Characteristics of Modern Roundabouts in the United States: Summary of NCHRP Project 3-65 Operations Database

This paper describes the basic characteristics of the more than 300 modern roundabouts that have been constructed in the United States since 1990. The paper also describes the traffic operations data that were collected at a subset of these sites, 474 hours of data recorded at 31 sites throughout the U.S. The data base that has been assembled is the most extensive to date for U.S. conditions. Changes to the Highway Capacity Manual, as well as other standard traffic engineering references, will be made based on the conclusions drawn from this data base. The data base will eventually be made available to other researchers to investigate the many other research problems that remain unanswered regarding roundabout operations and it will also serve as a benchmark for changes in capacity flow rates at U.S. roundabouts as design, usage, and driver behavior mature over time.