Research is complete. The final report is available HERE
The AASHTO Highway Safety Manual (HSM) 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 current edition of 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 was 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, 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 results of this research to be presented in such a way that they can easily be incorporated into (1) future editions of the HSM; (2) the AASHTO SafetyAnalyst software; and (3) the Interactive Highway Safety Design Model software.