Safety evaluation of four safety countermeasures on freeways in South Korea

We used data from seven freeways in South Korea to evaluate the safety impact of four safety countermeasures: (1) automated speed enforcement cameras, (2) chevron signs, (3) delineator posts, and (4) shoulder rumble strips. The evaluation was based on crash data from 2001 to 2008 and provided three years of before data and three years of after data for each countermeasure. We developed crash modification factors (CMFs) to show the safety impact of the countermeasures. We used the empirical Bayes before and after method to avoid regression-to-the-mean bias and take into account the impact of traffic volume changes in the before and after periods. We developed a safety performance function using a set of reference sites and a negative binomial regression technique. The CMFs for all crash data appeared that automated speed enforcement cameras and chevron signs decreased the frequency of crashes while delineator posts and shoulder rumble strips did not. The results were expanded by developing CMFs separately according to crash severity and crash type. The crash severity was divided into fatal and injury (FI) and property damage only (PDO) while the crash type was only focused on speeding-related crashes due to data limitations.