARACTERISTICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3 2.2.1 Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3 2.2.2 User Decisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-4 2.2.3 Traffic Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-5 2.2.4 Spatial Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-7 2.2.5 Access Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-9 2.2.6 Environmental Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-9 2.2.7 Operation and Maintenance Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-9 2.2.8 Traffic Calming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-10 2.2.9 Aesthetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-10 2.2.10 Summary of Advantages and Disadvantages . . . . . . . . . . . . . . . . . 2-11 2.3 USER CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-13 2.3.1 Pedestrians . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-13 2.3.2 Pedestrians with Disabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-15 2.3.3 Bicyclists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-17 2.3.4 Older Drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-18 2.3.5 Large Vehicles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-19 2.3.6 Transit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-20 2.3.7 Emergency Vehicles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-20 2.3.8 Rail Crossings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-20 2.4 POLICY AND LEGAL ISSUES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-21 2.4.1 Decision-Making Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-21 2.4.2 Rules of the Road . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-21 2.5 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-22
Roundabouts: An Informational Guide Page 2-2 Chapter 2/Roundabout Considerations LIST OF EXHIBITS Exhibit 2-1 Wide Nodes, Narrow Roads Concept . . . . . . . . . . . . . . . . . . . . . . . . . 2-8 Exhibit 2-2 Example of Wide Nodes, Narrow Roads Concept . . . . . . . . . . . . . . . 2-8 Exhibit 2-3 Example of Gateway Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-11 Exhibit 2-4 Examples of Aesthetic Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-11 Exhibit 2-5 Summary of Roundabout Advantages and Disadvantages . . . . . . 2-12
Chapter 2/Roundabout Considerations Page 2-3 Roundabouts: An Informational Guide 2.1 INTRODUCTION This chapter provides a general overview of the characteristics of roundabouts and considerations for all users. A discussion of legal considerations and user edu- cation provides policy makers with the information they need to make appropriate decisions and convey direction to the public. Understanding the advantages and disadvantages of roundabouts allows designers, policy makers, and the public to understand the trade-offs with this type of intersection treatment. While general information about roundabouts can be found in this chapter, the reader is encouraged to refer to later, more detailed chapters on the specifics associated with planning, operation, safety, and design of roundabouts. 2.2 GENERAL CHARACTERISTICS Many jurisdictions are looking for alternative intersection control methods to improve safety and carry more traffic without widening roadways. Roundabouts are becoming more popular based on the multiple advantages to safety, operations, and aesthetics. However, as agencies become increasingly familiar with these types of intersections, it is important to understand both advantages and disadvantages. 2.2.1 SAFETY Roundabouts have been demonstrated to be safer than other forms of at-grade intersections (1). The safety benefit is particularly notable for fatal and injury crashes. This section provides an overview of key safety issues; the reader is encouraged to refer to Chapter 5 for a more detailed discussion. The safety performance of a roundabout is a product of its design. At round- abouts, vehicles travel in the same direction, eliminating the right-angle and left- turn conflicts associated with traditional intersections. In addition, good roundabout design places a high priority on speed control. Speed control is provided by geometric features, not just by traffic control devices or by the impedance of other traffic. Because of this, speed control can be achieved at all times of day. If achieved by good design, in principle, lower vehicle speeds should provide the following safety benefits: ⢠Provide more time for entering drivers to judge, adjust speed for, and enter a gap in circulating traffic, allowing for safer merges; ⢠Reduce the size of sight triangles needed for users to see one another; ⢠Increase the likelihood of drivers yielding to pedestrians (compared to an uncontrolled crossing); ⢠Provide more time for all users to detect and correct for their mistakes or mistakes of others; ⢠Make crashes less frequent and less severe, including crashes involving pedestrians and bicyclists; and ⢠Make the intersection safer for novice users. Roundabouts have been demonstrated to be safer for motor vehicles and pedestrians than other forms of at-grade intersections. Good roundabout designs encourage speed control.
Roundabouts: An Informational Guide Page 2-4 Chapter 2/Roundabout Considerations Single-lane roundabouts designed for low-speed operation are one of the safest treatments available for at-grade intersections. Drivers have no lane use decisions to make. Pedestrians cross one lane of traffic at a time. Roadway speeds and widths are low enough to allow comfortable mixed bicycle and motor vehi- cle flow. Due to the increased number of conflicting and interacting movements, multi- lane roundabouts often cannot achieve the same levels of safety improvement as their single-lane counterparts. Driver decisions are more complex at multilane roundabouts, with the most important being proper lane selection before entering the intersection. Pedestrians face potential multiple-threat conflicts as they cross more than one lane of traffic at a time. Visually impaired pedestrians face a signifi- cantly more complex auditory environment that may reduce the accessibility of the intersection without additional treatments. Cyclists traveling as vehicles must select the correct lane for circulating; if traveling as pedestrians, they face the same conflicts as other pedestrians. Despite these challenges, the overall safety performance of multilane roundabouts is often better than comparable signalized intersections, particularly in terms of fatal and injury crashes. 2.2.2 USER DECISIONS User decisionsâthat is, decisions by drivers, pedestrians, and cyclistsâare generally simpler at roundabouts than at other intersection treatments. However, roundabouts also place more reliance on individuals to make decisions rather than directing them by a traffic control device. 2.2.2.1 Drivers Drivers approaching a single-lane roundabout have two basic decisions regarding other users: select the appropriate lane (as applicable) for their intended destination, and yield to those who have the right-of-way. Making navigating decisions in roundabouts is generally more complex than for other intersection types, mainly because the driver cannot always see the exit or destination and the fact that the intersection is curved requiring drivers to gradually change direction, potentially disorienting a driver as to their origin and destination. As a consequence the designer may need to provide additional guidance in the form of signs and markings to aid in driver navigation. The latter of the two decisionsâyielding to those who have the right-of-wayâ occurs at several points when negotiating the roundabout: ⢠Drivers must be mindful of any bicyclists merging into motor vehicle traf- fic from the right side of the road, a bicycle lane, or shoulder. ⢠Drivers must yield to any pedestrians crossing at the entry (the laws on this vary somewhat from state to state). ⢠Drivers must choose an acceptable gap in which to enter the roundabout. ⢠Drivers must yield to any pedestrians crossing the exit (the laws on this vary somewhat from state to state). By contrast, a driver making a left turn from the minor leg of a two-way stop- controlled intersection yields to pedestrians and bicyclists and judges gaps in the Single-lane roundabouts designed for low-speed opera- tion are one of the safest treat- ments available for at-grade intersections.
Chapter 2/Roundabout Considerations Page 2-5 Roundabouts: An Informational Guide major street through movements from both directions, as well as the major street left and right turns and opposing minor through and right turns. Signalized intersections attempt to simplify the decision-making process for drivers, especially at locations where protected left-turn phasing is provided, by separating conflicts in time and space. However, the rules and driver decisions for negotiating signalized intersections are still quite complex in many cases. For signals with permissive left-turn phasing, the driver must be cognizant of the opposing vehicular traffic and its speed, presence of pedestrians, and the signal indication itself (to ensure a legal maneuver). In addition, at traffic signals, failure on the part of a driver can be associated with occasionally severe consequences for those involved. By contrast, once at the yield line, the entering driver at a roundabout can focus attention entirely on the circulating traffic stream approaching from the left. A driver behind the entering driver can focus entirely on crossing pedestrians. While operation in a roundabout requires increased user vigilance, as compared to traffic signals, the consequence of an error at a roundabout is less severe by comparison. 2.2.2.2 Pedestrians The design of a roundabout allows pedestrians to cross one direction of traffic at a time on each leg of the roundabout. This is significantly simpler than two-way stop-controlled intersections, where pedestrians cross parallel with the major street and contend with potential conflicts in front of and behind them (e.g., major-street left and right turns). Although signalized intersections can provide indication of when pedestrians have the right-of-way (through a WALK indication), potential conflicts can come from multiple directions: left turns on green, right turns on green, right turns on red, and red-light-running through vehicles. 2.2.2.3 Bicyclists Bicyclist decisions at roundabouts depend on how the bicyclist chooses to travel through the intersection. If traveling as a vehicle, as is often the case for experienced cyclists and cyclists in lower volume and speed environments, the decision process mirrors that of motorized vehicles. If traveling as a pedestrian, as is often the case for less experienced cyclists and cyclists in higher volume environments, the decision process mirrors that of pedestrians. 2.2.3 TRAFFIC OPERATIONS The operation of vehicular traffic at a roundabout is determined by gap accept- ance: entering vehicles look for and accept gaps in circulating traffic. The low speeds of a roundabout facilitate this gap acceptance process. Furthermore, the operational efficiency (capacity) of roundabouts is greater at lower circulating speed because of the following two phenomena: 1. The faster the circulating traffic, the larger the gaps that entering traffic will comfortably accept. This translates to fewer acceptable gaps and therefore more instances of entering vehicles stopping at the yield line. 2. Entering traffic, which is first stopped at the yield line, requires even larger gaps in the circulating traffic in order to accelerate and merge with
Roundabouts: An Informational Guide Page 2-6 Chapter 2/Roundabout Considerations the circulating traffic. The faster the circulating traffic, the larger this gap must be. This translates into fewer acceptable gaps and therefore longer delays for entering traffic. 2.2.3.1 Vehicle Delay and Queue Storage When operating within their capacity, roundabouts typically operate with lower vehicle delays than other intersection forms and control types. With a roundabout, it is unnecessary for traffic to come to a complete stop when no conflicts are present. When there are queues on one or more approaches, traffic within the queues usually continues to move, and this is typically more tolerable to drivers than a stopped or standing queue. The performance of roundabouts during off-peak periods is particularly good compared with other intersection forms, usually with very low average delays. 2.2.3.2 Delay of Major Movements Roundabouts tend to treat all movements at an intersection equally, with no priority provided to major movements over minor movements. Each approach is required to yield to circulating traffic, regardless of whether the approach is a local street or major arterial. This may result in more delay to the major movements than might otherwise be desired. This problem is most acute at the intersection of high-volume major streets with low- to medium-volume minor streets (e.g., major arterial streets with minor collectors or local streets). Therefore, the overall street classification system and hierarchy should be considered before selecting a roundabout (or stop-controlled) intersection. This limitation should be specifically considered on emergency response routes in comparison with other intersection types and control. The delays depend on the volume of turning movements and should be analyzed individually for each approach, according to the procedures in Chapter 4. 2.2.3.3 Signal Progression It is common practice to coordinate traffic signals on arterial roads to minimize stops and travel time delay for through traffic on the major road. A roundabout with only yield control cannot be actively managed to provide priority to major street movements in the same way. As a result, the coordinated platoons of traffic that improve the efficiency of traffic signals can be disrupted by roundabouts, thus reducing the efficiency of downstream intersections. Roundabouts cannot be managed using a centralized traffic management system to facilitate special events, diverted traffic flows, and so on unless signals at the roundabout or in the vicinity are used for such a purpose. On the other hand, roundabouts may present an opportunity to make more efficient use of the existing traffic signals in the vicinity. An example is the use of a roundabout at the highest-volume junction in the system, either a single large at-grade intersection or at the ramp terminals of an interchange. In many cases, the minimum cycle length needed for an entire system is governed by the highest-volume junction in the system. To minimize overall system delay, it may be beneficial to divide the signal system into subsystems separated by the roundabout, assigning each subsystem a cycle length that Since all intersection move- ments at a roundabout have equal priority, major street movements may be delayed more than desired.
Chapter 2/Roundabout Considerations Page 2-7 Roundabouts: An Informational Guide may be lower than before. In these cases the overall total delay, stops, and queues may be reduced. 2.2.4 SPATIAL REQUIREMENTS Roundabouts often require more space in the immediate vicinity of the intersection than comparable stop-controlled or signalized intersections. This space requirement is dictated by a number of factors, including the size and shape of the roundabout (e.g., circular versus noncircular). However, as dis- cussed previously in the context of a corridor, the additional space needed in the vicinity of a roundabout may be offset by reduced space needed between intersections. To the extent that a comparable roundabout would outperform a signal in terms of reduced delay and thus shorter queues, it will require less queue storage space on the approach legs. If a signalized intersection requires long or multiple turn lanes to provide sufficient capacity or storage, a roundabout with similar capacity may require less space on the approaches. As a result, roundabouts may reduce the need for additional right-of-way on the links between intersections, at the expense of additional right-of-way requirements at the intersections them- selves. It may also be possible to space roundabouts closer together than traffic signals because of shorter queue lengths. Roundabouts present opportunities to shape the cross section of a corridor in ways that are perhaps different from those afforded by signalized intersections. Signalized intersections operate most efficiently when they progress platoons of traffic, allowing the maximum number of vehicles to pass through on green with- out stopping. These platoons maximize the use of green time by promoting shorter headways. However, lane continuity between signals is needed to sustain these platoons through a series of signals, and the links tend to be underused between platoons. Roundabouts, on the other hand, produce efficiency through a gap acceptance process. While the capacity for through traffic is limited by conflicting circulatory flow, drivers can accept gaps as they appear rather than waiting for their time in the cycle. The resulting flow between roundabouts tends to be more random and makes more efficient use of the links between intersections. As a result, roundabouts can be made as large as needed for node capacity, keeping the links between nodes more narrow. This concept is sometimes referred to as a âwide nodes, narrow roadsâ concept and is illustrated in Exhibit 2-1. The right-of-way savings between intersections may make it feasible to accommodate parking, wider sidewalks, planter strips, and/or bicycle lanes. Another space-saving strategy is the use of flared approach lanes to provide additional capacity at the intersection while maintaining the benefit of reduced spatial requirements upstream and downstream of an intersection. The wide nodes, narrow roads concept has a beneficial application at freeway interchanges. At interchange ramp terminals, paired roundabouts have been used to reduce the number of lanes in freeway overpasses and underpasses. In compact urban areas, there are typically signalized intersections at both ends of overpass bridges, necessitating additional overpass lanes to provide capacity and storage for left-turning vehicles. Exhibit 2-2 illustrates an example of this application in Vail, Colorado.
Roundabouts: An Informational Guide Page 2-8 Chapter 2/Roundabout Considerations Exhibit 2-1 Wide Nodes, Narrow Roads Concept Exhibit 2-2 Example of Wide Nodes, Narrow Roads Concept Vail, Colorado Most roundabouts on arterial streets are designed to accommodate traffic volume estimated for a future horizon year, which can extend 20 years or more from the construction date. Collector and local-street roundabouts are typically designed for full build-out conditions. While it is important to plan for future traffic volume and capacity needs, the immediate effects on pedestrian and bicycle users should also be considered. A roundabout constructed with a wide
Chapter 2/Roundabout Considerations Page 2-9 Roundabouts: An Informational Guide cross section can negatively impact bicycle and pedestrian movements. Therefore, a phased implementation may be an appropriate way to accommodate current usersâ needs while still providing an opportunity for the roundabout to be expanded for future traffic volume growth. In these cases, it is important to reserve right-of- way for future planned improvements and also plan the construction material to potentially allow for easier expansion in the future. More information on phased implementation can be found in Chapter 6. 2.2.5 ACCESS MANAGEMENT Roundabouts can be used at key public and private intersections to facilitate major movements and enhance access management. Minor public and private access points between roundabouts can be accommodated by partially or fully restricted two-way stop-controlled intersections, with the roundabouts providing U-turn opportunities. Most of the principles used for access management at con- ventional intersections can also be applied at roundabouts. While roundabouts may allow for fewer lanes between intersections, the traffic pattern that emerges from roundabouts can have a significant impact on existing midblock access. The more random departure pattern that emerges from a round- about and the potentially narrower cross section between roundabouts may reduce the number of available gaps for mid-block unsignalized intersections and drive- ways. As a result, an unsignalized intersection may have less capacity and more delay downstream of a roundabout than downstream of a signal, even accounting for the U-turns that roundabouts facilitate. This should be reviewed on a case-by- case basis with the given turning movement patterns of a corridor. More discussion of access management issues and techniques can be found in Chapter 6. 2.2.6 ENVIRONMENTAL FACTORS Roundabouts can provide environmental benefits if they reduce vehicle delay and the number and duration of stops compared with an alternative. Even when there are heavy volumes, vehicles continue to advance slowly in moving queues rather than coming to a complete stop. This may reduce noise and air quality impacts and fuel consumption significantly by reducing the number of acceleration/deceleration cycles and the time spent idling. Roundabouts may also be an alternative to help satisfy purpose and need requirements for envi- ronmental documents prepared under the National Environmental Policy Act (NEPA). 2.2.7 OPERATION AND MAINTENANCE COSTS The initial design and construction cost of a roundabout can vary signifi- cantly depending on the roundabout size, right-of-way impacts, illumination requirements, and other design or aesthetic features that may be desired. A new single-lane roundabout intersection in an unbuilt environment can have construc- tion costs comparable to a traffic signal. However, as the size of the roundabout increases, particularly in a fully built-out commercial or residential area, the cost of roundabout construction can be higher than that of a traffic signal, depending on the footprint of the roundabout relative to that needed for the signal. However,
Roundabouts: An Informational Guide Page 2-10 Chapter 2/Roundabout Considerations Landscaping issues are discussed in detail in Chapter 9. the ongoing operations and maintenance cost of a roundabout can be less than that for a signal, with the possible exception of increased illumination needs for a roundabout. Although the initial construction cost may be more, a roundabout can have less operating and maintenance costs than a traffic signal, and the service life of a roundabout is significantly longer, approximately 25 years, compared with 10 years for a typical signal (2). Roundabouts also provide substantial cost savings to society due to the reduction in crashes, particularly fatal and injury crashes, over their service life. Compared to signalized intersections, a roundabout does not have signal equipment that requires constant power, periodic light bulb and detection mainte- nance, and regular signal-timing updates. Roundabouts, however, can have higher landscape maintenance costs depending on the degree of landscaping provided on the central island, splitter islands, and perimeter. Illumination costs for round- abouts can be greater than for signalized intersections due to a larger area required for coverage. Drivers sometimes face a confusing situation when they approach a signalized intersection during a power failure, but such failures have minimal temporary effect on roundabouts or any other unsignalized intersections, other than the possible loss of illumination. 2.2.8 TRAFFIC CALMING Roundabouts can have traffic calming effects on streets by reducing vehicle speeds using geometric design rather than traffic control devices or traffic volume. Consequently, speed reduction can be realized at all times of day and on streets of any traffic volume. It is difficult for drivers to speed through an appropriately designed roundabout with raised channelization that forces vehicles to physically change direction. Example applications include using roundabouts at the transition from a rural, high-speed environment to a low-speed urban environment and to demarcate commercial uses from residential areas. Roundabouts have also been used successfully as gateway treatments at the interface between rural and urban areas where speed limits change or at freeway ramp terminals. In these applications, the traffic calming effect of roundabouts reduces traffic speeds and reinforces the notion of a significant change in the driving environment. These gateways also reduce unwanted vehicular intrusion into neighborhoods by providing a convenient U-turn location. Exhibit 2-3 shows a photo of a roundabout in Clearwater, Florida, that provides this gateway feature between commercial and residential land uses. 2.2.9 AESTHETICS Roundabouts offer the opportunity to provide attractive entries or centerpieces to communities. Landscaping is a desirable aesthetic feature and can be installed on the central island and splitter islands as long as sight-distance requirements are met. It may be possible to place monuments and art in some portions of the central island if they do not pose a significant safety hazard to errant vehicles. In addition, pave- ment textures and colors added to truck aprons or other elements improve the visual appearance of the intersection. When installing landscaping or other artistic features in the central island, clear distance and offsets should be considered to ensure that By reducing speeds, round- abouts complement other traf- fic calming measures.
Chapter 2/Roundabout Considerations Page 2-11 Roundabouts: An Informational Guide hard objects directly facing the entries do not create a safety hazard. Additional guidance for landscaping and art at roundabouts is presented in Chapter 9. Roundabouts are also used in tourist or shopping areas to aesthetically enhance the visual environment. They have been justified as a spur to economic development, conveying to developers that the area is favorable for investment in re-development. Some are exhibited as a signature feature on community postcards, advertisements, and travelogues. Exhibit 2-4 presents examples of the aesthetic treatments that have been applied to roundabouts. Additional examples and discussion are provided in Chapter 9. 2.2.10 SUMMARY OF ADVANTAGES AND DISADVANTAGES As described in the previous sections, roundabouts have unique features and characteristics, including safety, signal progression, environmental factors, spatial requirements, operation and maintenance costs, traffic calming, aesthetics, and Clearwater, Florida Exhibit 2-3 Example of Gateway Treatment (a) Ladera Ranch, California Exhibit 2-4 Examples of Aesthetic Treatments
Roundabouts: An Informational Guide Page 2-12 Chapter 2/Roundabout Considerations access management. The trade-offs involved when implementing a roundabout should be considered at a policy level when introducing roundabouts into a region or on a project-by-project basis at specific locations where a roundabout is one of the alternatives being considered. Exhibit 2-5 provides an overview of the primary advantages and disadvantages of roundabouts for users, policy makers, designers, and planners to understand when considering this type of intersection. Exhibit 2-5 Summary of Roundabout Advantages and Disadvantages Advantages Disadvantages Non-Motorized Users ⢠Pedestrians must consider only one direction of conflicting traffic at a time. ⢠Bicyclists have options for negotiating roundabouts, depending on their skill and comfort level. ⢠Pedestrians with vision impairments may have trouble finding crosswalks and determining when/if vehicles have yielded at crosswalks. ⢠Bicycle ramps at roundabouts have the potential to be confused with pedestrian ramps. Safety ⢠Reduce crash severity for all users, allow safer merges into circulating traffic, and provide more time for all users to detect and correct for their mistakes or the mistakes of others due to lower vehicle speeds. ⢠Fewer overall conflict points and no left-turn conflicts. ⢠Increase in single-vehicle and fixed-object crashes compared to other intersection treatments. ⢠Multilane roundabouts present more difficulties for individuals with blindness or low vision due to challenges in detecting gaps and determining that vehicles have yielded at crosswalks. Operations ⢠May have lower delays and queues than other forms of intersection control. ⢠Can reduce lane requirements between intersections, including bridges between interchange ramp terminals. ⢠Creates possibility for adjacent signals to operate with more efficient cycle lengths where the roundabout replaces a signal that is setting the controlling cycle length. ⢠Equal priority for all approaches can reduce the progression for high volume approaches. ⢠Cannot provide explicit priority to specific users (e.g., trains, emergency vehicles, transit, pedestrians) unless supplemental traffic control devices are provided. (b) Ottawa, Ontario, Canada Exhibit 2-4 (cont.) Examples of Aesthetic Treatments
Chapter 2/Roundabout Considerations Page 2-13 Roundabouts: An Informational Guide Exhibit 2-5 (cont.) Summary of Roundabout Advantages and Disadvantages Space ⢠Often require less queue storage space on intersection approachesâcan allow for closer intersection and access spacing. ⢠Reduce the need for additional right-of-way between links of intersection. ⢠More feasibility to accommodate parking, wider sidewalks, planter strips, wider outside lanes, and/or bicycle lanes on the approaches. ⢠Often requires more space at the intersection itself than other intersection treatments. Operation & Maintenance ⢠No signal hardware or equipment maintenance. ⢠May require landscape maintenance. Aesthetics ⢠Provide attractive entries or centerpieces to communities. ⢠Used in tourist or shopping areas to separate commercial uses from residential areas. ⢠Provide opportunity for landscaping and/or gateway feature to enhance the community. ⢠May create a safety hazard if hard objects are placed in the central island directly facing the entries. Access Management ⢠Facilitate U-turns that can substitute for more difficult midblock left turns. ⢠May reduce the number of available gaps for mid- block unsignalized intersections and driveways Environmental Factors ⢠Noise, air quality impacts, and fuel consumption may be reduced. ⢠Little stopping during off-peak periods. ⢠Possible impacts to natural and cultural resources due to greater spatial requirements at intersections. Traffic Calming ⢠Reduced vehicular speeds. ⢠Beneficial in transition areas by reinforcing the notion of a significant change in the driving environment. ⢠More expensive than other traffic calming treatments. 2.3 USER CONSIDERATIONS As with any intersection design, each transportation mode present requires careful consideration. This section offers some of the issues associated with each mode; additional detail on mode-specific safety and design issues is provided in subsequent chapters. Guidance on educating various users is provided in Chapter 3 and Appendix B. 2.3.1 PEDESTRIANS At roundabout locations where pedestrian access is provided, pedestrians are accommodated at crosswalks around the perimeter of the roundabout. By provid- ing 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 roundabout should be designed to discourage pedestrians from crossing to the central island, e.g., with landscape buffers on the corners. Crosswalks are set back from the yield line by one or more vehicle lengths to: ⢠Shorten the crossing distance compared to locations adjacent to the inscribed circle,
Roundabouts: An Informational Guide Page 2-14 Chapter 2/Roundabout Considerations ⢠Separate vehicleâvehicle and vehicleâpedestrian conflict points, and ⢠Allow the second entering driver to devote attention to crossing pedestri- ans while waiting for the driver ahead to enter the circulatory roadway. As discussed in Chapter 5, relatively slow vehicle speeds and a reduced num- ber of conflicts are two primary reasons that roundabouts are safer than most other intersection types. The slow speeds combined with well-defined crossings and split- ter islands result in relatively high rates of motorists yielding to pedestrians at most roundabouts, making it easy for pedestrians to cross. Research has found that pedes- trians often have very short waiting times to cross at roundabout crosswalks (3). Most intersections are two-way stop controlled. Compared to two-way stop-controlled intersections, roundabouts typically make it easier and safer for pedestrians to cross the major street. At both roundabouts and two-way stop-controlled intersections, pedestrians have to judge gaps in the major (uncontrolled) stream of traffic. At roundabouts, sighted pedestrians only have to look in one direction at a time, within a relatively small sight angle. At tradi- tional intersections, unless a raised median provides a refuge, pedestrians need to look in both directions on the major street. They must also be aware of vehicles turning off of the minor street, so their field of vision must be wide. Pedestrians with vision impairments can have difficulty assessing gaps at roundabouts and two-way stop-controlled intersections. By reducing stopping distance, the low vehicular speeds through a roundabout generally reduce the frequency of crashes involving pedestrians and increase the likelihood of vehicles yielding to pedestrians. The reduced kinetic energy reduces the severity of pedestrian crashes as well, if they occur. The comparison between roundabouts and all-way stop-controlled intersec- tions is less clear. All-way stop control is virtually nonexistent in most countries outside North America that have roundabouts, so there is little international experience with which to compare. All-way stop-controlled intersections may be preferred by pedestrians, especially those with vision impairments, because vehi- cles are required to stop before they enter the intersection. However, crossing an all-way stop-controlled intersection can also be intimidating, since traffic may be turning onto the exiting approach from multiple directions. Roundabouts, on the other hand, allow pedestrians to cross one direction of traffic at a time. However, traffic may be moving (albeit at a slow speed), thus making it challenging to judge gaps, especially for pedestrians who are blind or have low vision. All-way stop-controlled intersections normally have low incidence of severe pedestrian crashes due to the fact that motorists generally stop or at least slow down significantly before going through the stop signs. However, all-way stop- controlled intersections do not provide positive geometric features to slow vehicles and instead rely entirely on the authority of the traffic control device. The round- about geometry physically slows and deflects vehicles, reducing the likelihood of a high-speed crash due to a traffic control device violation. When properly designed to accommodate pedestrians, signalized intersections offer positive guidance to pedestrians by providing visual and audible pedestrian signal indications. In this respect, the decision process for pedestrians requires less judgment at signalized intersections than at roundabouts, particularly for visually
Chapter 2/Roundabout Considerations Page 2-15 Roundabouts: An Informational Guide impaired and elderly pedestrians. However, pedestrians at signalized intersections are vulnerable to unprotected right-turn and left-turn movements. In suburban environments with large intersections and large corner radii, these crashes occur at relatively high speeds, sometimes resulting in severe crashes. In addition, high- speed vehicleâpedestrian crashes occur when vehicles run through a red signal indication. In this respect, the roundabout provides a speed-constrained environ- ment for through traffic. At two-way and all-way stop intersections, right-turning motorists often look only to the left in order to check for vehicular conflicts, endangering or inconve- niencing pedestrians crossing from the right or on the right. The same situation occurs with motorists at signalized intersections making right turns on red. These crashes can be severe due to the fact that many of these drivers do not come to a complete stop if they do not perceive any vehicular conflicts. With crosswalks located back from the circulatory roadway, roundabouts place pedestrians in a more visible location. The two populations at opposite ends of the age continuumâchildren and the elderlyâand people with disabilities are particularly at risk at intersections. These pedestrians often find it more difficult to cross unprotected road crossings, walk at slower speeds than other pedestrians, and generally prefer larger gaps in the traffic stream. Children lack traffic experience, are impulsive, and have less developed cognitive abilities, and their small size limits their visibility. The elderly may have physical limitations including reduced visual acuity, hearing, and mobility. Crossing at multilane roundabouts is more difficult for all pedestrians, but especially for the more vulnerable users described above. Multilane roundabouts have longer crossing distances and pedestrians need assurance that all lanes are free of moving traffic before they can cross the street. Recent research indicates that two to three times more motorists do not yield to pedestrians at multilane round- abouts than at single-lane roundabouts (3). In addition, pedestrians are faced with the potential for multiple-threat crashes when the driver in the first lane stops to yield to a pedestrian, blocking the sight lines between the pedestrian and any vehi- cles in the next lane. If neither the driver in the next lane nor the pedestrian sees the other user in time to take evasive action, a crash can occur in the second lane. 2.3.2 PEDESTRIANS WITH DISABILITIES Pedestrians who are blind or have low vision have several areas of diffi- culty when crossing a roundabout. It is expected that a pedestrian with vision impairments who has good travel skills should be able to arrive at an unfamil- iar intersection and cross it without special intersection-specific training. For pedestrians with vision impairments, roundabouts pose problems at several locations throughout the crossing experience: ⢠Wayfinding. Pedestrians with vision impairments may have trouble find- ing crosswalks because crosswalks are located outside the projection of approaching side-walks, and the curvilinear nature of roundabouts alters the normal audible and tactile cues they use to find crosswalks. As described in Section 6.8.1, a landscape strip or other detectable edge treatment between sidewalks and roundabouts can help lead all pedestri- ans to a crosswalk, particularly those who are blind or have low vision. When crossing a roundabout, there are several areas of difficulty for the blind or visually impaired pedestrian.
Roundabouts: An Informational Guide Page 2-16 Chapter 2/Roundabout Considerations ⢠Alignment. Likewise, roundabouts do not typically include the normal audible and tactile cues used by pedestrians with vision impairments to align themselves with the crosswalk. This alignment task can be simplified if sidewalk ramps and splitter island cut-through walkways are aligned with the crosswalk and if detectable warnings are installed on curb ramps and splitter islands. ⢠Gap and yield detection. The most critical issue at roundabouts for pedestrians with vision impairments is the fact that the sound of circulating traffic masks the audible cues that blind pedestrians use to identify the appropriate time to enter the crosswalk (both gap detection and yield detection). It may be impossible to determine by sound alone whether a vehicle has actually stopped or intends to stop. This is especially problematic at roundabout exits because without visual confirmation, it is difficult to distinguish a circulating vehicle from an exiting vehicle. At multilane roundabouts, this problem is magnified by the need to assess traffic traveling in multiple directions in multiple lanes. Even if a vehicle in one lane has stopped and a blind pedestrian is able to discern this, the pedestrian will likely have diffi- culty assessing if motorists have stopped in all lanes of a roundabout exit. Although research has been conducted on other possible solutions and some research is still ongoing, the installation of accessible (audible and vibrotac- tile) pedestrian signals at roundabout pedestrian crossings has been shown to be a treatment that consistently makes multilane roundabouts accessible to pedestrians who are blind or who have low vision. Any new or modified intersection in the United States that has pedestrian facili- ties must be accessible to and usable by all pedestrians per the requirements of the Americans with Disabilities Act (ADA) (4). Under the ADA the public right-of-way is a âprogramâ provided by state and local governments that must not discriminate against pedestrians with disabilities (28 CFR 35.150). Any facility or part of a facility that is newly constructed by a state or local government and that provides pedes- trian facilities must be designed and constructed so that it is readily accessible to and usable by people with disabilities [28 CFR 35.151(a)]. Alterations to existing facilities must include modifications to make altered areas accessible to individuals with disabilities [28 CFR 735.151 (b)]. As of this writing, the 1994 ADA Accessibility Guidelines (ADAAG) are the currently adopted standards that apply to the public right-of-way (5). These guide- lines, however, do not specifically address how to make roundabouts accessible. Nonetheless, these provisions mean providing information to safely cross streets in accessible format, including at roundabouts. The agency responsible for creating accessibility guidelines, the United States Access Board, has developed the draft Public Rights-of-Way Accessibility Guidelines (PROWAG), which address many accessibility issues found in the public right-of-way that are not addressed by ADAAG. Accessibility features at roundabouts include sidewalks and crosswalks that meet surface, slope, and clearance requirements; ramps connecting sidewalks and crosswalks; detectable edge treatments at ramps, splitter islands, and between sidewalks and round- abouts to guide pedestrians to crosswalks such as landscaping adjacent to the curb line; and signalized pedestrian crossings.
Chapter 2/Roundabout Considerations Page 2-17 Roundabouts: An Informational Guide Bicycle lanes are not recom- mended on the circulatory roadway. The Federal Highway Administration has issued a memo stating that âthe Draft Guidelines are the currently recommended best practices, and can be consid- ered the state of the practice that could be followed for areas not fully addressed by the present ADAAG standardsâ (6). These guidelines provide specific design guid- ance for making roundabouts and other intersections accessible to pedestrians with mobility impairments and vision impairments. The reader should refer to Chapters 6 and 7 for information about accessibility features and design details at roundabouts to improve access for pedestrians with disabilities. 2.3.3 BICYCLISTS Recent research of roundabouts in the United States has not found any substan- tial safety problems for bicyclists, as indicated by few crashes being reported in detailed crash reports (3). Nevertheless, roundabouts slow drivers to speeds more compatible with bicycle speeds, while reducing high-speed conflicts and simplify- ing turn movements for bicyclists. Typical on-road bicyclist speeds are 12 to 20 mph (19 to 32 km/h), so designing roundabouts for circulating traffic to flow at similar speeds will minimize the relative speeds between bicyclists and motorists and thereby improve safety and usability for cyclists. Bicyclists require particular atten- tion in two-lane roundabout design, especially in areas with moderate to heavy bicycle traffic. As with pedestrians, one of the difficulties in accommodating bicyclists is their wide range of skills and comfort levels in mixed traffic. Some of the least-skilled cyclists will choose to ride on sidewalks both along streets away from roundabouts and at the roundabouts. Since these cyclists are behaving like rolling pedestrians, no specific treatments are necessary at roundabouts besides what are provided for pedestrians. In general, cyclists who have the knowledge and skills to ride effectively and safely on roadways can navigate low-speed single lane roundabouts without much difficulty. The most experienced and skilled on-road cyclists will be comfortable traveling through all roundabouts like other vehicles, even at multi- lane roundabouts. Single-lane roundabouts are much simpler for cyclists than multilane roundabouts since they do not require cyclists to change lanes to make left turn movements or otherwise select the appropriate lane for their direction of travel. In addition, at single-lane roundabouts, motorists are less likely to cut off cyclists when exiting the roundabout. Therefore, care should be exercised when selecting a multilane roundabout over a single-lane roundabout in the short term, even when long-term traffic predictions suggest that a multilane roundabout may be desirable. In addition, the use of a roundabout with two-lane entries and exits on the major roadway and one-lane entries and exits for the minor roadway can be a good solution to reduce complexity for bicyclists where a roundabout is pro- posed at an intersection of a major multilane street and a minor street. Where bicycle lanes or shoulders are used on approach roadways, they should be terminated in advance of roundabouts to merge cyclists into traffic for appropri- ate circulation with other vehicles. In addition, bicycle lanes should not be located within the circulatory roadway of roundabouts as this would suggest that bicyclists should ride at the outer edge of the circulatory roadway, which can increase crashes with cyclists and both entering and exiting motor vehicles. Because some cyclists
Roundabouts: An Informational Guide Page 2-18 Chapter 2/Roundabout Considerations 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. Bicycle ramps at roundabouts have the potential to be confused as pedestrian ramps, particularly for pedestrians who are blind or who have low vision. Therefore, bicycle ramps should be reserved for those situations where the roundabout complexity or design speed may result in less comfort for some bicyclists. Ramps should not normally be used at urban single-lane round- abouts. More details about bicycle design treatments at roundabouts can be found in Chapter 6. 2.3.4 OLDER DRIVERS There is a trend in the United States of individuals continuing to drive until an older age than in years past. This trend has implications for all roadway design, including roundabout design, ranging from operations through geometric and sign design. In this regard, designers should consult available documents such as the FHWA Highway Design Handbook for Older Drivers and Pedestrians (7), which presents the following considerations for understanding the differences of older drivers and pedestrians with understanding and navigating through intersections. ⢠The single greatest concern in accommodating older road users, both driv- ers and pedestrians, is the ability of these persons to safely maneuver through intersections. ⢠Driving situations involving complex speedâdistance judgments under time constraints are more problematic for older drivers and pedestrians than for their younger counterparts. ⢠Older drivers are much more likely to be involved in crashes where the drivers were driving too fast for the curve or, more significantly, were surprised by the curved alignment. ⢠Left-turn maneuvers are difficult for older drivers since they have difficulty in selecting acceptable gaps due to reduced ability to judge oncoming speeds and slower response times (8â11). They also have more difficulty understanding left-turn displays (12â14). ⢠Left-turn crashes are particularly problematic for older drivers. Research has shown that the potential of being involved in left-turn crashes increases with age (15â16). ⢠Many studies have shown that loss-of-control crashes result from an inabil- ity to maintain lateral position through the curve because of excessive speed with inadequate deceleration in the approach zone. These problems stem from a combination of factors, including poor anticipation of vehicle control requirements, induced by the driverâs prior speed, and inadequate perception of the demands of the curve. ⢠Older drivers have difficulties in allocating attention to the most relevant aspects of novel driving situations. ⢠Older drivers generally need more time than average drivers to react to events.
Chapter 2/Roundabout Considerations Page 2-19 Roundabouts: An Informational Guide These findings apply to older drivers and pedestrians encountering all types of intersections, including roundabouts. The excerpts above all imply that lower, more conservative design speeds are appropriate. Research indicates that roundabouts may address some of the problems drivers experience in dealing with intersections. One of the key design features of a round- about is that all traffic must slow down as it enters the intersection. Slower speeds can benefit both the novice and older driver as they navigate the roadways. Some of the potential benefits of slower intersection speeds include a reduction in crash severity (for a given crash type), safer merges, and more opportunities to correctly judge and enter gaps (17). The slower and consistent speeds at roundabouts can cater to the preferences of older drivers by: ⢠Allowing more time to make decisions, act, and react; ⢠Providing less complicated situations to interpret; ⢠Requiring simpler decision-making; ⢠Reducing the need to look over oneâs shoulder; ⢠Reducing the need to judge closing speeds of fast traffic accurately; and ⢠Reducing the need to judge gaps in fast traffic accurately. The benefits a roundabout provides to older drivers can be an important fac- tor in reducing the number of crashes at an intersection. For example, two-way stop-controlled intersections may be appropriate for replacement with a round- about when a crash analysis indicates that age-related crashes are prevalent. It is important that older drivers understand the key operating characteristics of roundabouts, such as determining a safe approach speed, identifying the number of lanes and which lane to be in, understanding the direction of travel on the circula- tory roadway, yielding to vehicles upon entry, and understanding the street/route signs at each exit. Research shows that proper use of roundabout advance warning signs with arrows indicating direction of traffic flow, yield signs, directional signs, and road name signs can improve older driversâ understanding of roundabouts (18). Overhead lane use signs, recommended by the Highway Design Handbook for Older Drivers and Pedestrians for signals (7), can aid navigation choices on multilane roundabout approaches. 2.3.5 LARGE VEHICLES Large vehicles have a direct impact on the design of a roundabout. Single-lane roundabouts often employ a traversable apron around the perimeter of the central island to provide the additional width needed for tracking the trailer wheels of large vehicles. Multilane roundabouts are designed either to allow large vehicles to track across more than one lane while entering, circulating, and exiting or to stay within their lane. In some cases, roundabouts have been designed with aprons or gated roadways through the central island to accommodate over-sized trucks, emergency vehicles, or trains. Details on treatments for large vehicles can be found in Chapter 6. Design roundabouts to accom- modate the largest vehicle that can reasonably be expected.
Roundabouts: An Informational Guide Page 2-20 Chapter 2/Roundabout Considerations 2.3.6 TRANSIT Transit vehicles are a special type of large vehicle and have unique require- ments, many of which are similar to those at other types of intersection treat- ments. If the roundabout has been designed using the appropriate design vehicle, a bus should have no physical difficulty negotiating the intersection. To minimize passenger discomfort, it is preferable for buses to not need to use a truck apron if present. Bus stops should be located carefully to minimize the probability of vehi- cle queues spilling back into the circulatory roadway. This typically means that bus stops located on the far side of the intersection need to have pullouts or be further downstream than the splitter island, located in a way that is mindful of the bus driverâs ability to merge into the traffic stream. Pedestrian access routes to transit should be designed for safety, comfort, and convenience. Pedestrian cross- ing capacity should be accounted for if demand is significant, such as near a sta- tion or terminus. When combined with signals, roundabouts may provide opportunities for giving transit (including rail) and emergency vehicles priority. For example, these could include signals holding entering traffic while the transit vehicle enters in its own right-of-way or in mixed traffic. Chapters 6 and 7 provide more detail on transit treatments. 2.3.7 EMERGENCY VEHICLES The passage of large emergency vehicles through a roundabout is the same as for other large vehicles and may require use of a traversable apron. On emergency response routes, the delay for the relevant movements at a planned roundabout should be compared with alternative intersection types and control. Just as they are required to do at conventional intersections, drivers 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. Roundabouts provide emergency vehicles the benefit of lower vehicle speeds, which may make roundabouts safer for them to negotiate than signalized cross- ings. Unlike at signalized intersections, emergency vehicle drivers are not faced with through vehicles unexpectedly running the intersection and hitting them at high speed. 2.3.8 RAIL CROSSINGS Rail crossings through or near a roundabout may involve many of the same design challenges as at other intersections. In retrofit, the rail track may be designed to pass through the central island or across one of the legs. Queues spilling back from a rail blockage into the roundabout can fill the circulatory roadway and temporarily prevent movement on any approach. However, to the extent that a roundabout approach capacity exceeds that of a signal at the same location, queues will dissipate faster. Therefore, a case-specific capacity and safety analysis is recommended. Chapter 7 addresses the design of at-grade rail crossings. Buses should not need to use a truck apron to negotiate a roundabout. Chapters 6 and 7 provide more detail on transit treatments.
Chapter 2/Roundabout Considerations Page 2-21 Roundabouts: An Informational Guide 2.4 POLICY AND LEGAL ISSUES Policy plays an important role in the implementation of roundabouts, particu- larly at the state level. There are two key aspects to policy implementations: ⢠Decision-making process and ⢠Legal issues, including rules of the road. 2.4.1 DECISION-MAKING PROCESS Many state agencies have developed roundabout policies to help guide design- ers and planners in making appropriate decisions when considering a roundabout intersection. In some cases, these states have established task forces to establish a policy for implementing roundabouts on state facilities. These policies often include background information about the geometric, safety, and operational characteristics of roundabouts; example locations where roundabouts may be considered; opera- tional and safety evaluation discussions; and an overview of the trade-offs and gen- eral considerations for this type of intersection control. Some states have made internal decisions about prioritizing the way round- abouts are used compared to other traditional intersection types. Some have adopted a âroundabout firstâ policy that requires designers and planners to consider roundabouts as a first priority during any intersection improvements or construction. Other states encourage designers or planners to only use roundabouts as solutions to unique situations. Some states have developed their own roundabout guidelines and standards, including Kansas, New York, Washington, and Wisconsin. These allow states to include design, operation, and planning information that is specific to their state practices and policies. Where there are no specific state guidelines, the guidance provided in this document is typically used. As jurisdictions continue to implement roundabouts, all users need to understand their unique features and operational characteristics, including safety, relationship to signal progression, environmental factors, spatial require- ments, operation and maintenance costs, traffic calming effects, aesthetics, and access management benefits. 2.4.2 RULES OF THE ROAD The legal environment in which roundabouts operate is an important area for jurisdictions to consider when developing a roundabout program or set of guide- lines. The rules of the road that govern the operation of motor vehicles in a given state can have a significant influence on the way a roundabout operates and on how legal issues, such as crashes involving roundabouts, are handled. Local jurisdictions that are building roundabouts should be aware of the governing state regulations in effect. The 2000 Uniform Vehicle Code (UVC) is the primary resource guidance per- taining to roadways and intersections. However, the UVC does not provide specific guidance for roundabouts. Some states have begun to update their state code to include guidance for roundabouts. For example, in Oregon, Chapter 811âRules of
Roundabouts: An Informational Guide Page 2-22 Chapter 2/Roundabout Considerations The Road of the Oregon Revised Statutes (ORS 811.400) (19) creates certain traffic procedures for roundabouts; creates offense of failure to yield right-of-way within a roundabout; modifies offense of failure to use the appropriate signal for turns, lane changes, or stops to include exiting from a roundabout; and defines a roundabout and circulatory roadway. Further detail can be found in Appendix C. 2.5 REFERENCES 1. Brown, M. TRL State of the Art Review: The Design of Roundabouts. HMSO, London, 1995. 2. Niederhauser, M. E., B. A. Collins, and E. J. Myers. âThe Use of Roundabouts: Comparison with Alternate Design Solution.â In Compendium of Technical Papers for the 67th ITE Annual Meeting, Boston, Institute of Transportation Engineers, Washington, D.C., 1997. 3. Rodegerdts, L., M. Blogg, E. Wemple, E. Myers, M. Kyte, M. Dixon, G. List, A. Flannery, R. Troutbeck, W. Brilon, N. Wu, B. Persaud, C. Lyon, D. Harkey, and D. Carter. NCHRP Report 572: Roundabouts in the United States. Transportation Research Board of the National Academies, Washington, D.C., 2007. 4. Americans with Disabilities Act. www.ada.gov/pubs/ada.htm. Accessed March 2009. 5. Americans with Disabilities Act Accessibility Guidelines. www.access-board. gov/adaag/html/adaag.htm. Accessed March 2009. 6. Isler, F. D. âINFORMATION: Public Rights-of-Way Access Advisory.â Memo from FHWA Associate Administrator for Civil Rights to Division Adminis- trators, Resource Center Directors, and Federal Lands Highway Division Engineers. January 23, 2006. www.fhwa.dot.gov/environment/bikeped/ prwaa.htm. Accessed January 19, 2009. 7. Staplin, L., K. Lococo, S. Byington, and D. Harkey. Highway Design Handbook for Older Drivers and Pedestrians. Publication No. FHWA-RD-01-103. FHWA, Washington, D.C., May 2001. 8. Traffic Maneuver Problems of Older Drivers: Final Technical Report. Publication No. FHWA-RD-92-092. FHWA, Washington, D.C., 1993. 9. Staplin, L. âSimulator and Field Measures of Driver Age Differences in Left- Turn Gap Judgments.â Transportation Research Record 1485. TRB, National Research Council, Washington, D.C., 1995, pp. 49â55. 10. Scialfa, C. T., L. T. Guzy, H. W. Leibowitz, P. M. Garvey, and R. A. Tyrrell. âAge Differences in Estimating Vehicle Velocity.â Psychology and Aging, Vol. 6, No. 1, 1991, pp. 60â66. 11. Oxley, J., B. Corben, and B. Fildes. âOlder Driver Highway Design: The Development of a Handbook and Training Workshop to Design Safe Road
Chapter 2/Roundabout Considerations Page 2-23 Roundabouts: An Informational Guide Environments for Older Drivers.â Proc., Traffic Safety on Three Continents Confer- ence, Moscow, Russia, 2001. 12. Williams, J. C., S. A. Ardekani, and S. Adu Asante. âMotorist Understanding of Left-Turn Signal Indications and Auxiliary Signs.â Transportation Research Record 1376. TRB, National Research Council, Washington, D.C., 1992, pp. 57â63. 13. Drakopoulos, A. and R. W. Lyles. âDriver Age as a Factor in Comprehension of Left-Turn Signals.â Transportation Research Record 1573. TRB, National Research Council, Washington, D.C., 1997, pp. 76â85. 14. Noyce, D. A. and K. C. Kacir. âDriversâ Understanding of Protected-Permitted Left-Turn Signal Displays.â Transportation Research Record 1754. TRB, National Research Council, Washington, D.C., 2001, pp. 1â10. 15. Garber, N., and R. Srinivasan. âCharacteristics of Accidents Involving Elderly Drivers at Intersections.â Transportation Research Record No. 1325, TRB, National Research Council, Washington, D.C., 1991, pp. 8â16. 16. Matthias, J., M. De Nicholas, and G. Thomas. A Study of the Relationship between Left Turn Accidents and Driver Age in Arizona. Report No. AZ-SP-9603. Arizona Department of Transportation, Phoenix, Arizona, 1996. 17. Stutts, J. NCHRP Synthesis 348: Improving the Safety of Older Road Users. Trans- portation Research Board of the National Academies, Washington D.C., 2005. 18. Lord, D., I. van Schalkwyk, L. Staplin, and S. Chrysler. Reducing Older Driver Injuries at Intersections Using More Accommodating Roundabout Design Practices. Texas Transportation Institute, College Station, Texas, 2005. 19. State of Oregon. Oregon Revised Statute 811.400. www.leg.state.or.us/ors/ 811.html. Accessed March 2009.
