Guidance for the Development and Application of Crash Modification Factors

Final products of National Cooperative Highway Research Program (NCHRP), Project 17-63

NCHRP Project 17-63 developed guidance 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. The final products of this project are presented below.

These materials have not yet been published by NCHRP and are presented in here as unpublished materials. NCHRP has given permission for these materials to be hosted on this site until they are published.

Final Report

This report presents the results of a study on recommended methods for applying and developing crash modification factors (CMFs) in road safety practice. The guidance was developed in three parts. The first provides a process to select and adjust a CMF for use at a site of interest which may differ in one or more characteristics from the sites where the CMF was developed. The second provides a process by which an analyst can determine a combined effectiveness of two countermeasures that are implemented at the same location. The third provides guidance on developing CMFs that account for differences in key site characteristics, particularly in creating crash modification functions.

Procedure for Estimating the Effect of a Proposed Treatment at a Subject Site (Appendix A)

This appendix provides the recommended procedure for selecting and adjusting a 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 an appropriate CMF(s) 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.

Procedure for Estimating the Combined Safety Effect of Two Treatments (Appendix B)

This guidance presents 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.

Guidance for Developing Crash Modification Functions (Appendix C)

The appendix provides guidance on CMFunction development. It addresses how to develop CMFunctions either from cross-sectional regression models or from a set of CMF point estimates. The guidance is illustrated through four case studies that demonstrate how to develop CMFunctions from individual CMFs and from cross-sectional regression analysis.

User Guide for CMF Regression Spreadsheet Tool (Appendix D)

This appendix serves as the user guide for the CMF Regression Software (hyperlink this) (one of the Microsoft Excelâ„¢ tools developed under this project). This tool facilitates the procedure in Appendix A. The tool can be used to conduct a statistical evaluation of crash modification factors (CMFs) for a common treatment or change in site characteristic. 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.

Resource: CMF Regression Software (Excel, 200 KB)

User Guide for CMF Combination Spreadsheet Tool (Appendix E)

This appendix serves as the user guide for the CMF Combination Tool (hyperlink this) (one of the Microsoft Excelâ„¢ tools developed under this project). This tool can be used to test two or more CMFs for the same treatment to determine if they are similar enough to be combined into a single CMF value. The CMFs must also address the same crash type (i.e., total crashes, rear end crashes, etc.). If the CMFs are able to be combined, the tool produces a value for the combined CMF and gives cautions for the use of the combined value.

Resource: CMF Combination Tool (Excel, 40 KB)

Enhancing Future CMF Research (Appendix F)

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 a single-equation models to cross-section 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.

Developing Consensus in Research about the Safety Effect of Manipulations (Appendix G)

This appendix describe the complexity of the issue surrounding consensus in research regarding the effects of safety treatments. As a demonstration of 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 a better use of future research.

The information contained in the Crash Modification Factors (CMF) Clearinghouse is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for the use of the information contained in the CMF Clearinghouse. The information contained in the CMF Clearinghouse does not constitute a standard, specification, or regulation, nor is it a substitute for sound engineering judgment.