Crash Modification Factors in the Highway Safety Manual: A Review (2023)

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4 Introduction 1.1 Background Highway safety practitioners were given a significant new tool in 2010 with the publication of the AASHTO Highway Safety Manual (HSM) (AASHTO 2010). In Part D of the HSM, crash modi­ fication factors (CMFs) are provided to estimate the safety effects for a variety of treatments or countermeasures. Some of these CMFs are also presented in Part C to complement the predictive methodology in assessing the safety effects of design decisions. By the time the second edition of the HSM (HSM2) is published, many more important CMFs will have been developed, and additional CMFs will be needed to support enhancements to the predictive methodologies—for example, to better predict crashes by type and severity. It is critical that the next edition of the HSM incorporate these new CMFs to fill voids and continue to push forward this significant tool. The original objectives of this research were to • Assess the current criteria and existing process for evaluating the quality of CMFs and identi­ fying CMFs for appropriate use with the HSM; • Develop proposed revisions to the criteria and process, including how existing and new CMFs may be incorporated in the HSM, and provide guidance for practitioner application of the revised process; • Apply the evaluation criteria to identify and assess CMFs and develop a list of appropriate CMFs for the HSM; and • Conduct a CMF gap analysis to identify treatments for which quality CMFs are needed. Subsequently, the scope of the research was expanded to include the following additional activities: • Review and rate the complete list of CMFs that could not be reviewed with the funds allocated initially; • Estimate crash modification functions (CMFunctions) for select treatments for which detailed data on site characteristics were available from the original studies; • Estimate adjustment factors for safety performance functions (SPFs) that were estimated in NCHRP 17­62, “Improved Prediction Models for Crash Types and Severities” (Ivan et al. 2021); • Calibrate the prediction models planned for inclusion in the HSM2 (also called single state calibration); • Validate the combination of SPFs estimated in NCHRP 17­62 (Ivan et al. 2021) with the SPF adjustment factors estimated in this effort; and • Develop CMFs for roadside crashes, building on results from NCHRP 17­54, “Consideration of Roadside Features in the Highway Safety Manual” (Carrigan and Ray 2018). C H A P T E R 1

Introduction 5 1.2 Structure of Final Report The remaining chapters of this report are as follows: • Chapter 2: Review of CMF Inclusion Criteria and Star Rating System in the CMF Clearing- house. One of the initial activities of this project was to conduct a critical review of the inclusion criteria used for CMFs in the HSM and a review of the star rating system used by the CMF Clearinghouse to evaluate CMF quality. The findings from this review were used to guide the development of a recommended procedure for (1) evaluating the quality of CMFs and (2) determining which CMFs are appropriate for use with the HSM. These activities were conducted simultaneously, and a report was developed to document the findings. Chapter 2 summarizes these findings. Further information is provided in Appendices A and A.1. • Chapter 3: Determination of User Preferences and Practices. The preferences and practices of the CMF user community constitute another key consideration. CMF users include safety and design engineers, planners, researchers, academics, and other professionals at the national, state, and local levels. It is critical to consider how they currently use CMFs and how best to present CMF information in the HSM2. This effort involved a questionnaire that was distrib­ uted electronically late in 2015 and a focus group of experienced users that was conducted in February 2016. Chapter 3 summarizes the findings, and additional detail is given in Appendix B. • Chapter 4: Recommendations for Incorporating CMFs in the HSM and Possible Framework for Practitioner Use. Chapter 4 discusses how a new rating system for CMFs was developed. It also includes recommendations for how CMFs may be incorporated in the HSM and a proposed framework applicable for practitioner use. Appendices C, D, D.1, and D.2 provide further details. • Chapter 5: Development of CMFunctions for Select Treatments. It is difficult to think of any CMF that could categorically be said to be constant from application to application. There are numerous examples in the recent and not­so­recent literature that suggest that CMFs do vary by application circumstance, such as traffic volume and roadway curvature. Chapter 5 provides an overview of the methodologies that were used to estimate CMFunctions for four treatments and a summary of the findings. Appendices E through H provide the details of the estimation of the CMFunctions. • Chapter 6: Identification of Adjustment Factors for SPFs Estimated in NCHRP Project 17-62. NCHRP 17­62, “Improved Prediction Models for Crash Types and Crash Severities,” esti­ mated base SPFs for segments and intersections on rural two­lane roads, rural multilane roads, and urban and suburban arterials (Ivan et al. 2021). NCHRP 17­62 did not estimate CMFs (also called SPF adjustment factors) for these base SPFs. NCHRP 17­72 was tasked with iden­ tifying CMFs from the CMF Clearinghouse that could be used as adjustment factors with the base SPFs estimated in NCHRP 17­62. Chapter 6 provides an overview of the approach used to identify the adjustment factors, which are documented in Appendices I and J. • Chapter 7: Calibration of Prediction Models for Inclusion in the HSM2. Before the pre­ diction models were included in Part C of the HSM, Highway Safety Information System (HSIS) data from California were used to calibrate the models predicting intersection crashes, and data from Washington State were used to calibrate the models predicting segment crashes (R. Srinivasan, F. Council, and D. Harkey, “Calibration Factors for HSM Part C Predictive Models,” submitted to HSM Task Force in 2008). It was argued that these recalibrated models could be used by the end user to directly compare the expected safety performance of different facility types. NCHRP 17­72 was tasked with recalibrating models that would be included in the HSM2. This exercise is also referred to as “common state calibration” or “single state calibration.” Chapter 7 provides the list of facility types that were included in the calibration and some insights into the approach that was used. The calibration factors can be found in Appendices K, L, and M.

6 Crash Modification Factors in the Highway Safety Manual: A Review • Chapter 8: Validation of SPF Adjustment Factors. NCHRP 17­62 developed crash­type and severity­specific SPFs for inclusion in Part C of the HSM2. These SPFs were developed to replace the crash prediction models (CPMs) in the HSM. They were estimated for specific base conditions, but NCHRP 17­62 did not develop CMFs for these SPFs. As discussed earlier, NCHRP 17­72 was tasked with developing CMFs from the high­quality CMFs documented in the FHWA CMF Clearinghouse. Because the CMFs and the SPFs were estimated indepen­ dently, these CMFs need to be validated for the SPFs. Chapter 8 provides an overview of the approaches that were used for validation and some initial results. • Chapter 9: Development of CMFs for Roadside Crashes on Rural Two-Lane Two-Way Roads. NCHRP 17­72 was tasked with developing a procedure for estimating a CMF for roadside design. Chapter 9 provides a brief overview of the development of the procedure for esti­ mating the CMF. A detailed description of the development of the CMF is provided in Appendix N, and the procedure for implementation is provided in Appendix O. • Chapter 10: CMF Gap Analysis. The objective of the CMF gap analysis was to identify the research needed to guide future CMF development efforts. Chapter 10 describes the approach that was used to identify the CMF gaps. • Chapter 11: Development of Guidance on the Selection, Application, and Development of CMFs. There is a need for explicit guidance on the proper selection and application of the CMFs to be included in Part D of the HSM. The intent is for this guidance to provide an over­ view of CMF selection, application, and development, with a focus on countermeasure­specific CMFs that are a key part of the processes and methods described in Part B of the HSM. Chap­ ter 11 provides a brief outline of the guidance document that was developed in NCHRP 17­72.