1. Home
  2. RIP
  3. View Record
https://www.nationalacademies.org/trb/events

Updated Calibration Factors for Highway Safety Manual Crash Prediction Models (2020-2023)

Traffic safety is the primary objective for transportation professionals. However, according to the traffic crash data released by the U.S. Department of Transportation’s National Highway Traffic Safety Administration, 39,007 lives were taken in traffic crashes in 2020, which represents a 6.8% increase in fatal crashes and a 21% increase in the fatality rate per 100 million vehicle miles traveled, respectively, over the previous year. The Highway Safety Manual (HSM), published by the American Association of State Highway and Transportation Officials (AASHTO) in 2010, provides a set of quantitative tools that help transportation practitioners make more informed decisions regarding highway safety performance. One of the most critical tools are Safety Performance Functions (SPFs), models that relate expected safety performance of individual facilities (measured in terms of expected annual crash frequency) with its specific features, such as traffic volume or geometric characteristics. The design-level SPFs included in the HSM were developed using data from a limited number of states. Since a variety of features, including traffic flow characteristics, crash reporting system, and climate vary significantly across different states, using the HSM SPFs or other national-level SPFs directly for any state will likely lead to biased predictions. For this reason, jurisdictions need to calibrate these SPFs before they can be applied for meaningful predictive analysis. This calibration is typically done using a calibration factor. The purpose of this study is to develop updated calibration factors and accompanying crash proportion tables for the SPFs in the HSM using the most recent data and develop calibration factors for newer SPFs that are now available via other national-level projects. The segment facility types that calibration factors will be generated for include the following: two-lane rural roads (Chapter 10 of HSM); rural multilane roads (Chapter 11 of HSM); urban and suburban arterials (2-5 lanes from Chapter 12 of HSM); urban and suburban arterials (6-8 lanes and one-way from the results of NCHRP (5) 17-58); and urban and rural freeways, and ramps (Chapters 18 and 19 of HSM). The intersection types that calibration factors will be generated for include the following: intersections on rural two-lane highways; intersections on rural multilane highways; intersections on urban and suburban arterials and one-way streets; ramp terminals (Chapter 19 of HSM); and roundabouts (results of NCHRP 17-70). SPF calibration factors will be generated by following the calibration procedure in the HSM using 2020 through 2023 data. The research team will identify study sites for inclusion in this effort by beginning with sites used in previous North Carolina SPF calibration efforts and then supplementing those locations to build a robust database meeting sample size requirements suggested by the HSM. Then, the research team will collect roadway geometric and operational data and gather and assemble crash data for each site. Finally, the research team will use the SPFs to predict crash frequency for each site and calculate statewide calibration factors and crash proportion tables, as well as regional calibrations for the Mountain, Piedmont, and Coastal regions. The outcomes of this project are anticipated to: (1) provide North Carolina Department of Transportation (NCDOT) the ability to generate North Carolina-specific predictions for all facility types; (2) incorporate economic analysis into decision-making by determining expected changes in safety performance among alternatives; (3) increased reliability in decision-making by using state-of-the-practice methods for evaluating expected safety performance; and (4) develop a broader dataset for North Carolina to serve as a base for future applications in calibration efforts.