National Academies Press: OpenBook

Roundabout Practices (2016)

Chapter: Chapter Five - Conclusions and Suggestions for Future Research

« Previous: Chapter Four - Case Examples of Roundabout Practices
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Suggested Citation:"Chapter Five - Conclusions and Suggestions for Future Research ." National Academies of Sciences, Engineering, and Medicine. 2016. Roundabout Practices. Washington, DC: The National Academies Press. doi: 10.17226/23477.
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Page 32
Page 33
Suggested Citation:"Chapter Five - Conclusions and Suggestions for Future Research ." National Academies of Sciences, Engineering, and Medicine. 2016. Roundabout Practices. Washington, DC: The National Academies Press. doi: 10.17226/23477.
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Page 33

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32 multilane roundabouts averaging $249,000, $1.30 mil- lion, and $2.05 million, respectively. • The majority of reporting states have public education materials to support roundabout projects. Most have cre- ated a roundabout website and a flyer and/or pamphlet. About half have developed videos. Only a few respon- dents indicated they had not developed any public out- reach material. • To analyze roundabout performance, about three quar- ters of the reporting states use some form of the Highway Capacity Manual 2010 model and SIDRA’s Standard Model, and about a one-quarter of the reporting states use some form of the United Kingdom equations. • Most of the reporting states analyze safety performance at roundabouts. About half of the reporting states use crash modification factors, crash reduction factors, and/ or the Highway Safety Manual predictive methodology to measure safety performance. A handful of states have also developed state-specific procedures, and a couple of states use information published by the Insurance Institute for Highway Safety (IIHS). • NCHRP Report 672: Roundabouts: An Informational Guide, 2nd edition, is used by most of the reporting state agencies for design guidance. About one-quarter of the states only use NCHRP Report 672, another one- quarter uses material from other sources to supplement their use of NCHRP Report 672, and a third quarter have developed their own guidance to supplement this report. Only a few states either do not use, or rarely use, NCHRP Report 672; two use materials from other sources, and one has developed their own guidelines. • Most states use a design life of 20 years when developing the ultimate roundabout design. However, more than half of the states use a phased implementation approach over the project life, indicating they prefer to open a round- about in a single-lane or other smaller configuration, with the ability to expand the roundabout when traffic volume increases. About a quarter of the state reported developing criteria for the use of a phased implementation approach, and a handful of states indicated they would move for- ward with a single-lane roundabout if it were projected to operate “acceptably” for at least 10 years. • Many states, especially the early-adopter states (defined as having built a roundabout before the year 2000), have experienced the need to modify existing roundabouts owing to identified safety or operational problems. Most This synthesis provides state-of-the-practice information about roundabout practices within state departments of transportation (DOTs) across the United States. Information about current roundabout practices was taken from literature, from 40 state responses to a questionnaire, and interviews with state DOT personnel from seven states. The growth in roundabouts over the past 25 years, both in total numbers and their broad application and use across the country, is notable. The 1998 Synthesis of Highway Prac- tice 264: Modern Roundabout Practice in the United States provides a useful comparison between roundabout practices prior to the publication of the first FHWA roundabout guide in 2000 and today. In 1997, only nine state agencies had a roundabout in operation, and only a third of the states with- out a roundabout on their state highway system were con- sidering the construction of roundabouts. Today, 38 of the 40 reporting states have a roundabout in operation. Of the two states reporting that their agency has no roundabout on the state highway system, one reported that a roundabout has been planned or designed but has not yet been built, and the other is considering the construction of roundabouts. According to the state questionnaire responses and inter- views, current roundabout practices can be summarized as follows: • Improved safety performance is the primary reason cited for the selection of roundabouts compared with other intersection options. Shorter vehicular delays and higher capacity were the next most frequent responses. The rea- sons least frequently reported for selection of round- abouts were lower initial capital costs and requests from an election official. • However, despite the documented safety performance of roundabouts compared with other intersection control options, several interviewed states cited continuing chal- lenges with evaluating roundabouts on an equal footing with other options. Several states have developed poli- cies requiring the evaluation of roundabouts to varying degrees of success depending on how well the policy is enforced, In other states, the use of Intersection Control Evaluation policies has had the effect of increasing the number of roundabouts evaluated. • State agencies provided planning-level costs (screening or feasibility level estimate) of mini-, single-lane, and chapter five CONCLUSIONS AND SUGGESTIONS FOR FUTURE RESEARCH

33 of the modifications involved reducing the number of lanes owing to excess capacity at multilane facilities that were overbuilt. Largely because of this experience, all seven interview participants indicated they use a phased approach to roundabout implementation and are actively attempting to size their initial constructed configuration for the vehicle demand that will be pres- ent at the intersection in the immediate and near future. • Half of the reporting states indicated their agency had developed design-vehicle guidance. Several states sup- ply guidance to the use of a large tractor-trailer truck (WB-67) design vehicle, and a few others are currently in the process of developing guidance for the accom- modation of large vehicles. The interview participants indicated they are using different techniques to accom- modate oversize/overweight vehicles at roundabouts, depending on how oversize/overweight vehicles are tracked across their state highway system. • All reporting states follow developed illumination guid- ance, with only one state indicating it does not require the illumination of roundabouts. More than half of the reporting states use the AASHTO Roadway Lighting Guide, about one-quarter use the IES Design Guide, and about one-third use NCHRP Report 672. A handful of states have developed their own state-specific standard. • Several states have installed accelerated, low-cost round- abouts, which involves roundabouts implemented at a substantially reduced cost or time. For instance, the Geor- gia DOT installed a mini-roundabout in a rural location over several weekends. The Washington State DOT and Maryland State Highway Administration also have expe- rience with accelerated, low-cost roundabouts. After reviewing the responses to the questionnaire and interviews, some key questions still remain: What are effec- tive strategies for ensuring roundabouts and other intersection forms are consistently evaluated using state of the practice methods and measures? What strategies can be used to reduce the total cost of a roundabout installation? Is it better to con- struct roundabouts without lighting than to not construct a roundabout at all? The following key topics emerged from this synthesis as areas recommended for future research: • There is a general need continually to advance the plan- ning, evaluation, and design guidance available related to roundabouts. NCHRP Report 672, used by almost all of the reporting states, provided the most up-to-date guidance available when it was published in 2010. A sig- nificant amount of new material, including this synthe- sis, has been developed in the interim, and several states have developed their own design guidance to supple- ment and/or advance the information. As documented in the responses to survey questions and state interviews, there is also a need for continuing information about definitions and safety issues related to roundabouts. The production of a third edition of Roundabouts: An Infor- mational Guide would help to disseminate this updated information to practitioners. • Several state agencies expressed a desire to identify and disseminate strategies for limiting the installation costs of roundabouts. Specific items identified as contributing to the rising costs if roundabout implementation include the maintenance of traffic during construction, the use of curb-and-gutter sections, and illumination. Although research has been conducted documenting the safety benefits of roundabouts, limited research has been con- ducted on whether individual items, such as illumination, can be excluded from the roundabout without sacrific- ing the overall safety benefits. In addition, identifying particular applications, such as interchange ramp termi- nals where roundabouts may have additional cost ben- efits, would be beneficial for states looking to maximize cost-effectiveness. • States are searching for ways to ensure roundabouts and other intersection forms are evaluated using state- of-the-practice methods and measures. A comparison of rate of implementation among states with different roundabout policies and particularly Intersection Con- trol Evaluation policies might shed more light on their effectiveness. Furthermore, research on the enforce- ment of policies and the development and use of round- about programs at the state level may identify effective strategies for ensuring the consideration of reasonable intersection control options. • Almost all reporting states have developed public out- reach materials related to roundabouts. However, addi- tional research on their effectiveness could be helpful in identifying successful strategies and prioritizing resources towards them, and may help alleviate push- back against roundabouts. • The majority of states are now using a phased implemen- tation approach in the design of roundabouts. However, because of the small number of roundabouts expanded to date, before/after data related to the planned expansion of roundabouts are limited. If the number of roundabouts that are expanded to their ultimate footprint increases over the next few years, additional research on the most effective practices for planning and funding expansion, along with managing traffic during reconstruction, could be helpful to state agencies.

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TRB's National Cooperative Highway Research Program (NCHRP) Synthesis 488: Roundabout Practices summarizes roundabout policies, guidance, and practices within state departments of transportation (DOTs) as of 2015. The synthesis may be used as a reference for state agencies that are creating or updating their roundabout and intersection control policies.

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