Guidelines for the Development and Application of Crash Modification Factors (2022)

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1   Crash modification factors (CMFs) 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 Administra- tion (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 guidelines 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 charac- teristics, and adjusting them to produce a final CMF to be used at the site of interest. The procedure produces aggregate 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 the proposed treatment and subject site • Step 2. Identify characteristics of the subject site • Step 3. Identify existing CMFs for the proposed treatment • Step 4. Compare the subject site characteristics to factors known to influence the CMF • Step 5. Convert the CMF to disaggregate CMFs • Step 6. Process the disaggregate CMFs – Step 6a. Test for homogeneity – Step 6b. Combine CMFs – Step 6c. Select the one CMF that is the best match to the subject site characteristics or develop a crash modification function (CMFunction) • Step 7. Develop an aggregate CMF using the crash distribution at the subject site – Step 7a. Determine the local crash distribution – Step 7b. Calculate the aggregate CMF based on the local crash distribution S U M M A R Y Guidelines for the Development and Application of Crash Modification Factors

2 Guidelines for the Development and Application of Crash Modification Factors Quantifying the Effect of Multiple Treatments at a Single Location Often a transportation agency will apply multiple treatments to a location, such as sig- nalizing a stop-controlled intersection, adding turn lanes, and reducing intersection skew. The preferred approach to estimate the combined safety effect of multiple treatments is to apply a single CMF that represents the combined treatments. In the absence of such a CMF, transportation professionals need to know how to apply CMFs that represent individual treatment effects to estimate the combined safety effect. Before this project, the “rule of thumb” was to multiply the CMF values to arrive at the single combined effect. However, this sim- plified approach did not reflect certain major issues, such as how much interaction there is between the treatments in question. These guidelines present a recommended method to estimate the combined safety effect of two treatments at the same location, presented in Appendix B. The method hinges on the determination of how much overlap there is between the treatments. The steps of this method are: • Step 1. Determine the potential overlap of the individual treatment effects • Step 2. Determine the magnitude of the individual treatment effects • Step 3. Define the applicability of individual CMFs (i.e., what crash types and severities the individual CMFs apply to) • Step 4. Select the appropriate method for calculating a combined value based on whether the CMFs address the same crash type and severity or different crash types and severities Developing CMFunctions that Identify Key Influential Site Characteristics While single value estimates of CMFs are prevalent in current practice, it is widely recog- nized that single values cannot account for other factors that may change the effectiveness of the treatment. This research provides guidelines for researchers to develop CMFunctions that identify key influential site characteristics and account for various biases that may arise. These guidelines are presented in Appendix C. The guidelines on CMFunction development address how to develop CMFunctions either from cross-sectional regression models or from a set of CMF point estimates. The guidelines are accompanied by four case studies that demonstrate the development of CMFunctions in various situations, from cross-sectional data on curve radius to CMFs on applying center- line and shoulder rumble strips. The guidelines also provide recommendations for design- ing future evaluation studies that are configured to produce the most accurate estimate of countermeasure effectiveness. Appendixes to the Report Appendix A—Procedure for Estimating the Effect of a Proposed Treatment at a Subject Site This appendix provides the recommended procedure for selecting and adjusting a crash modification factor (CMF) to be used at a site of interest. It is structured in a step-by-step manner to guide the user through the process of identifying one or more appropriate CMFs for a treatment of interest, combining similar CMFs if needed, and adjusting the CMF based on crash distribution differences between the site of interest and the sites used to develop the CMF. These are the prime guidelines from Task 2 of this project.

Summary 3   Appendix B—Procedure for Estimating the Combined Safety Effect of Two Treatments These guidelines present a recommended method to estimate the combined safety effect of two treatments at the same location. It is structured in a step-by-step method to guide the user through the process of determining the potential overlap of the effects of the treatments, the magnitudes of the individual treatments, and the method for calculating the magnitude of the combined effect. These are the prime guidelines from Task 3 of this project. Appendix C—Guidelines for Developing Crash Modification Functions This appendix provides guidelines on crash modification function (CMFunction) devel- opment. It addresses how to develop CMFunctions either from cross-sectional regression models or from a set of CMF point estimates. The guidelines are illustrated through four case studies that demonstrate how to develop CMFunctions from individual CMFs and from cross-sectional regression analysis. These are the prime guidelines from Task 4 of this project. Appendix D—User Guide for CMF Regression Software This appendix serves as the user guide for the CMF Regression Software (one of the Microsoft Excel™ tools developed under this project). This tool facilitates the procedure in Appendix A. The tool can be used to conduct a statistical evaluation of CMFs for a common treatment or change in site characteristics. The evaluation can consist of (1) computing the overall average CMF, (2) computing CMFs by crash type or severity category using reported aggregate CMFs, or (3) computing CMFs as a function of site characteristics. Appendix E—User Guide for the CMF Combination Tool Spreadsheet This appendix serves as the user guide for the CMF Combination Tool (one of the Micro- soft Excel™ tools developed under this project). This tool can be used to test two or more CMFs for the same treatment to determine whether they are similar enough to be combined into a single CMF value. The CMFs must also address the same crash type (e.g., total crashes or rear-end crashes). If the CMFs can be combined, the tool produces a value for the com- bined CMF and gives cautions for the use of the combined value. Appendix F—Enhancing Future CMF Research This appendix addresses the need to change the way that CMF research is conducted. It presents a strong argument that continued attempts to extract reliable CMFs by fitting single-equation models to cross-sectional data are unlikely to bring about consensus. It describes the need to accurately attribute an observed safety effect to the cause related to the CMF of interest and how research studies should be designed to create conditions in which nuisance influences are minimized or well accounted for. Appendix G—Developing Consensus in Research About the Safety Effect of Manipulations This appendix describes the complexity of consensus in research regarding the effects of safety treatments. To demonstrate this complexity, it presents the results of eight studies on the effect of pavement marking reflectivity. It discusses the identification of alternative ways for determining the safety effect of manipulations and better use of future research.