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

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27   Identification of Adjustment Factors for SPFs Estimated in NCHRP Project 17-62 6.1 Background NCHRP 17­62, “Improved Prediction Models for Crash Types and Crash Severities,” estimated 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 identifying CMFs from the CMF Clearinghouse that can be used as adjustment factors with the base SPFs esti­ mated in NCHRP 17­62. 6.2 Approach and Outcomes The process used to identify the CMFs was based on (1) the identification of segment and intersection SPFs developed in NCHRP 17­62 and (2) a review of the CMFs in the clearinghouse on an SPF­by­SPF basis to identify those CMFs that match the SPF base conditions. In the case of intersections, with a couple of exceptions, the base conditions included in the signal SPFs of the HSM were extended to the NCHRP 17­62 SPFs. The exceptions were (1) a change in the base condition for lighting presence at signalized intersections from “lighting not present” to “lighting is present” and (2) removal of the “left­turn phase not present” base condition for the Chapter 12 signal SPFs. In general, the CMFs were required to have a quality rating that met or exceeded one or both of the following threshold values: • Legacy star rating in the CMF Clearinghouse of 3, 4, or 5 stars and • NCHRP 17­72 CMF rating of ≥100. In most cases, the recommended CMFs were based on CMFs from one study. In a few cases, when two more CMFs were identified as applicable to a specific NCHRP 17­62 SPF and associ­ ated base condition, they were tested for suitability for combination and, if suitable, combined to produce an overall average CMF. This test and combination used the procedure described in Appendix A of the final report for NCHRP Project 17­63 (Carter et al. 2022). For segments, CMFs could be identified for the following roadway elements: • Modify shoulder width on rural two­lane undivided segments; • Modify right shoulder width of rural nonfreeway four­lane divided or undivided segments; • Install snowplowable, permanent raised pavement markers on rural two­lane undivided segments; • Change median width on rural and urban four­lane nonfreeway divided segments; • Change vertical grade of road on rural two­lane undivided segments; C H A P T E R 6

28 Crash Modification Factors in the Highway Safety Manual: A Review • Install passing or climbing lane on rural two­lane segments; • Install a short four­lane section on rural two­lane segments; • Modify lane width on rural two­lane undivided segments; • Modify horizontal curvature on rural two­lane undivided segments; • Modify driveway density on rural two­lane undivided segments; • Improve roadside on rural two­lane undivided segments; • Install TWLTLs on rural two­lane undivided segments; and • Install shoulder or centerline rumble strips on rural or urban two­lane undivided segments. For intersections, CMFs could be identified for the following intersection design elements or traffic control features: • Left­turn bay, • Right­turn bay, • Skew angle, • Enforcement of red­light­running cameras, • Prohibition of right turn on red, and • Intersection lighting. Appendix I provides the recommended CMFs for roadway segments, and Appendix J provides the recommended CMFs for intersections. For each treatment, the following information is provided: • Description of the treatment, • Description of the application circumstances to which the CMF applies (i.e., area type, number of lanes, median type, treatment location, intersection vs. nonintersection), • Details on the study that provides the CMF and the CMF values and formulas, • Recommended CMF for each crash/severity type in the NCHRP 17­62 SPFs to which the CMF may apply, • Comments and assumptions related to applying the CMF or a description of how the recom­ mended CMF was derived in order to apply to the NCHRP 17­62 SPFs, and • A reference to the original study.