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3 STUDY APPROACH The synthesis was completed in three stages, shown in Fig- ure 1. The first stage included a comprehensive literature review of relevant published national materials and ongoing research projects focusing on the design, operational analy- sis, and safety analysis practices of roundabouts. During the second stage, a questionnaire concerning the selection, per- formance analysis, and design of roundabouts was sent to all 50 states, as well as Washington, D.C. and Puerto Rico. In the final stage, state DOT personnel from seven states were inter- viewed to create case examples and more specific information. SYNTHESIS ORGANIZATION This synthesis document is divided in sections based on how the material was sourced. The literature review is summarized in chapter two; the survey results are presented in chapter three; and the case examples are summarized in chapter four. The report concludes with the identification of key findings and suggestions for future research. Consistent with all syn- thesis projects, the report does not contain policy or design recommendations. In mid-1997, the Synthesis of Highway Practice 264: Mod- ern Roundabout Practice in the United States (1) identified 50 round abouts known to have been constructed since the beginning of that decade. As of 2013, there were an estimated 3,200 roundabouts in use throughout the country (2). To deal with the continual expansion of roundabouts in the United States, states and municipal roadway agencies now use a range of sources and engineering judgment to establish policies and design guidelines for roundabouts. OBJECTIVES OF STUDY The purpose of this synthesis was to document and summa- rize roundabout policies, guidance, and practices in place as of 2015 within state departments of transportation (DOTs). Its primary focus was on roundabout selection and design with a secondary focus on performance analysis. The synthesis was intended to be a useful reference to agen- cies that are creating or updating roundabout and inter- section control policies. The specific objectives included the documentation of roundabout growth in the United States, statewide roundabout policies, and public outreach efforts. A survey sent to all state DOTs provided useful informa- tion on reasons that agencies have selected roundabouts, planning-level cost estimates of roundabouts, state efforts to quantify roundabout operational performance, the extent to which agencies used NCHRP Report 672 for round- about design guidelines, whether agencies had existing standards or guidance for lighting at roundabouts, and how states accommodated design vehicles (specifically, trucks). Interviews with selected state DOTs addressed questions related to modifications to existing roundabouts, whether agencies were using a phased roundabout design approach over the project life, and the use of accelerated low-cost roundabouts. chapter one INTRODUCTION Literature Review Survey Case Examples FIGURE 1 Study approach stages.