Citation

Carter D, Srinivasan R, Gross F, Himes S, Le T, Persaud B, Lyon C, Bonneson J. NCHRP Res Rep. 2021; (991): 1-454.

Copyright

(Copyright © 2021, National Cooperative Highway Research Program (NCHRP); National Academies Press)

DOI

10.17226/26408

PMID

unavailable

Abstract

Crash modification factors provide transportation professionals with the kind of quantitative information they need to make decisions on where best to invest limited safety funds. Resources such as the American Association of State Highway and Transportation Officials (AASHTO) Highway Safety Manual (HSM) and the Federal Highway Administration (FHWA) CMF Clearinghouse provide a rich source of CMFs, but there is a pressing need for guidelines on how best to apply CMFs and how to further develop quality CMFs for additional countermeasures. This project developed guideline on selecting and adjusting CMFs for sites at which key characteristics may be different, applying CMFs for multiple treatments in a single location, and developing future CMFs in functions that account for differences in key characteristics.

Calibration of Existing CMFs for Different Site Characteristics

The guidelines in this report describe a procedure for estimating the effect of a proposed treatment on a site of interest, presented in Appendix A. The procedure consists of a step-by-step approach using CMFs reported in the literature, matching on major site characteristics, and adjusting them to produce a final CMF to be used at the site of interest. The procedure produces aggregated CMFs that are locally calibrated using the crash distribution representative of the site of interest. The procedure consists of the following steps:
Step 1. Identify proposed treatment and subject site
Step 2. Identify characteristics of subject site
Step 3. Identify existing CMFs for proposed treatment
Step 4. Compare subject site characteristics to known influential factors on the CMF Step 5. Convert CMF to disaggregate CMFs
Step 6. Process disaggregate CMFs
Step 6a. Test for homogeneity
Step 6b. Combine CMFs
Step 6c. Select the one CMF that is a best match to the subject site characteristics OR develop CMFunction
Step 7. Develop aggregate CMF using crash distribution at subject site
Step 7a. Determine the local crash distribution
Step 7b. Calculate the aggregate CMF based on the local crash distribution

Language: en

Keywords

Transportation and Infrastructure