NCHRP | 2021

NCHRP Report 572: Roundabouts in the United States

  • Published by:  NCHRP
  • Authored by:  L. Rodegerdts
  • Co-authored by:  M. Blogg , Elizabeth Wemple , E. Myers , M. Kyte , M. Dixon , G. List , A. Flannery , R. Troutbeck , W. Brilon , N. Wu , B. Persaud , C. Lyon , D. Harkey , D. Carter

See NCHRP Report 672: Roundaboutsand Information Guide Second Edition for up-to-date info

Based on a comprehensive evaluation of roundabouts in the United States, this report presents methods of estimating the safety and operational impacts of roundabouts and updates design criteria for them. The report will be useful to geometric designers and traffic engineers who are considering improvements to an intersection.

In general, roundabouts have improved both overall crash rates and, particularly, injury crash rates in a wide range of settings (urban, suburban, and rural) for all previous forms of traffic control except for all-way stop control, for which no statistically significant difference could be found. In addition, single-lane roundabouts have better safety performance than multilane roundabouts. The safety performance of multilane roundabouts appears to be especially sensitive to design details. This study produced a number of major safety findings:

  • Intersection-level crash prediction models for the prediction of the overall safety performance of the intersection. These models relate the crash prediction to the number of lanes, number of legs, and the average annual daily traffic.
  • Approach-level crash prediction models that relate common types of crashes (e.g., exitingcirculating crashes) to average annual daily traffic volumes and key geometric parameters that were demonstrated to influence the prediction.
  • An updated comparison of the performance of roundabouts to other forms of traffic control, disaggregated to a greater extent than any previous study of U.S. 

Rodegerdts, L., M. Blogg, E. Wemple, E. Myers, M. Kyte, M. Dixon, G. List, A. Flannery, R. Troutbeck, W. Brilon, N. Wu, B. Persaud, C. Lyon, D. Harkey, and D. Carter. NCHRP Report 572: Roundabouts in the United States. Transportation Research Board, National Research Council, Washington, D.C., (2007)