Developing Combined Crash Modification Factors: Challenges,Lessons Learned,and Recommendations

Combining crash modification factors (CMFs) often involves merging multiple CMFs to establish a unified CMF for either the same safety treatment or the overall effect of implementing multiple safety treatments simultaneously. This paper reports the combined CMF results for five safety countermeasures: change signal phasing, convert intersection to roundabout, install bicycle lane, change shoulder width, and change posted speed. More importantly, this paper discusses four challenges from the effort to create combined CMFs from the CMFs available in the CMF Clearinghouse that are related to CMF applicability, CMFs of different magnitudes with some showing a safety improvement and some showing non-improvement, the need of information beyond the CMF Clearinghouse, and issues with multiple CMFs from the same study. Based on the lessons learned, the paper also provides suggestions to mitigate these challenges in future research efforts to create combined CMFs, as well as recommendations to researchers who develop CMFs so that key information is reported and made available to facilitate similar work in the future.