Exposure-Adjusted Crash Rate Analysis Using Lighting Condition and Time-of-Day

Abstract

Accurate roadway crash rate estimation requires accounting for variations in traffic exposure across time and lighting conditions. Standard crash rate formulations, including those recommended by the Federal Highway Administration, rely on average annual daily traffic (AADT) and do not distinguish traffic volumes by time-of-day. This study develops an exposure-adjusted crash rate framework that incorporates lighting condition-specific traffic volumes by incorporating time-of-day into crash rate calculation to improve roadway safety assessment. The City of Saint Paul, MN, is used as a case study with its roadway network data, 5 years of geocoded crash records (2020–2024), and traffic volume data. Crashes were classified into daylight, sunrise, sunset, and dark conditions. Lighting condition time ranges were defined using hourly crash distributions and highest density interval methods. Hourly traffic volume distributions derived from Minnesota Department of Transportation Automatic Traffic Recorder data were used to allocate AADT into lighting-specific exposure estimates. Results indicate that exposure-adjusted crash rates are higher and more variable during dark hours, while daylight crash rates remain relatively stable throughout the day. These findings underscore the importance of incorporating time-of-day exposure into crash rate analysis and provide a foundation for future work focused on reducing nighttime crash rate variability through improved and adaptive roadway lighting strategies.