The estimation of crash modification factors of Color Guidance Markings near exit ramps in motorways using the Comparison Group method

OBJECTIVE: Exit ramps are the most crash-prone locations on motorways due to frequent conflicts between exiting vehicles and through traffic, often resulting in rear-end and angle crashes. In South Korea, Color Guidance Markings (CGMs) were introduced as an innovative countermeasure to mitigate such conflicts by visually guiding drivers to appropriate exit ramps in advance. This study estimates crash modification factors of CGMs on Korean motorways. METHOD: This study estimates crash modification factors of CGMs using a robust analytical approach. The Comparison Group method was applied to before-and-after crash data from 20 treatment and 87 comparison sites on Korean motorways. In addition, Regression-To-The-Mean (RTM) violations were evaluated using an established RTM assessment method to determine whether the observed crash reductions exceeded the expected RTM effects. RESULTS: The results show that CGMs installed at exit ramps with low traffic volumes of less than 20,000 vehicles per lane per day reduced crashes by 47.5% and fatalities and serious injuries by 74.4%. However, reductions were lower, at 31.0% and 54.7%, respectively, at exit ramps with higher traffic volumes exceeding 20,000 vehicles per lane per day. The analysis showed that observed crash reductions exceeded expected RTM effects at up to 75% of treatment sites. CONCLUSIONS: These findings indicate that CGMs effectively reduce the likelihood and severity of crashes at exit ramps. The extensive implementation of CGMs is plausible for reducing crashes and crash severity on Korean motorways. However, because the Comparison Group method cannot inherently account for RTM effects and may over- or underestimate treatment effects, the results need to be confirmed using the Empirical Bayes (EB) method. The application of the EB method requires developing safety performance functions for Korean motorways, which should be established in future research.