Evaluating Safety Effectiveness of Truck Climbing Lanes using Cross-Sectional Analysis and Propensity Score Models

Abstract

The State of Wyoming is characterized by heavy truck traffic flow, especially along Interstate 80 (I-80). A large portion of I-80 in Wyoming goes through mountainous and rolling terrain, resulting in significant vertical grades. About 9% of I-80 in each direction is within vertical grades of more than 3%, with certain sections reaching grades of close to 7%. Currently, there are 14 miles of climbing lanes in both directions. This study investigates the effects of climbing lanes on traffic safety using sections of I-80 in Wyoming. Cross-sectional analysis and propensity score methods were applied to evaluate the safety effectiveness and calibrate the Crash Modification Factor (CMF) and Relative Risk (RR) for climbing lanes. Data were collected from different sources and Wyoming-specific safety performance functions were developed using crash data from 2008 to 2016 for total crashes and truck-related crashes. All the segments were selected from I-80 in Wyoming with climbing lanes as treatment sites, and segments with similar geometrical characteristics without climbing lanes as comparison sites. Aggregated data were used to develop Negative Binomial and Zero-Inflated Negative Binomial models for performing cross-sectional analysis as they were found to fit better for the crash data. On the other hand, panel count data were used to conduct a propensity scores-potential outcomes framework. The CMFs and RR for climbing lanes from both analyses were found to be effective in reducing total and truck-related crashes. This is a first study that develops CMFs for climbing lanes in Wyoming.