Study Details

Study Title: Crash Modification Factors Based on Crash Histories of 148 Modern Roundabouts in Five States

Authors: Zhang and Wang

Publication Date:MAY, 2017

Abstract: The concept of modern roundabout design was conceived by Frank Blackmore 1966 as a way to combat traffic congestion in Great Britain. Implementation of this design in the U.S. started in 1990. As of 2015, it was estimated that around 4,800 modern roundabouts have been installed in the U.S., with the majority of them exhibiting good to excellent operational and safety improvements. How much crash reduction can one expected after converting an existing intersection into a modern roundabout is an everlasting question that will never disappear, but will encompass new and different context as roundabouts are being implemented under broader ranges of traffic and environmental conditions. Crash modification factor is derived by computing the ratio of average annual crashes under roundabout condition over the average annual crashes under prior traffic control condition. This multiplicative factor can be derived for different types of roundabouts, different types of crash severity, and different types of crashes, etc. In this study, a total of 656 modern roundabouts were identified from five states, Florida, Indiana, New York, Pennsylvania, and Washington. They are the participating states of SHRP2 Safety study and each supplied 8 years’ statewide crash data as part of the SHRP2 RID database. After cleaning and processing the data, 148 roundabouts were found to have enough before and after period crash history data, as well as additional information to allow for rigorous statistical analysis. The roundabouts were grouped single-lane and multi-lane categories, each category was further divided into five sub-categories based on prior traffic control type, and the crash modification factor of each subcategory was derived using the before and after crash analysis approach. The crash modification factors derived from this study for different roundabout types converted from different prior traffic controls are higher than equivalent values reported in previous studies, but show similar trends in safety impacts between single-lane and multi-lane roundabouts. Given the fact that nowadays, roundabouts are being implemented at more types of locations, and many of them not due to safety concern, it may not be a surprise to see higher crash modification factors reported in later studies. When reporting this multiplicative factor, it is important to properly document the intersections’ prior condition. Furthermore, how well a roundabout was designed and constructed do affect it safety outcome, and including poorly designed/built roundabouts in this type of study can also tip the scale.

Study Citation: Zhang, W., and Y. Wang. "Crash Modification Factors Based on Crash Histories of 148 Modern Roundabouts in Five States". 2017 TRB International Roundabout Conference, Green Bay, WI, May 2017.

CMFs Associated With This Study

Category: Intersection geometry

Countermeasure: Conversion of intersection into multi-lane roundabout

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
2.29-1291 StarAllAllNot specifiedNot specified
3.06-2061 StarAllAllNot specifiedNot specified
0.79211 StarAllAllNot specifiedNot specified
0.9911 StarAllAllNot specifiedNot specified
0.8201 StarAllAllNot specifiedNot specified

Countermeasure: Conversion of intersection into single-lane roundabout

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
1.15-151 StarAllAllNot specifiedNot specified
0.5501 StarAllAllNot specifiedNot specified
0.9731 StarAllAllNot specifiedNot specified
0.9731 StarAllAllNot specifiedNot specified
1.04-41 StarAllAllNot specifiedNot specified