With the publication in 2000 of the first edition of the Federal Highway Administration’s (FHWA) Roundabouts: An Informational Guide, roundabout implementation in the United States grew exponentially. Demand continues for more information about appropriate performance analysis methods and design parameters for roundabouts, with a particular need for that information to be based on the performance of U.S. drivers, bicyclists and pedestrians rather than relying exclusively on international experience and procedures as surrogates.


Kittelson led an international team of experts to develop methods for estimating the safety and operational impacts of roundabouts in the United States and to refine roundabout design criteria. Several parallel efforts were needed to address the comprehensive nature of this project. The team updated a national inventory of roundabouts across the United States and selected a sample for collection of field data related to operational performance and geometric factors, as well as a larger national set for collection of safety performance data. The team reviewed international operational and safety performance models for use in developing new capacity models, safety performance functions, and crash modification factors anchored to the new data. The team also refined the fastest path speed prediction method in Roundabouts: An Informational Guide and conducted an observational analysis of pedestrian and bicycle behavior at roundabouts.

The Outcome

Moving Beyond Speculation into Measurement

The product of this project, NCHRP Report 572: Roundabouts in the United States, formed the basis of roundabout components of many national reference documents, including the Highway Capacity Manual, Highway Safety Manual, and AASHTO Green Book. The project also produced marketing materials to communicate roundabout concepts to elected officials.




National Cooperative Highway Research Program, Transportation Research Board, National Academies of Science