Safety Performance Functions and Crash Modification Factors for Skid Resistance on Interstate and Non-Interstate Highways

Pavement friction is crucial for road safety, especially in adverse weather. This study investigates the relationship between pavement friction—measured as skid number (SN)—and crash frequency on Utah highways. Using data from 2016 to 2019 for I-15 (interstate) and US-89 (non-interstate), negative binomial models were estimated to establish safety performance functions (SPFs) and crash modification factors (CMFs). The models accounted for traffic volume, segment length, and roadway geometric characteristics, examining various crash types, including dry- and wet-weather, property damage only, and injury-related crashes. Results show a significant negative association between SN and crash frequency for all crash types on both highway types. Higher SN values (more friction) were linked to fewer crashes. A 10-unit increase in SN was associated with a 7% to 8% reduction in dry-weather crashes on both highway segments. For wet-weather crashes, the same increase in SN resulted in a 14% decrease on non-interstate and a 21% decrease on interstate highways. The impact of SN on reducing crashes was particularly strong on interstate highways during wet conditions, indicating that pavement friction is vital for safety in these scenarios. Additionally, the safety impact of skid resistance was greater on segments with more curved portions. These findings suggest that enhancing pavement friction through measures such as high friction surface treatments could significantly improve traffic safety. The results support the continued collection of skid data by transportation agencies to identify high-risk locations and prioritize friction improvement efforts to enhance roadway safety.