Traffic safety remains the foremost objective of transportation professionals, yet roadway fatalities continue to present a major public safety challenge. The Highway Safety Manual (HSM) provides a critical framework for quantitatively evaluating highway safety performance, with Safety Performance Functions (SPFs) serving as one of its central analytical tools. SPFs relate predicted crash frequency to roadway, traffic, and geometric characteristics, allowing practitioners to estimate the safety performance of specific facility types and assess the effects of design and operational alternatives. Because the design-level SPFs in the HSM were developed using data from a limited number of states, direct application in North Carolina may produce biased results due to differences in traffic conditions, roadway characteristics, crash reporting practices, and other state-specific factors.
This project developed updated North Carolina-specific calibration factors and accompanying crash proportion tables for HSM SPFs using 2020 through 2023 data. The study addressed a comprehensive set of roadway segment and intersection facility types, including rural two-lane roads, rural multilane highways, urban and suburban arterials, freeways and ramps, ramp terminals, and roundabouts, as well as newer SPF forms introduced through national research efforts. Following HSM calibration procedures, the research team identified and verified study sites from previous North Carolina calibration efforts, supplemented these locations to satisfy recommended sample size requirements, and assembled the geometric, operational, and crash data needed for calibration. The resulting analyses produced both statewide calibration factors and regional calibration factors for the Mountain, Piedmont, and Coastal regions, where applicable.
