Abstract Manager

MnDOT engaged HDR to conduct a comprehensive roundabout (RAB) study to evaluate safety performance at RAB intersections with consideration to different design configurations. Research comparing RAB’s safety performance to traditional intersections such as all-way stops, signalized or thru-stops have consistently shown (OTE, 2017) reductions in total fatal and serious injury (KA) collisions but an increase in property damage (PDO) collisions. However, only a limited set of research exists that addresses how different geometric, or signage features unique to RABs influence safety performance relative to RABs without these features (NASEM, 2019; FHWA, 2000; MSU, 2023). With over 500 installed RABs on Minnesota’s city, county, and state roads, and significant historical record of collisions and AADT at each facility, MnDOT is in a good position to quantitatively evaluate the impact of design features on collisions by severity by means of crash prediction models, and the calculation of crash modification factors (CMF).

To date, HDR has developed CMFs for key RAB features including number of circulating lanes, approach counts, approach speed limits, inscribed circle diameter and features such as the teardrop or mini-roundabout design. We have identified which evaluated design features reduced crashes, increased crashes and which have had no notable impact.

These CMFs can aid traffic safety planners assessing the safety impacts and hence benefits of their proposed RABs, especially if outside the norm of RAB designs. In addition, the CMFs can aid municipalities to make informed decisions as to whether to invest in RAB intersections and if so, what design is appropriate for their level of traffic.

The presentation will discuss the methods to calibrate the results from Minnesota to Canadian provinces according to the Highway Safety Manual (HSM) guidance. Given Minnesota’s climate and population density are comparable to those of many Canadian regions, the application of Minnesota’s RAB-feature CMFs is reasonably transferable, provided that HSM protocols are followed and appropriate calibration factors are applied.

 Citations:

 Federal Highway Administration (2000). Ch. 5: Safety. In Roundabouts: An Informational Guide (pp. 103-126). McLean, Virginia: U.S. Department of Transportation.

 Office of Traffic Engineering (2017). A Study of the Traffic Safety at Roundabouts in Minnesota. Minnesota Department of Transportation, St. Paul

 Michigan State University (2023). Evaluating the Performance and Safety Effectiveness of Roundabouts - An Update. Lansing, MI: Michigan Department of Transportation.

National Academies of Sciences, Engineering, and Medicine (2019). Development of Roundabout Crash Prediction Models and Methods. Washington, DC: The National Academies Press.