Highway Safety Manual Transferability to Italian Low-Volume Roads

For decades, crashes have been studied as discrete events focusing on the circumstances of the crash. This type of analysis has been used to identify the characteristics of roadway features associated with higher crash experience, but other factors, such as traffic volumes, driver characteristics, land use, and environmental conditions are also needed to explain or describe crash events. The Highway Safety Manual (HSM) provides a predictive method to estimate the expected average crash frequency of a site in given geometric and geographic conditions over a specific period for a specific annual average daily traffic (AADT). The study presented here investigates whether the modeling results closely match the crash records. The HSM algorithms were used to assess transferability as a whole. The results obtained suggest that implementing the HSM techniques should foster the development of local Safety Performance Functions (SPFs) and accident modification factors (AMFs). The calibration preserves the original HSM model form and the relationship between independent variables and crashes. To adjust the base predicted crash frequency to meet the current conditions, the AMFs calculation for lane width, horizontal curve and vertical grade were made. Crash types (head-on/side collisions, single-vehicle crashes, rear end collisions) were investigated based on the vertical grade and the curvature indicator. The estimated model provides planners and designers with a tool better able to target and select countermeasures to address these specific aspects, resulting in improved project selection and improved safety.