Estimating Crash Modification Factors using Surrogate Measures of Safety

Author(s): Taha Saleem, Adrian Lorion

Slidedeck Presentation:

3A - Saleem


Crash Modification Factors (CMFs) are used to represent the effects of safety treatments on crashes and are usually obtained from observational studies based on reported crashes. Observational studies can be classified into before-after and cross-sectional studies. CMFs derived from before-after studies are based on the change in safety due to the implementation of a treatment, whereas, CMFs from cross-sectional studies are developed by comparing the safety of a group of sites with a feature with the safety of a group of sites without that feature. The results from these studies depend largely on the availability of crash data and collection of such data for a safety analysis requires real world “experimentation” at a large number of study sites and lengthy evaluation/observation periods due to the random and sparse nature of crashes. Traffic conflicts, on the other hand, can be used as a traffic safety surrogate (a quantifiable observation that can be used to replace or supplement crash records) for a less time-consuming measure to assess the safety effectiveness of a countermeasure. The main objective of this paper is to use traffic simulation to assess changes in traffic conflicts due to a change in road designs or features which would subsequently lead to the computation of Conflict Modification Factors (CfMFs). CMFs can then be estimated by using the crash-conflict relationship (i.e. the conflict based crash prediction models). For the purpose of this research a sample of intersections from the City of Toronto were used and a total of four treatment scenarios were considered. Treatment scenarios evaluated included installation of left turn lane on major road approaches at four leg signalized intersections, installation of left turn lane on major road approaches at three leg two way stop controlled intersections, installation of right turn lane on major road approaches at four leg signalized intersections, and changing left turn signal control from permissive to protected-permissive at four leg signalized intersections. All the scenarios were simulated in VISSIM using the updated designs and the most recent traffic volumes available. The results from VISSIM were then processed using SSAM to obtain the number of conflicts, and, subsequently the number of crashes in the updated design scenario using the crash-conflict relationship. The CMFs were then compared to the CMFs available from the Highway Safety Manual and the CMF Clearinghouse to assess the effectiveness of using simulation to derive CMFs for planned/future scenarios. The results are promising in that the proposed methodology yields CMFs that closely match CMFs obtained from observational studies.