Highway Safety Manual Safety Performance Functions & Roadway Calibration Factors: Roadway Segments

US Transportation Collection2020PDFTech ReportKhattak, AsadAhmad, NumanMohammadnazar, AminMahdiNia, ImanWali, BehramArvin, RaminUniversity of Tennessee, Knoxville. Center for Transportation ResearchTennessee. Department of TransportationUnited States. Department of Transportation. Federal Highway AdministrationTennessee. Department of TransportationUnited StatesTennesseeHighway safetyManualsCountermeasuresCalibrationSafety analysisHighway safety manualCalibration factorTennessee department of transportationRES 2016-27Final Report: October 1, 2015 - To: September 30, 2019To enhance safety, the Tennessee Department of Transportation (TDOT) is in the process of adopting the Highway Safety Manual (HSM) as a resource to facilitate decision making based on the safety performance of its roadways. The predictive models which are known as Safety Performance Functions (SPFs) are used to forecast the expected crash frequency for various roadway facility types. The HSM (2010) recommends transportation agencies such as TDOT either to develop their own SPFs using local data or develop calibration factors for use with the HSM default SPFs to reflect local conditions. This is because the HSM default SPFs were developed using data from a subset of states. Geographical conditions in Tennessee may differ substantially from the factors used to develop the default SPFs in the HSM such as terrain, weather, animal populations, driver populations, crash reporting thresholds, and crash reporting practices. Therefore, this study undertakes the task of developing 1) Tennessee-specific calibration factors, and 2) estimating Tennessee-specific SPFs. The calibration factors and locally estimated SPFs presented in this report are ready for implementation in Tennessee. Part 1 of the report focuses on rural multilane segments and urban/suburban arterial segments (Part 2 focuses on relevant intersections). Given the availability of relevant data in E-TRIMS, TDOT is in a good position to adopt the HSM procedures and benefit from software applications that make it easier to use the HSM. In this regard, the AASHTO Safety Analyst tool is discussed in detail. An example demonstrates how the Safety Analyst can use calibration factors and locally calibrated SPFs to make predictions of crashes with and without countermeasures. At the end of the report, recommendations are provided for advancing safety analysis in Tennessee.137