Evaluating Safety Treatments with Crash Modification Function: Case Study on Combined Safety Treatments for Rural Two-Lane Highways

A safety treatment of shoulder rumble strips, shoulder widening [from 0 to 0.6 m (2 ft)] and pavement resurfacing (RSR) was deployed to over 644 centerline km (400 mi) of rural two-lane highways in Missouri. The effectiveness of the RSR treatment was evaluated with the empirical Bayesian before/after study method based on the roadway characteristics, traffic and crash data collected. Crash modification factors (CMF) for 20 categories of crash severity and/or collision type were calculated. Generally, the RSR treatment is effective for improving the safety performance of rural two-lane roads, especially for fatal and injury crashes. The RSR treatment is specifically effective for run-off road and head-on crashes. A cumulative residual (CURE) analysis reveals that fixed-value CMFs for all annual average daily traffics (AADT) range are likely to result in biased estimation on the RSR’s safety effectiveness. The crash modification function (CMFunction) was introduced in this study and a sample size of 32 was selected to develop the CMFunction based on the sensitivity analysis results. CMFunctions were developed for two collision types of different crash severities. The CURE plot indicates that crashes predicted with CMFunctions are more accurate than those with CMFs and therefore CMFunctions are more reliable than fixed-value CMFs for evaluating the effectiveness of safety treatments.

The roadway departure crashes, in which the vehicles left the roadways and hit trees or other fixed objects on roadway sides, can cause fatalities or serious injuries and is a big safety concern for rural highways. Many state and local agencies deployed various safety treatments to eliminate the roadway departure crashes or alleviate the crash severities. The Missouri Department of Transportation installed shoulder rumble strips together with shoulder widening and pavement resurfacing on rural two-lane highways to reduce the roadway departure crashes. Results from statistical analysis indicate that totally it will reduce 9% of roadway departure crashes; however, the treatment can be more effective for high-volume roadways than for low-volume roadways (specifically, roadways with AADT less than 2,000). To provide more reliable results, the authors developed a series of function to estimate how many crashes can be reduced with this safety treatments. State and local agencies can use the tool to prioritize roadway segments for safety improvements and quantify their safety benefits.