The National Academies

NCHRP 17-70 [Final]

Development of Roundabout Crash Prediction Models and Methods

  Project Data
Funds: $600,000
Research Agency: Kittelson and Associates
Principal Investigator: Erin Ferguson
Effective Date: 7/14/2014
Completion Date: 2/28/2018

STATUS:  Research is complete. The final report and additional resources are available HERE.

NCHRP Report 888 provides crash prediction models that quantify the expected safety performance of roundabouts for motorized and non-motorized road users. The report will be of interest to highway design engineers, highway safety analysts and researchers, and transportation planners, as well as those responsible for maintaining highway safety guidance, including the Highway Safety Manual (HSM).

Safety performance factors (SPFs) and crash modification factors (CMFs) are predictive models that estimate expected crash frequencies. Transportation professionals use these models to identify locations where crash rates are higher than expected, to estimate safety benefits of a proposed project, and to compare the safety benefits of design alternatives. The models help identify and prioritize locations for safety improvements, compare project alternatives by their expected safety benefits, and guide detailed design decisions to optimize safety.

Research indicates that roundabouts provide substantial reductions in crashes, especially crashes that result in serious injuries or fatalities. However, the 1st edition of the HSM did not include a crash prediction method for roundabouts; therefore, practitioners were not able to quantitatively assess the crash reduction benefits of providing a roundabout at a specific intersection or to investigate the safety effects of complex design decisions at single- lane and multilane roundabouts.

NCHRP Report 888 provides practitioners with SPFs and CMFs for roundabouts. The models include variables for geometric features and operational characteristics, as well as their expected effects on crash rates for motorized and nonmotorized users. The models can be used for planning-level analysis, and, as a project develops and more detailed information is available, for analysis at the intersection level and leg level of an intersection. The research was conducted by Kittelson and Associates, with support from Persaud and Lyon, Inc., and Write Rhetoric. NCHRP Report 888 is accompanied by online resources, including recommended revisions for the 2nd edition of the HSM, detailed information on the data sources and data pre-processing, and a downloadable slide presentation summarizing the results of the research. These materials are available on the TRB website.

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