• Updated and Regional Calibration Factors for Highway Safety Manual Crash Prediction Models (2016-2019)

    NCDOT Research Project Number: 2020-27

 Executive Summary

  • One of the objectives of state agencies is to reduce the number and severity of crashes within the limits of available resources, science, technology, and legislatively mandated priorities. To achieve the greatest return on the investment of limited budgets, it is imperative that decisions are made based on the best information regarding the safety implications of various design alternatives and engineering treatments. The Highway Safety Manual (HSM), developed through funding from the American Association of State and Highway Transportation Officials (AASHTO) and the Federal Highway Administration (FHWA) provides analytical tools and techniques for quantifying the safety effects of decisions made in planning, design, operations, and maintenance.  

    To be able to use the advanced tools in the HSM, it is necessary for each jurisdiction to employ crash prediction models (also called safety performance functions, SPFs) that relate crash frequency and severity to roadway characteristics for different types of facilities. The HSM does not recommend using the SPFs directly from the HSM without calibration because the general level of crash frequencies may vary substantially from one jurisdiction to another for a variety of reasons including climate, driver populations, animal populations, accident reporting thresholds, and crash report system procedures. Four previous NCDOT projects (2009-06, 2009-07, 2010-09 and 2016-09) produced North Carolina-specific calibration factors for the prediction models from Part C of the HSM. 

    This effort aims to update these previous efforts as well as including new models which have not yet had calibration factors estimated. Results are shown in the following tables. Factors that are based on the HSM desired sample size of at least 100 observed crashes per year are indicated in bold italics. In these tables, 2U represent two-lane undivided roads, 4U represents four-lane undivided roads, 4D represents four-lane divided roads, 3T represents roads with two through lanes and a center TWLTL, and 5T represents roads with four through lanes with a center TWLTL. For freeways, MV F&I represents multiple-vehicle fatal and injury, SV F&I represents single-vehicle fatal and injury, MV PDO represents multiple-vehicle PDO, and SV PDO represents single-vehicle PDO. For intersections, 3ST represents 3-leg intersections with a stop sign on the minor leg, 4ST represents 4-leg intersections with a stop sign on the minor legs, 3SG represents 3-leg signalized intersections, and 4SG represents 4-leg signalized intersections.

Taha Saleem
Taha Saleem; Raghavan Srinivasan; Bo Lan
Carrie Simpson
Lisa E. Penny
UNC Chapel Hill - HSRC

 Report Period

  • August 1, 2019 - June 30, 2021


  • Complete


  • Traffic, Mobility, Safety and Roadway Design

 Sub Category

  • Traffic Forecasting

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