Evaluating the Safety Effectiveness of Advance Downgrade Warning Signs in Preventing Downgrade Truck Crashes using a Propensity Scores Framework

Abstract

Truck crashes on steep downgrades exact devastating tolls on lives and property. An important intervention to reduce the frequency of truck crashes on downgrades has been to present quality information to drivers about upcoming downgrades through warning signs. The use of advance warning signs on downgrades to prevent truck crashes is the current state of practice for most highway agencies; thus, it is critical to assess the safety effectiveness of such warning signs. Though several studies have evaluated the efficacies of warning signs, the safety effectiveness of advance downgrade signs is less clear. This study uses a propensity scores framework to assess the effectiveness of current downgrade warning signs by matching treated and untreated downgrade segments in Wyoming. The advantage of propensity score matching over other traditional analysis techniques is that it reduces bias in safety evaluation by mimicking randomization and accounts for confounding factors. Using the risk ratio computed from the matched data, the study found that advance downgrade warning signs are effective in reducing downgrade truck crashes. The results indicate that truck crash risks on downgrades without advance warning signs are an estimated 15% higher than downgrades with advance downgrade signs. The 90% bootstrap confidence interval for the risk ratio was found to be from 1.04 to 1.53.