The concept of the modern roundabout to move traffic more efficiently through unsignalised intersections has evolved from conventional traffic circles. This report of the Transportation Research Board presents a discussion of modern roundabout applications in the United States, based on a survey of state and local transportation agencies who provided information on 38 individual roundabouts. Case examples of three roundabouts, each representing a different type of roundabout, are described in detail. The synthesis presents information on the design guidelines used in the United States as well as those of other countries. Other major areas of interest with regard to roundabouts include safety issues; traffic capacities and delays; issues related to pedestrians, bicyclists, and the visually impaired; costs; and location criteria to be considered for roundabouts. To develop this synthesis in a comprehensive manner and to ensure inclusion of significant knowledge, the Board analysed available information assembled from numerous sources, including a large number of state highway and transportation departments. A topic panel of experts in the subject area was established to guide the research in organising and evaluating the collected data, and to review the final synthesis report. This synthesis is an immediately useful document that records the practices that were acceptable within the limitations of the knowledge available at the time of its preparation. As the processes of advancement continue, new knowledge can be expected to be added to that now at hand. (A) This report may be accessed by Internet users at http://trb.org/publications/nchrp/nchrp_syn_264.pdf
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