Roundabout Practices

Abstract

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, focus-ing 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 SUMMARY Roundabout Practices