Identifying the Threshold Discrepancy of Rear-End Conflicts under Clear and Rainy Weather Conditions Using Trajectory Data

Surrogate safety measures (SSMs) have drawn considerable attention in the field of traffic safety. Previous studies have characterized SSMs based on road geometries and conflict types, exploring various SSM thresholds. Adverse weather conditions have been examined for their impact on safety events estimated from surrogate metrics, however, limited research focuses on discrepancies in SSM thresholds under various weather characteristics. This study investigated specific thresholds for modified time to collision (MTTC), deceleration rate to avoid a crash (DRAC), and single-step probabilistic driving risk field (S-PDRF) under clear and light rainy weather conditions. A total of 1,048 rear-end events in the expressway (724 clear and 324 rainy) were obtained from the CitySim dataset. A statistical test was conducted to examine the significance of three indicators under two weather scenarios. Subsequently, the peak over threshold (POT) and bimodal histogram threshold methods were employed to recommend appropriate thresholds. The results indicate that thresholds of MTTC and DRAC metrics have statistically significant differences under two weather scenarios. Specifically, the recommended MTTC thresholds were 4 s for rainy conditions and 2.3 s for clear conditions. Similarly, the DRAC thresholds were 3.2 m/s 2 for clear weather and 2.4 m/s 2 for rainy weather. Conversely, the threshold of S-PDRF was statistically insignificant between two weather conditions, suggesting a single threshold is appropriate for this measure regardless of weather. These findings can inform the design of active safety systems and traffic safety policies by suggesting different safety thresholds for triggering safety systems under clear and rainy conditions, thereby reducing false alarms.