Development of Roundabout Crash Prediction Models and Methods

The AASHTO Highway Safety Manual (HSM) currently includes safety performance functions (SPFs)—predictive models—for traditional intersection forms (e.g., four-legged, signalized intersections) found on rural two-lane roads, rural multilane highways, and urban/suburban arterials. These prediction models allow transportation professionals to assess the benefits of providing turn lanes, modifying intersection phasing, adding intersection lighting, and other such design decisions. The HSM does not currently include a crash prediction method for roundabouts; therefore, practitioners are 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. Research consistently indicates that roundabouts generally provide substantial reductions in crashes—and specifically reductions in serious injuries and fatalities—but critical attributes that make some roundabouts more successful than others at reducing crashes is not understood. While crash modification factors (CMFs) for roundabouts have been developed, these CMFs do not reflect the safety effects of site-specific roundabout design features. The predictive methods in HSM Chapter 10 (Predictive Method for Rural Two-Lane, Two-Way Roads) or Chapter 11 (Predictive Method for Rural Multilane Highways) do not address roundabouts at all; predictive methods are needed for roundabouts on these roadway types. The interim procedure for roundabouts based on CMFs, in HSM Chapter 12 (Predictive Method for Urban and Suburban Arterials) should be replaced with a full predictive method.  Developing crash prediction models that enable practitioners to understand and compare key design trade-offs will result in better informed decisions and higher returns on safety investments. Potential users of this research include state and local agencies responsible for safety management, and for planning and implementing more effective safety programs and projects. The objective of this research is to develop SPFs and CMFs for all road users (including non-motorized users) that can be used to estimate the severity and number of crashes likely to occur at roundabouts (a roundabout is defined in NCHRP Report 672: Roundabouts: An Informational Guide – Second Edition), under a variety of rural and urban contexts for single-lane and multilane roundabouts. The results of this research should enable transportation agencies to quantify the safety effects of roundabout design features and the safety performance of implementing a roundabout at a particular location so that safety performance can be compared to alternative intersection configurations for the same location. The research should answer the following questions at a minimum: (1) How do geometric features—and combinations of features—influence the number and severity of crashes at the roundabout? (2) How do operational features—and combinations of features—influence the number and severity of crashes at the roundabout? (3) How do driver learning curves influence the number and severity of crashes at any age roundabout? The results of this research should be presented in such a way that they can easily be incorporated into (1) a future edition of the HSM; (2) the AASHTO SafetyAnalyst software; and (3) the Interactive Highway Safety Design Model software. Research is complete. Final Report anticipated summer 2018.


  • English


  • Status: Completed
  • Funding: $600000
  • Contract Numbers:

    Project 17-70

  • Sponsor Organizations:

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590

    American Association of State Highway & Transportation Officials (AASHTO)

    444 North Capitol Street, NW, Suite 225
    Washington, DC  United States  20001

    National Cooperative Highway Research Program

    Transportation Research Board
    500 Fifth Street, NW
    Washington, DC  United States  20001
  • Project Managers:

    Hartell, Ann

  • Performing Organizations:

    Kittelson and Associates

    1818 Library Street, Suite 500
    Reston, VA  United States  20190
  • Principal Investigators:

    Ferguson, Erin

  • Start Date: 20140714
  • Expected Completion Date: 20180228
  • Actual Completion Date: 20180228
  • Source Data: RiP Project 37698

Subject/Index Terms

Filing Info

  • Accession Number: 01545247
  • Record Type: Research project
  • Source Agency: Transportation Research Board
  • Contract Numbers: Project 17-70
  • Files: TRB, RiP
  • Created Date: Nov 26 2014 1:00AM