Consideration of Roadside Features in the Highway Safety Manual (2022)

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NCHRP Web-Only Document 325: Consideration of Roadside Features in the Highway Safety Manual Christine E. Carrigan Malcolm H. Ray RoadSafe LLC Canton, ME Conduct of Research Report for NCHRP Project 17-54 Submitted September 2018 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 TRB, AASHTO, FAA, FHWA, FTA, GHSA, NHTSA, or TDC endorsement 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 The opinions and conclusions expressed or implied in this report 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 information contained in this document was taken directly from the submission of the author(s). This material has not been edited by TRB.

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C O O P E R A T I V E R E S E A R C H P R O G R A M S CRP STAFF FOR NCHRP WEB-ONLY DOCUMENT 325 Christopher J. Hedges, Director, Cooperative Research Programs Lori L. Sundstrom, Deputy Director, Cooperative Research Programs Waseem Dekelbab, Associate Program Manager, National Cooperative Highway Research Program David Jared, Senior Program Officer Traci Caldwell, Senior Program Assistant Natalie Barnes, Director of Publications Heather DiAngelis, Associate Director of Publications Jennifer J. Weeks, Publishing Projects Manager NCHRP PROJECT 17-54 PANEL Field of Traffic—Area of Safety David McCormick, Washington State Department of Transportation, Shoreline, WA (Chair) Steven Buckley, JEO Consulting Group, Topeka, KS Craig Copelan, American Society of Civil Engineers, Winters, CA Daniel M. Dulaski, Northeastern University, Boston, MA Wilfred Hernandez, Rhode Island Department of Transportation, Providence, RI Shuo Li, Indiana Department of Transportation, West Lafayette, IN Hadi Shirazi, Louisiana Department of Transportation and Development, Baton Rouge, LA J. Richard Young, Jr., Atkins, Jackson, MS Richard Albin, FHWA Liaison Kelly Hardy, AASHTO Liaison Nelson Gibson, TRB Liaison AUTHOR ACKNOWLEDGMENTS The research reported herein was performed under NCHRP Project 17-54 by RoadSafe LLC, with subcontracting and consulting services provided by H. Clay Gabler and Allison Daniello with Virginia Tech and Karen Dixon and Raul Avelar now with Texas Transportation Institute formally with Oregon State University. Several current and former RoadSafe LLC personal contributed to this effort: T. Olaf Johnson assembled the databases used in the formulation of the regression models, Archie Ray supplemented the data available through the states with field data collection, and Ethan Ray reviewed the final products for consistency and usability while also assembling the Highway Safety Manual (HSM) chapter. Roadside asset inventories collected by the Washington Department of Transportation and the Ohio Department of Transportation, which were consider groundbreaking data at the onset of this research, were graciously provided by these departments and made this research possible. These databases set the benchmark which many other departments are now following at the close of this research.

TABLE OF CONTENTS Introduction ..................................................................................................................................... 1 Background ..................................................................................................................................... 2 Roadside Safety Analysis Program ............................................................................................. 3 Highway Safety Manual ............................................................................................................. 3 Road Segments and Intersections ........................................................................................... 4 Special Facilities and Geometric Situations ........................................................................... 4 Road Networks........................................................................................................................ 5 Predicting Crash Frequency and Modifying Predictions for Roadway Segments .................. 5 Predictive Method for Rural Two-Lane, Two-Way Roads .................................................... 6 Predictive Method for Rural Multilane Highways................................................................ 14 Predictive Method for Urban and Suburban Arterials .......................................................... 18 Summary of Predictive Methods and Roadside Design in the HSM .................................... 24 Summary ................................................................................................................................... 25 Method .......................................................................................................................................... 26 Run-Off-Road Crash Prediction Model Form .......................................................................... 26 Available Data .......................................................................................................................... 27 Database Compilation ............................................................................................................... 28 Divided Roadways ................................................................................................................ 30 Undivided Roadways ............................................................................................................ 30 Types of ROR Crashes.......................................................................................................... 30 Roadside Inventories ............................................................................................................. 31 SPF Modeling ........................................................................................................................... 31 CMF Modeling.......................................................................................................................... 32 CMFROADWAY ........................................................................................................................ 33 CMFROADSIDE ......................................................................................................................... 34 SPFEDGE Derivation ....................................................................................................................... 36 Rural Divided Roadways .......................................................................................................... 37 Rural Undivided Roadways ...................................................................................................... 41 Urban Divided Roadways ......................................................................................................... 44 Urban Undivided Roadways ..................................................................................................... 47 SPFEDGE Results ........................................................................................................................ 50 Comparison to Other Studies ................................................................................................ 56 Divided Model Comparison ............................................................................................... 60 Undivided Model Comparison ........................................................................................... 61 CMFROADWAY Derivation .............................................................................................................. 63 Rural Divided Highway Model ................................................................................................. 63 Rural Undivided Highway Model ............................................................................................. 68 Urban Divided Highway Model................................................................................................ 72 Urban Undivided Highway Model ............................................................................................ 77 CMFROADWAY Results ............................................................................................................... 81 Non-Directional CMFs ......................................................................................................... 81 CMF for Average Lane Width ........................................................................................... 81 CMF for Right Shoulder Width ......................................................................................... 83 CMF for Posted Speed Limit ............................................................................................. 84

CMF for Number of Lanes ................................................................................................ 86 Directional Divided and Undivided CMFs ........................................................................... 87 Undivided Highway CMF for Horizontal Curvature ......................................................... 88 Divided Highway CMF for Horizontal Curvature ............................................................. 89 Undivided Highway CMF for Vertical Grade ................................................................... 92 Divided Highway CMF for Vertical Grade ....................................................................... 93 Discussion of CMFROADWAY Results .................................................................................... 94 CMFROADSIDE Derivation ............................................................................................................... 95 CMFROADSIDE Results ................................................................................................................ 99 CMFJ and CMFK Development .............................................................................................. 103 CMFJ for Change to Barrier Type ...................................................................................... 103 Crash Data ........................................................................................................................ 103 Methodology .................................................................................................................... 105 Results .............................................................................................................................. 107 CMFK for Change to Roadside Slope ................................................................................. 108 Simulated Trajectory Data ............................................................................................... 108 Methodology .................................................................................................................... 109 Statistical Analysis ........................................................................................................... 109 CMFJ and CMFK Developed from Simulated Before/After Study ..................................... 114 CMFJ for Offset to Barriers ............................................................................................. 115 CMFK for Narrow Fixed Objects ..................................................................................... 115 CMFK for Miscellaneous Obstacles ................................................................................. 115 Results .............................................................................................................................. 116 Summary .......................................................................................................................... 119 Conclusions ................................................................................................................................. 120 References ................................................................................................................................... 121 The following appendices are available on the National Academies Press website (www.nap.edu) by searching for NCHRP Web-Only Document 325: Consideration of Roadside Features in the Highway Safety Manual. Appendix A. Draft HSM Chapter Appendix B. Modifications to RSAPv3 Appendix C. Databases Documentation Appendix D. RSAPv3 Simulations Appendix E. Characterization Of The Roadsides Edges Appendix F. Results of Survey of Practice

LIST OF FIGURES Figure 1. Roadways and Segment Definitions. [AASHTO10] ...................................................... 4 Figure 2. Default Distribution of Crash Frequency for Rural Two-Lane Roads. .......................... 7 Figure 3. Photographic Representation of Roadside Hazard Ratings (RHR). [Harwood00] ..... 11 Figure 4. Rural Two-Lane Road Example 1. ............................................................................... 12 Figure 5. Rural Two-Lane Road Example 2. ............................................................................... 12 Figure 6. Example 1 All Crash Types Modified for Roadside Design Over Possible RHR. ...... 14 Figure 7. Fixed-Object CMF for Various Offsets and Spacing Along Two-Lane Urban and Suburban Arterials. ............................................................................................... 24 Figure 8. Fixed-Object CMF for Two Foot Offset for Various Urban and Suburban Arterial Road Types. .......................................................................................................... 25 Figure 9. Possible Vehicle Crashes by Direction of Travel and Highway Type. ........................ 29 Figure 10. Proportion of Rural Divided Highway Vehicle ROR Crashes by Edge and AADT. . 51 Figure 11. Proportion of Urban Divided Highway Vehicle ROR Crashes by Edge and AADT. 52 Figure 12. Proportion of Rural Undivided Highway Vehicle ROR Crashes by Edge Mile and AADT. ............................................................................................................ 52 Figure 13. Proportion of Urban Undivided Highway Vehicle ROR Crashes by Edge Mile and AADT. ............................................................................................................ 53 Figure 14. Frequency of Rural Divided Highway ROR Crashes by Edge. ................................. 53 Figure 15. Frequency of Urban Divided Highway ROR Crashes by Edge. ................................ 54 Figure 16. Comparison of Urban and Rural Divided SPFEDGE. ................................................... 54 Figure 17. Frequency of Rural Undivided Highway ROR Crashes by Edge. ............................. 55 Figure 18. Frequency of Urban Undivided Highway ROR Crashes by Edge. ............................ 55 Figure 19. Comparison of Urban and Rural Undivided SPFEDGE. ............................................... 56 Figure 20. Hutchinson and Kennedy Encroachment Frequency. ................................................ 57 Figure 21. Cooper Base Encroachment Rate.[Cooper80] ............................................................ 58 Figure 22. Findings of FHWA-RD-01-124. [Miaou01] .............................................................. 58 Figure 23. RSAPv3 Tabulated Encroachment Model.................................................................. 60 Figure 24. Comparison of Divided Models for Encroachments and Crashes per Mile per Year. ....................................................................................................... 61 Figure 25. Comparison of Undivided Models for Encroachments and Crashes per Mile per Year. ....................................................................................................... 62 Figure 26. Lane Width CMF (CMFLW). ...................................................................................... 82 Figure 27. Right Shoulder Width CMF (CMFSW). ...................................................................... 84 Figure 28. Posted Speed Limit CMF (CMFPSL). .......................................................................... 86 Figure 29. Number of Lanes CMF (CMFNL). .............................................................................. 87 Figure 30. Undivided Highway Horizontal Curve CMF. ............................................................ 89 Figure 31. Visual Assessment of Rural and Urban Divided Highway Horizontal Curve CMF (CMFDOC). ............................................................................................................. 91 Figure 32. Undivided Highway Vertical Grade CMF. ................................................................ 92 Figure 33. Visual Assessment of Rural and Urban Divided Highway Vertical Grade CMF (CMFPG). ............................................................................................................... 93 Figure 34. Rural Divided CMFROADSIDE for the Each Severity Distribution. ............................ 101 Figure 35. Rural Undivided CMFROADSIDE for the Each Severity Distribution. ........................ 101 Figure 36. Urban Divided CMFROADSIDE for the Each Severity Distribution. ........................... 102 Figure 37. Urban Undivided CMFROADSIDE for the Each Severity Distribution. ....................... 102

Figure 38. WHO Suggested Design of a Case-Control Study. [Bonita06] ................................ 106  Figure 39. Predicted Probability of Rollover for Population Factors. ....................................... 112  LIST OF TABLES Table 1. Rural Two-Lane Road CMFs and the Applicable Crash Types. ..................................... 8  Table 2. Roadside Design CMFs by RHR ................................................................................... 13  Table 3. Recommended Median Width Rounding for Segmenting. [AASHTO10] .................... 15  Table 4. Recommended Shoulder Width Rounding for Segmenting. [AASHTO10] .................. 15  Table 5. Recommended Lane Width Rounding for Segmenting. [AASHTO10] ........................ 16  Table 6. SPF Coefficients for Various Injury Crash Frequencies on Rural Multilane Road Segments. [AASHTO10] ...................................................................................... 16  Table 7. Rural Multilane Road Base Conditions. [AASHTO10] ................................................ 17  Table 8. Rural Multilane Road CMFs and the Applicable Crash Types. .................................... 18  Table 9. Coefficients for Urban and Suburban Single-Vehicle SPF.[AASHTO10] .................... 21  Table 10. Proportion of Crash Severities for Road Types within the Urban and Suburban Models. [AASHTO10] ......................................................................................... 21  Table 11. Urban and Suburban Arterial CMFs and the Applicable Crash Types. ....................... 22  Table 12. Fixed-Object Offset Factor.[AASHTO10] .................................................................. 23  Table 13. Proportion of Fixed-Object Collisions.[AASHTO10] ................................................. 23  Table 14. Descriptive Statistics of Filtered Rural Divided Highway Dataset. ............................ 38  Table 15. Correlation Matrix for Divided Highway ROR Events and Data Elements. ............... 39  Table 16. Resulting Rural Divided Highway SPFEDGE Coefficients and Fit Statistics. ............... 40  Table 17. Derivation of Rural Divided Highway SPFEDGE Coefficients. .................................... 41  Table 18. Descriptive Statistics of Filtered Undivided Highway Dataset. .................................. 41  Table 19. Correlation Matrix for Undivided Highway ROR Events and Data Elements. ........... 42  Table 20. Resulting Rural Undivided Highway SPFEDGE Coefficients and Fit Statistics. ........... 43  Table 21. Rural Undivided Highway SPFEDGE Coefficients. ....................................................... 44  Table 22. Descriptive Statistics of Filtered Multi-State Urban Divided Highway Dataset. ........ 45  Table 23. Correlation Matrix for Multi-State Urban Divided Highway Dataset. ........................ 45  Table 24. Resulting Urban Divided Highway SPFEDGE Coefficients and Fit Statistics. .............. 46  Table 25. Derivation of Urban Divided Highway SPFEDGE Coefficients. ................................... 47  Table 26. Descriptive Statistics of Filtered Multi-State Urban Undivided Highway Dataset. .... 47  Table 27. Correlation Matrix for Multi-State Urban Undivided Highway Dataset. .................... 48  Table 28. Resulting Urban Undivided Highway SPFEDGE Coefficients and Fit Statistics........... 49  Table 29. Derivation of Urban Undivided Highway SPFEDGE Coefficients. ............................... 50  Table 30. Rural and Urban Base Conditions for SPFEDGE. .......................................................... 50  Table 31. Undivided Highway SPFEDGE Coefficients. ................................................................. 51  Table 32. Divided Highway SPFEDGE Coefficients. ..................................................................... 51  Table 33. Descriptive Statistics for Rural Divided Highway Dataset. ........................................ 64  Table 34. Negative Binomial Model for All ROR Crashes on Rural Divided Roadways. .......... 65  Table 35. Negative Binomial Model for Right Edge Only ROR Crashes on Rural Divided Highways. ............................................................................................................. 66  Table 36. Negative Binomial Model for Left Edge Only ROR Crashes on Rural Divided Roadways. ............................................................................................................. 67 

Table 37. Descriptive Statistics for Rural Undivided Highway Dataset. .................................... 69 Table 38. Negative Binomial Model for Right Edge ROR Crashes on Rural Undivided Roadways. ............................................................................................................. 70 Table 39. Negative Binomial Model for Primary Right Departure Only ROR Crashes on Rural Undivided Roadways. ........................................................................................... 71 Table 40. Descriptive Statistics for Urban Divided Highway Dataset. ....................................... 73 Table 41. Negative Binomial Model for All ROR Crashes on Urban Divided Roadways. ........ 74 Table 42. Negative Binomial Model for Right Edge Only ROR Crashes on Urban Divided Roadways. ............................................................................................................. 75 Table 43. Negative Binomial Model for Left Edge Only ROR Crashes on Urban Divided Roadways. ............................................................................................................. 76 Table 44. Descriptive Statistics for Urban Undivided Highway Dataset. ................................... 78 Table 45. Negative Binomial Model for Right Edge ROR Crashes on Urban Undivided Roadways. ............................................................................................................. 79 Table 46. Negative Binomial Model for Primary Right Departures Only on Urban Undivided Roadways. ............................................................................................................. 80 Table 47. Lane Width Database Representation. ......................................................................... 81 Table 48. Rural Average Lane Width CMF (CMFLW). ............................................................... 82 Table 49. Urban Average Lane Width CMF (CMFLW). .............................................................. 82 Table 50. Right Shoulder Width Database Representation. ........................................................ 83 Table 51. Rural Average Right Shoulder Width CMF (CMFSW). ............................................... 83 Table 52. Urban Right Shoulder Width CMF (CMFSW).............................................................. 83 Table 53. Posted Speed Limit Database Representation. ............................................................ 84 Table 54. Rural Posted Speed Limit CMF (CMFPSL). ................................................................. 85 Table 55. Urban Posted Speed Limit CMF (CMFPSL). ................................................................ 85 Table 56. Number of Lanes by Proportion within Databases. ..................................................... 86 Table 57. Rural Number of Lanes CMF (CMFNL). ..................................................................... 87 Table 58. Urban Number of Lanes CMF (CMFNL). .................................................................... 87 Table 59. DOC Database Representation. ................................................................................... 88 Table 60. PG Database Representation. ....................................................................................... 88 Table 61. Horizontal Curve CMF for Undivided Right Edge in Direction Under Evaluation (CMFDOC). ............................................................................................................. 89 Table 62. Rural Divided Highway Horizontal Curve CMF (CMFDOC). ...................................... 90 Table 63. Urban Divided Highway Horizontal Curve CMF (CMFDOC). ..................................... 90 Table 64. Rural and Urban Undivided Highway Vertical Grade CMF (CMFPG). ....................... 92 Table 65. Rural and Urban Divided Highway Vertical Grade CMF (CMFPG). ........................... 93 Table 66. Descriptive Statistics for Rural Divided Dataset. ........................................................ 96 Table 67. Descriptive Statistics for Rural Undivided Dataset. .................................................... 96 Table 68. Descriptive Statistics for Urban Divided Dataset. ....................................................... 96 Table 69. Descriptive Statistics for Urban Undivided Dataset. ................................................... 96 Table 70. Rural Divided Highway Longitudinal Barrier to All Other ROR Crashes Proportional Log Odds Modeling Results. ................................................................................ 97 Table 71. Rural Undivided Highway Longitudinal Barrier to All Other ROR Crashes Proportional Log Odds Modeling Results. ........................................................... 98 Table 72. Urban Divided Highway Longitudinal Barrier to All Other ROR Crashes Proportional Log Odds Modeling Results. ................................................................................ 98

Table 73. Urban Undivided Highway Longitudinal Barrier to All Other ROR Crashes Proportional Log Odds Modeling Results. ........................................................... 99 Table 74. Rural Divided Highway Estimated Probability of Crashes by Severity and Type. ..... 99 Table 75. Rural Undivided Highway Estimated Probability of Crashes by Severity and Type. . 99 Table 76. Urban Divided Highway Estimated Probability of Crashes by Severity and Type. .... 99 Table 77. Urban Undivided Highway Estimated Probability of Crashes by Severity and Type. .............................................................................................. 100 Table 78. CMFROADSIDE Coefficients. ........................................................................................ 100 Table 79. Summary of Crash Severity Distribution Data for Strong Post W-beam. ................. 104 Table 80. Summary of Crash Severity Distribution Data for Weak Post W-beam. .................. 104 Table 81. Summary of Crash Severity Distribution Data for High Tension Cable. .................. 104 Table 82. Summary of Crash Severity Distribution Data for Low Tension Cable. ................... 104 Table 83. Summary of Crash Severity Distribution Data for New Jersey Shape Barrier. ......... 105 Table 84. Summary of Crash Severity Distribution Data for Vertical Rail Barrier. ................. 105 Table 85. Summary of Crash Severity Distribution Data for F-shape Barrier. ......................... 105 Table 86. Summary of Crash Severity Distribution Data for Single-Slope Barrier. ................. 105 Table 87. Tabulation for Simple Case-Control Analysis. [Gross10] ......................................... 107 Table 88. CMFj for Changes to Barrier Type. ........................................................................... 108 Table 89. Simulated Trajectory Study Population. .................................................................... 109 Table 90. Model Coefficients on Rollover Proportion. ............................................................. 111 Table 91. Slope CMF Recommended for the HSM and EAF for use in RSAPv3. ................... 114 Table 92. Longitudinal Barrier Model Coefficients on Crash Proportion. ................................ 115 Table 93. NFO Model Coefficients on Crash Proportion. ......................................................... 115 Table 94. Misc. Model Coefficients on Crash Proportion. ........................................................ 115 Table 95. Measured Roadside Base Conditions. ....................................................................... 116 Table 96. Summary of βi for Calculating CMFs. ...................................................................... 116 Table 97. Resulting Modifiers for Changes to Offset of Longitudinal Barriers. ....................... 117 Table 98. Resulting Modifiers for Changes to Offset of NFOs. ................................................ 117 Table 99. Resulting Modifiers for Changes to Density of NFOs. ............................................. 118 Table 100. Resulting Modifiers for Changes to Offset of Misc Obstacles. ............................... 118 Table 101. Resulting Modifiers for Changes to Density of Misc Obstacles. ............................ 119