Over the past 25 years, traffic roundabouts have gone from nonexistent in the United States to being found in every state. In that time, state agencies have continued to update their roundabout policies, guidance, and practices, and this synthesis documents and summarizes these practices within state departments of transportation (DOTs) as of 2015. The intent of the synthesis is twofold: to be a useful reference to agencies that are creating or updating roundabout and/or intersection control policies; and to provide updated information about current roundabout practices. The synthesis was completed in three stages. The first stage comprised a comprehensive literature review of relevant published national materials and ongoing research projects, focusing on the design, operational analysis, and safety analysis practices of roundabouts. During the second stage, a questionnaire concerning the selection, performance analysis, and design of roundabouts was sent to the 52 AASHTO member departments (50 states, plus the District of Columbia and Puerto Rico), and responses were received from 40 states, an 80% response rate. In the final stage, state DOT personnel from seven states–California, Connecticut, Georgia, Kansas, Maryland, Minnesota, and Washington–were interviewed to provide case examples and to elaborate on survey responses. From 1990, when the first roundabout in the United States was constructed, to 2013, approximately 3,200 roundabouts are estimated to have been built. In general, roundabouts have been constructed at an increasing rate each year, and by 2010, all 50 states and the District of Columbia had a roundabout. Single-lane roundabouts have consistently been the most common type constructed, although the ratio of multilane roundabouts to single-lane roundabouts has increased slightly over the last decade. Agency policies, guidance, and practices regarding the selection and design of roundabouts vary across the United States. Currently, 11 states formally require the analysis of roundabout alternatives, and 19 additional states encourage the analysis of roundabout alternatives. Although some states make reference to NCHRP Report 672 for design guidance, other states go further than simply referencing the federal guidance and provide supplemental material or have developed standalone guidance. Of the 40 states that responded to the questionnaire, 38 have a roundabout in operation. Of the two states that reported their agency has not built a 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. By comparison, the 1998 Synthesis of Highway Practice 264: Modern Roundabout Practice in the United States identified only nine state agencies with a roundabout in operation, under construction, or in design as of 1997; only about one-third of the states without a roundabout on their state highway system was considering the construction of roundabouts. The questionnaire addressed practices within state DOTs, including the primary reasons for the selection of roundabouts, cost, public education material, and roundabout performance analysis. In addition, questions were asked regarding the design of roundabouts, including the use of design guidance, design life, design-vehicle guidance, and illumination of roundabouts. A brief summary of responses, detailed in chapter three, is provided here: • The primary reason cited for the selection of roundabouts is improved safety performance compared with other intersection options, followed by shorter vehicular delays and higher capacity. • Respondents provided planning-level cost estimates (screening or feasibility level estimates) for mini-, single-lane, and multilane roundabouts, which range from $249,000 to $2.05 million. • The majority of reporting states have developed some public education materials to support roundabout projects, such as websites, videos, and flyers and/or pamphlets. • To analyze roundabout performance, about three-quarters of the reporting states use some form of the Highway Capacity Manual 2010 model and SIDRA’s Standard Model; about one-quarter use some form of the United Kingdom equations. (Because states were allowed to select more than one operational performance model, there is some overlap.) • Most of the reporting states analyze safety performance at roundabouts, using crash modification factors, crash reduction factors, and/or the Highway Safety Manual predictive methodology. • NCHRP Report 672: Roundabouts: An Informational Guide, 2nd edition, is used by most of the reporting state agencies for at least some design guidance. • Most states use a design life of 20 years when developing the ultimate roundabout design. However, the majority uses a phased implementation approach, and a few indicated they would be open to an operational projection of 10 years. • Half of the reporting states indicated that their agency had developed design-vehicle guidance that typically conforms to large tractor-trailer trucks. • All of the reporting states follow developed illumination guidance, with only one state indicating it does not require the illumination of roundabouts. To expand on the information collected in the questionnaire, and to get more in-depth information on topics such as the selection of roundabouts, experience with using a phased roundabout implementation approach, modifications to existing roundabouts, and the use of accelerated low-cost roundabouts, several states were asked to participate in interviews. A summary of information obtained from the state interviews, also provided in more detail in chapter four and Appendix D, is provided here: • Although interview participants cited safety as the number-one driver behind the selection of roundabouts, several of the interview participants cited challenges with evaluating roundabouts on an equal footing with other intersection control options. • All of the interviewed early adopter states (defined as having built a roundabout before the year 2000) had experienced the need to modify existing roundabouts resulting from identified safety or operational problems. • Most interview participants indicated they use a default design vehicle of a large tractortrailer truck for roundabout movements along state routes. • Several interview participants reported experience with accelerated, low-cost roundabouts, which involves roundabouts implemented at a substantially reduced cost or time period compared with a typical roundabout installation. The early adopter states planned for, constructed, and built roundabouts prior to any type of federal design guidance, and while the interview participants indicated they experienced some challenges and the need to modify their early designs, they indicated that their journey helped pave the way for roundabouts to be installed throughout the United States. This synthesis documents some of the past and present experiences of these early adopter states. In addition, the current state of the practice at other state DOTs documented in this synthesis emphasizes the lessons learned from these early adopter states. However, this synthesis also suggests topics for further research, including the development of a third edition of Roundabouts: An Informational Guide, strategies for increasing the cost-effectiveness of roundabout installations, and the effectiveness of intersection control policies. (Author/publisher)
Samenvatting