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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2021. Guidelines for the Development and Application of Crash Modification Factors. Washington, DC: The National Academies Press. doi: 10.17226/26408.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2021. Guidelines for the Development and Application of Crash Modification Factors. Washington, DC: The National Academies Press. doi: 10.17226/26408.
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© 2021 National Academy of Sciences. All rights reserved. NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM Systematic, well-designed, and implementable research is the most effective way to solve many problems facing state departments of transportation (DOTs) administrators and engineers. Often, highway problems are of local or regional interest and can best be studied by state DOTs individually or in cooperation with their state universities and others. However, the accelerating growth of highway transportation results in increasingly complex problems of wide interest to highway authorities. These problems are best studied through a coordinated program of cooperative research. Recognizing this need, the leadership of the American Association of State Highway and Transportation Officials (AASHTO) in 1962 initiated an objective national highway research program using modern scientific techniques— the National Cooperative Highway Research Program (NCHRP). NCHRP is supported on a continuing basis by funds from participating member states of AASHTO and receives the full cooperation and support of the Federal Highway Administration (FHWA), United States Department of Transportation, under Agreement No. 693JJ31950003. COPYRIGHT INFORMATION Authors herein are responsible for the authenticity of their materials and for obtaining written permissions from publishers or persons who own the copyright to any previously published or copyrighted material used herein. Cooperative Research Programs (CRP) grants permission to reproduce material in this publication for classroom and not-for-profit purposes. Permission is given with the understanding that none of the material will be used to imply endorsement by TRB and any of its program sponsors of a particular product, method, or practice. It is expected that those reproducing the material in this document for educational and not-for-profit uses will give appropriate acknowledgment of the source of any reprinted or reproduced material. For other uses of the material, request permission from CRP. DISCLAIMER To facilitate more timely dissemination of research findings, this pre-publication document is taken directly from the submission of the research agency. The material has not been edited by TRB. The opinions and conclusions expressed or implied in this document are those of the researchers who performed the research. They are not necessarily those of the Transportation Research Board; the National Academies of Sciences, Engineering, and Medicine; the FHWA; or the program sponsors. The Transportation Research Board, the National Academies, and the sponsors of the National Cooperative Highway Research Program do not endorse products or manufacturers. Trade or manufacturers’ names appear herein solely because they are considered essential to the object of the report. This pre-publication document IS NOT an official publication of the Cooperative Research Programs; the Transportation Research Board; or the National Academies of Sciences, Engineering, and Medicine. Recommended citation: Carter, D., R. Srinivasan, F. Gross, S. Himes, T. Le, B. Persaud, C. Lyon, and J. Bonneson. 2021. Guidelines for the Development and Application of Crash Modification Factors. Pre- publication draft of NCHRP Research Report 991. Transportation Research Board, Washington, D.C.

3 Contents SUMMARY ..................................................................................................................................................... 8  CALIBRATION OF EXISTING CMFS FOR DIFFERENT SITE CHARACTERISTICS .............................................................................. 8  QUANTIFYING THE EFFECT OF MULTIPLE TREATMENTS AT A SINGLE LOCATION ....................................................................... 8  DEVELOPING CMFUNCTIONS THAT IDENTIFY KEY INFLUENTIAL SITE CHARACTERISTICS ............................................................. 9  BACKGROUND............................................................................................................................................... 10  INTRODUCTION ........................................................................................................................................................ 10  RESEARCH APPROACH ................................................................................................................................... 11  FINDINGS AND APPLICATIONS ....................................................................................................................... 12  TASK 2: GUIDELINES FOR CALIBRATION OF EXISTING CMFS FOR DIFFERENT SITE CHARACTERISTICS ......................................... 12  Overview of Task and Products ....................................................................................................................... 12  Subtask 2.1: Literature Review ....................................................................................................................... 13  Subtask 2.2: Develop Framework for Guidelines ............................................................................................ 14  Subtask 2.3: Identify Potential Data Sources for Phase 2 ............................................................................... 16  Subtask 2.4: Develop Plan for Phase 2 ............................................................................................................ 17  Subtask 2.5: Collect and Assemble Data ......................................................................................................... 17  Subtask 2.6: Develop Guidelines and Demonstrate Their Use ........................................................................ 17  TASK 3: QUANTIFYING THE EFFECT OF MULTIPLE TREATMENTS AT A SINGLE LOCATION ......................................................... 34  Overview of Task and Products ....................................................................................................................... 34  Subtask 3.1: Literature Review ....................................................................................................................... 34  Subtask 3.2: Develop Framework for Guidelines ............................................................................................ 43  Subtask 3.3: Identify Potential Data Sources for Phase 2 ............................................................................... 44  Subtask 3.4: Develop Plan for Phase 2 ............................................................................................................ 48  Subtask 3.5: Implement Plan for Phase 2 ....................................................................................................... 60  TASK 4: RECOMMENDED PROCEDURES FOR CALIBRATING AND FORMULATING FUTURE CMFS THAT IDENTIFY KEY INFLUENTIAL SITE CHARACTERISTICS  ................................................................................................................................................... 109  Overview of Task and Products ..................................................................................................................... 109  Subtask 4.1: Literature Review ..................................................................................................................... 109  Subtask 4.2: Develop Framework for Alternate Procedures ......................................................................... 110  Subtask 4.3: Identify Potential Data Sources for Phase 2 ............................................................................. 110  Subtask 4.4: Develop Plan for Phase 2 .......................................................................................................... 110  Subtask 4.5: Implement Plan for Phase 2 ..................................................................................................... 111  CONCLUSIONS AND SUGGESTED RESEARCH ................................................................................................. 115  Combined Effect of Multiple Treatments ...................................................................................................... 115  Enhancing Future CMF Research .................................................................................................................. 116  REFERENCES ................................................................................................................................................ 117  APPENDIX A: Procedure for Estimating the Effect of a Proposed Treatment at a Subject Site…….…………………………A‐1  APPENDIX B: Procedure for Estimating the Combined Safety Effect of Two Treatments……………………………………….B‐1  APPENDIX C: Guidelines for Developing Crash Modification Functions………………………….……………………………………..C‐1  APPENDIX D: User Guide for CMF Regression Software…………………………………………………………………………………..…...D‐1  APPENDIX E: User Guide for the CMF Combination Tool Spreadsheet…………………………………………………………………..E‐1  APPENDIX F: Enhancing Future CMF Research………………………………….…………………………………………………………………..F‐1  APPENDIX G: Developing Consensus in Research About the Safety Effect of Manipulations…………………………………G‐1 

4 Tables TABLE 1. EXAMPLE OF CMFS COMPARED USING HOMOGENEITY TEST AND FOUND TO BE SIGNIFICANTLY DIFFERENT .......................... 20  TABLE 2. REVIEW OF STUDIES USING DATA FROM MULTIPLE STATES .......................................................................................... 22  TABLE 3. CMF VARIATION DUE TO DIFFERENCES IN CRASH TYPE DISTRIBUTION. ............................................................................. 26  TABLE 4. REPORTED SAFETY EFFECT OF SHOULDER RUMBLE STRIPS. ............................................................................................. 28  TABLE 5. CMF VARIATION DUE TO DIFFERENCES IN CRASH LOCATION DISTRIBUTION. ...................................................................... 30  TABLE 6. REPORTED SAFETY EFFECT OF INTERSECTION TURN BAYS ............................................................................................... 32  TABLE 7. SUMMARY OF EXISTING CMFS FOR COMBINED TREATMENTS ....................................................................................... 49  TABLE 8. STUDY SITES CATEGORIZED BY TREATMENT TYPE ....................................................................................................... 53  TABLE 9. SAMPLE DATA FOR ESTIMATING COMBINED EFFECTS BY CRASH PROPORTION.................................................................. 58  TABLE 10. SUMMARY OF BEFORE AND AFTER CONDITIONS ....................................................................................................... 61  TABLE 11. SUMMARY STATISTICS FOR TREATED SITES FROM WASHINGTON RUMBLE STRIP DATASET ................................................ 64  TABLE 12. SUMMARY STATISTICS FOR REFERENCE SITES FROM WASHINGTON RUMBLE STRIP DATASET ............................................ 65  TABLE 13. SUMMARY OF LANE WIDTH AND SHOULDER WIDTH ON TANGENTS ............................................................................. 66  TABLE 14. SUMMARY OF LANE WIDTH AND SHOULDER WIDTH ON HORIZONTAL CURVES .............................................................. 67  TABLE 15. SUMMARY OF THE DATASET ................................................................................................................................. 72  TABLE 16. CMFS FOR CLRS ONLY ....................................................................................................................................... 73  TABLE 17. CMFS FOR SRS ONLY......................................................................................................................................... 73  TABLE 18. CMFS FOR CLRS + SRS ...................................................................................................................................... 73  TABLE 19. CMFS FOR WIDENING SHOULDER WIDTH TO FOUR FEET ON TANGENTS ...................................................................... 74  TABLE 20. CMFS FOR WIDENING SHOULDER WIDTH TO EIGHT FEET ON TANGENTS ...................................................................... 74  TABLE 21. CMFS FOR WIDENING LANE WIDTH TO 12 FEET ON TANGENTS .................................................................................. 74  TABLE 22. CMFS FOR COMBINATION OF WIDENING LANE WIDTH TO 12 FEET AND SHOULDER WIDTH TO FOUR FEET ON TANGENTS .... 74  TABLE 23. CMFS FOR COMBINATION OF WIDENING LANE WIDTH TO 12 FEET AND SHOULDER WIDTH TO EIGHT FEET ON TANGENTS .... 75  TABLE 24. CMFS FOR WIDENING SHOULDER WIDTH TO EIGHT FEET ON CURVES ......................................................................... 75  TABLE 25. CMFS FOR WIDENING LANE WIDTH TO 12 FEET ON CURVES ..................................................................................... 75  TABLE 26. CMFS FOR COMBINATION OF WIDENING LANE WIDTH TO 12 FEET AND SHOULDER WIDTH TO EIGHT FEET ON CURVES ........ 75  TABLE 27. CMFS FOR INCREASING ISD TO 1,320+ FT (FROM 500 ‐ 750 FT) .............................................................................. 76  TABLE 28. CMFS FOR INCREASING INTERSECTION ANGLE TO 85 ‐ 90 DEGREES (FROM 50 ‐ 75 DEGREES) ......................................... 76  TABLE 29. CMFS FOR COMBINATION OF INCREASING ISD TO 1,320+ FT (FROM 500 ‐ 750 FT) AND INTERSECTION ANGLE TO 85 ‐ 90 DEGREES (FROM 50 ‐ 75 DEGREES) ............................................................................................................................. 76  TABLE 30. METHODS ASSESSMENT FOR CLRS + SRS (TOTAL CRASHES) ..................................................................................... 77  TABLE 31. METHODS ASSESSMENT FOR CLRS + SRS (FATAL + INJURY CRASHES) ......................................................................... 78  TABLE 32. METHODS ASSESSMENT FOR CLRS + SRS (RUN‐OFF‐ROAD CRASHES) ........................................................................ 78  TABLE 33. METHODS ASSESSMENT FOR CLRS + SRS (TARGET CRASHES) .................................................................................... 78  TABLE 34. SUMMARY OF MEAN SQUARE ERROR BY METHOD FOR CLRS + SRS ........................................................................... 79  TABLE 35. SUMMARY OF DIFFERENCES BY METHOD FOR CLRS + SRS ........................................................................................ 80  TABLE 36. METHODS ASSESSMENT FOR LANE AND SHOULDER WIDTH COMBINATION 1 (TOTAL CRASHES ON TANGENTS) .................... 80  TABLE 37. METHODS ASSESSMENT FOR LANE AND SHOULDER WIDTH COMBINATION 1 (FATAL & INJURY CRASHES ON TANGENTS) ....... 81  TABLE 38. METHODS ASSESSMENT FOR LANE AND SHOULDER WIDTH COMBINATION 1 (RUN‐OFF‐ROAD CRASHES ON TANGENTS) ....... 81  TABLE 39. METHODS ASSESSMENT FOR LANE AND SHOULDER WIDTH COMBINATION 1 (TARGET CRASHES ON TANGENTS) .................. 82  TABLE 40. METHODS ASSESSMENT FOR LANE AND SHOULDER WIDTH COMBINATION 2 (TOTAL CRASHES ON TANGENTS) .................... 82  TABLE 41. METHODS ASSESSMENT FOR LANE AND SHOULDER WIDTH COMBINATION 2 (FATAL & INJURY CRASHES ON TANGENTS) ....... 83  TABLE 42. METHODS ASSESSMENT FOR LANE AND SHOULDER WIDTH COMBINATION 2 (RUN‐OFF‐ROAD CRASHES ON TANGENTS) ....... 83  TABLE 43. METHODS ASSESSMENT FOR LANE AND SHOULDER WIDTH COMBINATION 2 (TARGET CRASHES ON TANGENTS) .................. 83  TABLE 44. METHODS ASSESSMENT FOR 12‐FT LANE AND 8‐FT SHOULDER WIDTH COMBINATION (TOTAL CRASHES ON CURVES) .......... 84  TABLE 45. METHODS ASSESSMENT FOR 12‐FT LANE AND 8‐FT SHOULDER WIDTH COMBINATION (FATAL & INJURY CRASHES ON CURVES)  ............................................................................................................................................................................ 84 

5 TABLE 46. METHODS ASSESSMENT FOR 12‐FT LANE AND 8‐FT SHOULDER WIDTH COMBINATION (RUN‐OFF‐ROAD CRASHES ON CURVES)  ............................................................................................................................................................................ 85  TABLE 47. METHODS ASSESSMENT FOR 12‐FT LANE AND 8‐FT SHOULDER WIDTH COMBINATION (TARGET CRASHES ON CURVES) ........ 85  TABLE 48. SUMMARY OF MEAN SQUARE ERROR BY METHOD FOR LANE AND SHOULDER WIDTH COMBINATION 1 (TANGENTS) ............ 86  TABLE 49. SUMMARY OF MEAN SQUARE ERROR BY METHOD FOR LANE AND SHOULDER WIDTH COMBINATION 2 (TANGENTS) ............ 86  TABLE 50. SUMMARY OF MEAN SQUARE ERROR BY METHOD FOR LANE AND SHOULDER WIDTH COMBINATION (CURVES) .................. 87  TABLE 51. SUMMARY OF DIFFERENCES BY METHOD FOR LANE AND SHOULDER WIDTH COMBINATION 1 (TANGENTS) ......................... 87  TABLE 52. SUMMARY OF DIFFERENCES BY METHOD FOR LANE AND SHOULDER WIDTH COMBINATION 2 (TANGENTS) ......................... 88  TABLE 53. SUMMARY OF DIFFERENCES BY METHOD FOR LANE AND SHOULDER WIDTH COMBINATION (CURVES) ............................... 88  TABLE 54. METHODS ASSESSMENT FOR ISD AND INTERSECTION ANGLE (TOTAL TARGET CRASHES) .................................................. 89  TABLE 55. METHODS ASSESSMENT FOR ISD AND INTERSECTION ANGLE (FATAL AND INJURY TARGET CRASHES) ................................. 89  TABLE 56. METHODS ASSESSMENT FOR ISD AND INTERSECTION ANGLE (RIGHT ANGLE CRASHES) ................................................... 90  TABLE 57. SUMMARY OF MEAN SQUARE ERROR BY METHOD FOR AVAILABLE ISD AND INTERSECTION ANGLE ................................... 90  TABLE 58. SUMMARY OF DIFFERENCES BY METHOD FOR AVAILABLE ISD AND INTERSECTION ANGLE ................................................ 91  TABLE 59. SUMMARY OF AVERAGE MSE BY METHOD BY FACILITY TYPE ..................................................................................... 91  TABLE 60. SUMMARY OF AVERAGE MSE BY METHOD BY CRASH TYPE ........................................................................................ 92  TABLE 61. SUMMARY OF AVERAGE MSE BY METHOD BY MAGNITUDE OF TREATMENT EFFECT ....................................................... 93  TABLE 62. SUMMARY OF AVERAGE MSE BY METHOD BY INCLUSION OF CMFS > 1.0 ................................................................... 93  TABLE 63. SUMMARY OF METHOD ASSESSMENT FOR ESTIMATING THE STANDARD ERROR OF THE COMBINED EFFECT ......................... 96  TABLE 64. DIFFERENCE IN DECISIONS COMPARING ESTIMATED AND GROUND TRUTH CMFS AND STANDARD ERRORS OF THE COMBINED  EFFECT ................................................................................................................................................................... 97  TABLE 65. EXAMPLE 1 INTERSECTION DATABASE .................................................................................................................. 100  TABLE 66. EXAMPLE 2 HORIZONTAL CURVE DATABASE .......................................................................................................... 102  TABLE 67. CMFS FOR INSTALLING CENTERLINE AND SHOULDER RUMBLE STRIPS ........................................................................ 103  TABLE 68. CMFS FOR WIDENING LANE WIDTH FROM 11 TO 12 FEET AND SHOULDER WIDTH FROM THREE TO FOUR FEET ON TANGENTS  .......................................................................................................................................................................... 103  TABLE 69. CMFS FOR WIDENING LANE WIDTH FROM 11 TO 12 FEET AND SHOULDER WIDTH FROM THREE TO EIGHT FEET ON TANGENTS  .......................................................................................................................................................................... 103  TABLE 70. CMFS FOR WIDENING LANE WIDTH FROM 11 TO 12 FEET AND SHOULDER WIDTH FROM TWO TO EIGHT FEET ON CURVES . 104  TABLE 71. CMFS FOR INCREASING INTERSECTION SIGHT DISTANCE TO 1,320+ FEET AND INTERSECTION ANGLE TO 85 ‐ 90 DEGREES .. 104  TABLE 72. METHOD SELECTION FOR SAME CRASH TYPE AND SEVERITY ..................................................................................... 107  TABLE 73. METHOD SELECTION FOR DIFFERENT CRASH TYPE AND SEVERITY .............................................................................. 107  Figures FIGURE 1. INITIAL FLOWCHART OF PROCESS FOR SELECTING AND ADJUSTING CMF ....................................................................... 15  FIGURE 2. EXAMPLE FLOW CHART FOR SELECTING APPROPRIATE METHOD .................................................................................. 44  FIGURE 3. ILLUSTRATION OF MERGING ROADWAY SEGMENTS .................................................................................................... 62  FIGURE 4. ILLUSTRATION OF TAGGING ROADWAY SEGMENTS ..................................................................................................... 62  FIGURE 5. VISUAL VERIFICATION OF RUMBLE STRIP PRESENCE AND TYPE (SEGMENT WITH ONLY CLRS SHOWN) ................................. 63  FIGURE 6. LOCATION OF DECISION POINT AT THREE‐LEGGED INTERSECTION ................................................................................. 70  FIGURE 7. INTERSECTION ANGLE MEASUREMENTS .................................................................................................................. 71  FIGURE 8. FLOW CHART FOR SELECTING APPROPRIATE METHOD ............................................................................................. 106 

6 Appendices 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 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. * These are the prime guidelines from Task 2 of this project. Appendix B* - Procedure for Estimating the Combined Safety Effect of Two Treatments These guidelines 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. * These are the prime guidelines from Task 3 of this project. Appendix C* - Guidelines for Developing Crash Modification Functions The appendix provides guidelines on CMFunction development. 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 Spreadsheet Tool 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 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. Appendix E - User Guide for CMF Combination Spreadsheet Tool This appendix serves as the user guide for the CMF Combination Tool (one of the Microsoft Excel™ 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. 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 a single-equation models to cross-section data are unlikely to bring about consensus. It describes the need to accurately

7 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 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.

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Crash modification factors (CMF) provide transportation professionals with the kind of quantitative information they need to make decisions on where best to invest limited safety funds.

The TRB National Cooperative Highway Research Program's pre-publication draft of NCHRP Research Report 991: Guidelines for the Development and Application of Crash Modification Factors describes a procedure for estimating the effect of a proposed treatment on a site of interest.

Four supplemental files will accompany the final version of this pre-pub: a CMF regression tool, a CMF combination tool, a slide summary, and an implementation memo.

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