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NCHRP Web-Only Document 306 Safety Prediction Methodology and Analysis Tool for Freeways and Interchanges James A. Bonneson Srinivas Geedipally Michael P. Pratt Texas Transportation Institute College Station, TX Dominique Lord Texas A&M University College Station, TX Conduct of Research Report for NCHRP Project 17-45 Submitted May 2012 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.

The National Academy of Sciences was established in 1863 by an Act of Congress, signed by President Lincoln, as a private, non- governmental institution to advise the nation on issues related to science and technology. Members are elected by their peers for outstanding contributions to research. Dr. Marcia McNutt is president. The National Academy of Engineering was established in 1964 under the charter of the National Academy of Sciences to bring the practices of engineering to advising the nation. Members are elected by their peers for extraordinary contributions to engineering. Dr. John L. Anderson is president. The National Academy of Medicine (formerly the Institute of Medicine) was established in 1970 under the charter of the National Academy of Sciences to advise the nation on medical and health issues. Members are elected by their peers for distinguished contributions to medicine and health. Dr. Victor J. Dzau is president. The three Academies work together as the National Academies of Sciences, Engineering, and Medicine to provide independent, objective analysis and advice to the nation and conduct other activities to solve complex problems and inform public policy decisions. The National Academies also encourage education and research, recognize outstanding contributions to knowledge, and increase public understanding in matters of science, engineering, and medicine. Learn more about the National Academies of Sciences, Engineering, and Medicine at www.nationalacademies.org. The Transportation Research Board is one of seven major programs of the National Academies of Sciences, Engineering, and Medicine. The mission of the Transportation Research Board is to provide leadership in transportation improvements and innovation through trusted, timely, impartial, and evidence-based information exchange, research, and advice regarding all modes of transportation. The Board’s varied activities annually engage about 8,000 engineers, scientists, and other transportation researchers and practitioners from the public and private sectors and academia, all of whom contribute their expertise in the public interest. The program is supported by state transportation departments, federal agencies including the component administrations of the U.S. Department of Transportation, and other organizations and individuals interested in the development of transportation. Learn more about the Transportation Research Board at www.TRB.org.

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 306 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 Clara Schmetter, Program Associate Natalie Barnes, Director of Publications Heather DiAngelis, Associate Director of Publications Jennifer Correro, Assistant Editor NCHRP PROJECT 17-45 PANEL Field of Traffic—Area of Safety John Milton, Washington State Department of Transportation, Olympia, WA (Chair) James P. Allen, U.S. Army Corps of Engineers, Lincoln, IL Geni Bahar, NAVIGATS Inc., Toronto, Johnson Owusu-Amoako, Maryland State Highway Administration, Hanover, MD Daniel Turner, University of Alabama, Tuscaloosa, AL Michael Matzke, FHWA Liaison Bernardo Kleiner, TRB Liaison Richard Pain, TRB Liaison ACKNOWLEDGMENT The research reported herein was performed under NCHRP Project 17-45 by the Texas Transportation Institute (TTI); and CH2M-Hill. Dr. James Bonneson of TTI served as the Principal Investigator for the project and supervised the research conducted by TTI. Mr. Jason Moller supervised the research conducted by CH2M-Hill. The research supervisors were assisted by the following individuals: Dr. Srinivas Geedipally, Texas Transportation Institute; Dr. Dominique Lord, Texas A&M University; Mr. Tim Neuman, CH2M-Hill; and Mr. Michael P. Pratt, Texas Transportation Institute. .

iv CONTENTS LIST OF FIGURES ........................................................................................................................ v LIST OF TABLES ....................................................................................................................... viii SUMMARY .................................................................................................................................... 1 CHAPTER 1: INTRODUCTION ................................................................................................... 3 Research Objectives .................................................................................................................... 3 Research Scope ........................................................................................................................... 3 Organization of this Report ......................................................................................................... 4 CHAPTER 2: LITERATURE REVIEW ........................................................................................ 5 Background ................................................................................................................................. 5 Freeway and Interchange Safety ............................................................................................... 12 CHAPTER 3: FRAMEWORK FOR SAFETY PREDICTION ................................................... 57 Overview of HSM Methodology .............................................................................................. 57 Prioritized List of SPFs and CMFs ........................................................................................... 60 CHAPTER 4: DATABASE DEVELOPMENT ........................................................................... 69 Data Collection Procedures ....................................................................................................... 69 Database Summary ................................................................................................................... 79 CHAPTER 5: PREDICTIVE MODEL FOR FREEWAY SEGMENTS ................................... 100 Background ............................................................................................................................. 101 CMF Development.................................................................................................................. 107 Methodology ........................................................................................................................... 112 Model Calibration for FI Crashes ........................................................................................... 118 Model Calibration for PDO Crashes ....................................................................................... 157 Nomenclature .......................................................................................................................... 176 CHAPTER 6: PREDICTIVE MODEL FOR RAMP SEGMENTS ........................................... 181 Background ............................................................................................................................. 182 CMF Development.................................................................................................................. 186 Methodology ........................................................................................................................... 194 Model Calibration for FI Crashes ........................................................................................... 197 Model Calibration for PDO Crashes ....................................................................................... 221 Nomenclature .......................................................................................................................... 232 CHAPTER 7: PREDICTIVE MODEL FOR CROSSROAD RAMP TERMINALS ................. 235 Background ............................................................................................................................. 236 CMF Development.................................................................................................................. 240 Methodology ........................................................................................................................... 252

v Model Calibration for FI Crashes ........................................................................................... 257 Model Calibration for PDO Crashes ....................................................................................... 292 Nomenclature .......................................................................................................................... 316 CHAPTER 8: SEVERITY DISTRIBUTION FUNCTIONS ..................................................... 319 Literature Review.................................................................................................................... 319 Freeway Segments .................................................................................................................. 321 Ramp Segments ...................................................................................................................... 334 Crossroad Ramp Terminals ..................................................................................................... 340 CHAPTER 9: CONCLUSIONS AND RECOMMENDATIONS .............................................. 349 REFERENCES ........................................................................................................................... 351 APPENDIX A: Practitioner Interviews .......................................................................................363 APPENDIX B: Database Enhancement.......................................................................................372 APPENDIX C: Proposed HSM Freeways Chapter .....................................................................385 APPENDIX D: Proposed HSM Ramps Chapter .........................................................................523 APPENDIX E: Proposed HSM Appendix B for Part C ..............................................................693 APPENDIX F: Algorithm Description ........................................................................................727 LIST OF FIGURES Figure 1. Typical interchange types. ............................................................................................... 7 Figure 2. Distribution of interchange types used in the United States. ........................................... 7 Figure 3. Typical diamond and parclo interchange types. .............................................................. 8 Figure 4. Typical ramp configurations. ........................................................................................... 9 Figure 5. Freeway and interchange design components. .............................................................. 10 Figure 6. Disaggregated alignment for a diamond ramp. ............................................................. 12 Figure 7. Ramp-configuration-based SPFs based on total crashes. .............................................. 17 Figure 8. Relationship between CMF value and ramp radius based on total crashes. .................. 19 Figure 9. Relationship between CMF value and ramp lane width based on total crashes. ........... 20 Figure 10. Relationship between ramp crash frequency and crash type based on total crashes. .. 21 Figure 11. Typical speed-change lanes. ........................................................................................ 22 Figure 12. Relationship between CMF value and acceleration length based on total crashes. .... 25 Figure 13. Relationship between CMF value and deceleration length based on total crashes. .... 26 Figure 14. Relationship between crash rate, traffic demand, and number of lanes. ..................... 29 Figure 15. Freeway segment SPFs based on FI crashes. .............................................................. 30 Figure 16. Relationship between CMF value and curve radius. ................................................... 32 Figure 17. Relationship between CMF value and lane width based on total crashes. .................. 33 Figure 18. Illustrative interaction between shoulder width and lane width. ................................. 34 Figure 19. Relationship between CMF value and outside shoulder width based on total crashes. ....................................................................................................................................................... 34 Figure 20. Relationship between CMF value and inside shoulder width based on total crashes. 35 Figure 21. Relationship between CMF value and median width. ................................................. 35

vi Figure 22. Relationship between CMF value and horizontal clearance based on FI crashes. ...... 37 Figure 23. Percent of lane changes as a function of distance from ramp gore. ............................ 39 Figure 24. Total crash rate as a function of distance from ramp gore. ......................................... 40 Figure 25. Relationship between CMF value and distance from gore. ......................................... 41 Figure 26. Relationship between CMF value and weaving length. .............................................. 42 Figure 27. Relationship between CMF value and interchange spacing. ....................................... 43 Figure 28. Relationship between total crash rate and volume-to-capacity ratio for highways. .... 45 Figure 29. Relationship between total crash rate and volume-to-capacity ratio for freeways. ..... 45 Figure 30. Relationship between traffic demand, crash type, and total crash frequency. ............ 46 Figure 31. Relationship between CMF value and volume-to-capacity ratio. ............................... 47 Figure 32. High-occupancy vehicle travel direction types. .......................................................... 49 Figure 33. Relationship between FI crash rate and speed differential. ......................................... 51 Figure 34. SPF for two interchange types based on total crashes. ................................................ 53 Figure 35. Designs to discourage wrong-way maneuvers. ........................................................... 56 Figure 36. Highway Safety Manual predictive method. ............................................................... 58 Figure 37. Ramp terminal configurations. .................................................................................... 91 Figure 38. Illustrative freeway facility analysis sites. ................................................................. 102 Figure 39. Hourly volume distribution for two freeway segments. ............................................ 109 Figure 40. Proportion-of-volume statistic for freeway segments in three states. ....................... 110 Figure 41. Relationship between crash rate and side friction demand. ...................................... 111 Figure 42. Weaving section types. .............................................................................................. 113 Figure 43. Weaving section length measurement. ...................................................................... 114 Figure 44. Predicted vs. reported multiple-vehicle freeway FI crashes. ..................................... 134 Figure 45. Predicted vs. reported single-vehicle freeway FI crashes. ........................................ 136 Figure 46. Predicted vs. reported ramp-entrance-related FI crashes. ......................................... 137 Figure 47. Predicted vs. reported ramp-exit-related FI crashes. ................................................. 138 Figure 48. Calibrated freeway horizontal curve CMF for FI crashes. ........................................ 140 Figure 49. Calibrated freeway lane width CMF for FI crashes. ................................................. 141 Figure 50. Calibrated freeway outside shoulder width CMF for FI crashes. .............................. 142 Figure 51. Calibrated freeway inside shoulder width CMF for FI crashes. ................................ 143 Figure 52. Calibrated freeway median width CMF for FI crashes. ............................................ 144 Figure 53. Calibrated freeway outside clearance CMF for FI crashes. ...................................... 146 Figure 54. Lane change CMF as a function of distance from ramp gore. .................................. 148 Figure 55. Lane change CMF for segments between a pair of interchanges. ............................. 149 Figure 56. Average CMF value for FI crashes as a function of weaving section length. ........... 150 Figure 57. Calibrated freeway ramp entrance CMF for right-side ramps and FI crashes. ......... 151 Figure 58. Calibrated freeway ramp exit CMF for right-side ramps and FI crashes. ................. 152 Figure 59. Calibrated freeway high-volume CMF for FI crashes. .............................................. 153 Figure 60. Freeway FI model components. ................................................................................ 154 Figure 61. Freeway FI model. ..................................................................................................... 155 Figure 62. Predicted vs. reported multiple-vehicle freeway PDO crashes. ................................ 160 Figure 63. Predicted vs. reported single-vehicle freeway PDO crashes. .................................... 162 Figure 64. Predicted vs. reported ramp-entrance-related PDO crashes. ..................................... 163 Figure 65. Predicted vs. reported ramp-exit-related PDO crashes. ............................................. 164 Figure 66. Calibrated freeway horizontal curve CMF for PDO crashes. .................................... 166 Figure 67. Calibrated freeway outside shoulder width CMF for PDO crashes. ......................... 167

vii Figure 68. Calibrated freeway inside shoulder width CMF for PDO crashes. ........................... 167 Figure 69. Calibrated freeway median width CMF for PDO crashes. ........................................ 168 Figure 70. Calibrated freeway outside barrier CMF for PDO crashes. ...................................... 170 Figure 71. Average CMF value for PDO crashes as a function of weaving section length. ...... 172 Figure 72. Calibrated freeway ramp entrance CMF for right-side ramps and PDO crashes. ..... 173 Figure 73. Calibrated freeway high-volume CMF for PDO crashes. ......................................... 174 Figure 74. Freeway PDO model components. ............................................................................ 175 Figure 75. Freeway PDO model. ................................................................................................ 176 Figure 76. Illustrative interchange analysis sites. ....................................................................... 182 Figure 77. Starting milepost location on ramps and C-D roads. ................................................. 187 Figure 78. Diagonal ramp configuration for example application. ............................................. 193 Figure 79. Speed-change lane location on ramps and C-D roads. .............................................. 195 Figure 80. Predicted vs. reported multiple-vehicle ramp FI crashes. ......................................... 208 Figure 81. Predicted vs. reported single-vehicle ramp FI crashes. ............................................. 209 Figure 82. Calibrated ramp horizontal curve CMF for FI crashes. ............................................. 211 Figure 83. Calibrated ramp lane width CMF for FI crashes. ...................................................... 212 Figure 84. Calibrated ramp right shoulder width CMF for FI crashes. ...................................... 213 Figure 85. Calibrated ramp left shoulder width CMF for FI crashes. ......................................... 214 Figure 86. Calibrated ramp right side barrier CMF for FI crashes. ............................................ 215 Figure 87. Calibrated ramp left side barrier CMF for FI crashes. .............................................. 216 Figure 88. Calibrated ramp weaving section CMF for FI crashes. ............................................. 217 Figure 89. Calibrated ramp speed-change lane CMF for FI crashes. ......................................... 218 Figure 90. Calibrated ramp lane add or drop CMF for FI crashes. ............................................. 219 Figure 91. Ramp FI model components. ..................................................................................... 220 Figure 92. Ramp FI model. ......................................................................................................... 220 Figure 93. Predicted vs. reported multiple-vehicle ramp PDO crashes. ..................................... 223 Figure 94. Predicted vs. reported single-vehicle ramp PDO crashes. ......................................... 224 Figure 95. Calibrated ramp horizontal curve CMF for PDO crashes. ........................................ 226 Figure 96. Calibrated ramp right shoulder width CMF for PDO crashes. .................................. 226 Figure 97. Calibrated ramp left shoulder width CMF for PDO crashes. .................................... 227 Figure 98. Calibrated ramp right side barrier CMF for PDO crashes. ........................................ 228 Figure 99. Calibrated ramp left side barrier CMF for PDO crashes. .......................................... 229 Figure 100. Calibrated ramp weaving section CMF for PDO crashes. ...................................... 230 Figure 101. Ramp PDO model components. .............................................................................. 231 Figure 102. Ramp PDO model.................................................................................................... 232 Figure 103. Illustrative crossroad ramp terminal boundary. ....................................................... 237 Figure 104. Comparison of predicted turn lane CMFs with standardized residuals - total crashes. ..................................................................................................................................................... 245 Figure 105. Relationship between median width and CMF value at unsignalized intersections. ..................................................................................................................................................... 249 Figure 106. Effective number of lanes for various exit ramp configurations. ............................ 250 Figure 107. Relationship between exit ramp volume, control, and CMF value at signalized terminals. ..................................................................................................................................... 251 Figure 108. Relationship between skew angle and CMF value at unsignalized intersections. .. 252 Figure 109. Right-turn channelization length. ............................................................................ 253 Figure 110. Crossroad median width. ......................................................................................... 254

viii Figure 111. Simulation results for the inverse dispersion parameter. ......................................... 256 Figure 112. Predicted vs. reported FI crashes at signalized A2 and B2 configurations. ............ 267 Figure 113. Predicted vs. reported FI crashes at signalized A4 and D3ex configurations. ........ 268 Figure 114. Predicted vs. reported FI crashes at signalized B4 and D3en configurations. ........ 269 Figure 115. Predicted vs. reported FI crashes at signalized D4 configurations. ......................... 270 Figure 116. Calibrated segment length CMF for FI crashes–signalized. ................................... 275 Figure 117. Calibrated median width CMF for FI crashes–signalized. ...................................... 276 Figure 118. Terminal FI models–signalized. .............................................................................. 277 Figure 119. Predicted vs. reported FI crashes at unsignalized A2 and B2 configurations. ........ 285 Figure 120. Predicted vs. reported FI crashes at unsignalized A4 and D3ex configurations. .... 286 Figure 121. Predicted vs. reported FI crashes at unsignalized B4 and D3en configurations. .... 287 Figure 122. Predicted vs. reported FI crashes at unsignalized D4 configurations. ..................... 288 Figure 123. Calibrated exit ramp capacity CMF for FI crashes–unsignalized. .......................... 289 Figure 124. Calibrated segment length CMF for FI crashes–unsignalized. ............................... 291 Figure 125. Terminal FI models–unsignalized. .......................................................................... 293 Figure 126. Predicted vs. reported PDO crashes at signalized A2 and B2 configurations. ........ 296 Figure 127. Predicted vs. reported PDO crashes at signalized A4 and D3ex configurations. .... 297 Figure 128. Predicted vs. reported PDO crashes at signalized B4 and D3en configurations. .... 298 Figure 129. Predicted vs. reported PDO crashes at signalized D4 configurations. .................... 299 Figure 130. Calibrated segment length CMF for PDO crashes–signalized. ............................... 304 Figure 131. Calibrated median width CMF for PDO crashes–signalized. ................................. 305 Figure 132. Terminal PDO models–signalized. .......................................................................... 306 Figure 133. Predicted vs. reported PDO crashes at unsignalized A2 and B2 configurations. .... 309 Figure 134. Predicted vs. reported PDO crashes at unsignalized A4 and D3ex configurations. 310 Figure 135. Predicted vs. reported PDO crashes at unsignalized B4 and D3en configurations. 311 Figure 136. Predicted vs. reported PDO crashes at unsignalized D4 configurations. ................ 312 Figure 137. Terminal PDO models–unsignalized. ...................................................................... 315 Figure 138. Freeway severity distribution based on the proportion of segment with barrier. .... 327 Figure 139. Freeway severity distribution based on the proportion of AADT during high-volume hours. ........................................................................................................................................... 328 Figure 140. Freeway severity distribution based on the proportion of segment with rumble strips. ..................................................................................................................................................... 329 Figure 141. Freeway severity distribution based on the proportion of segment with horizontal curve. ........................................................................................................................................... 331 Figure 142. Freeway severity distribution based on lane width. ................................................ 332 Figure 143. Ramp severity distribution based on the proportion of segment with barrier. ........ 339 Figure 144. Crossroad ramp terminal severity distribution based on access point frequency. ... 346 LIST OF TABLES TABLE 1. Freeway mileage and number of interchanges in the United States ............................. 6 TABLE 2. Crash distribution among interchange ramp components ........................................... 13 TABLE 3. Crash distribution among interchange ramp components ........................................... 13 TABLE 4. Comparison of total crash rates for exit ramps with a lane drop ................................ 27 TABLE 5. CMFs for shoulder rumble strips ................................................................................ 36 TABLE 6. Elements that may influence the safety of crossroad ramp terminals ......................... 55

ix TABLE 7. Prioritized list of SPFs and CMFs for freeway segments ........................................... 63 TABLE 8. Prioritized list of SPFs and CMFs for interchange ramp ............................................ 65 TABLE 9. Prioritized list of SPFs and CMFs for crossroad ramp terminals ............................... 66 TABLE 10. Prioritized list of SPFs and CMFs for freeway speed-change lanes ......................... 67 TABLE 11. Base FI crash rates for minimum exposure criteria .................................................. 72 TABLE 12. SAS Processing Effort .............................................................................................. 73 TABLE 13. Database attributes .................................................................................................... 74 TABLE 14. Database target sample size ...................................................................................... 75 TABLE 15. Criteria for defining crossroad-ramp-terminal-related crashes ................................. 79 TABLE 16. Database sample size ................................................................................................ 80 TABLE 17. Summary characteristics for freeway segments ........................................................ 81 TABLE 18. Barrier and ramp characteristics for freeway segments ............................................ 82 TABLE 19. Crash data summary for freeway segments .............................................................. 84 TABLE 20. Summary characteristics for ramp segments ............................................................ 86 TABLE 21. Barrier and cross section characteristics for ramp segments .................................... 87 TABLE 22. Crash data summary for ramp segments ................................................................... 89 TABLE 23. Crash data summary by ramp configuration ............................................................. 90 TABLE 24. Summary characteristics for crossroad ramp terminals ............................................ 94 TABLE 25. Crash data summary for crossroad ramp terminals ................................................... 95 TABLE 26. Crash data summary by ramp terminal configuration ............................................... 97 TABLE 27. Sample size considerations for crossroad ramp terminals ........................................ 97 TABLE 28. Summary characteristics for freeway speed-change lane segments ......................... 99 TABLE 29. Freeway variables from HSIS database .................................................................. 104 TABLE 30. Freeway variables from supplemental data sources ................................................ 105 TABLE 31. Freeway FI model statistical description–combined model–two states .................. 129 TABLE 32. Freeway model validation statistics ........................................................................ 131 TABLE 33. Freeway FI model statistical description–combined model–three states ................ 132 TABLE 34. Freeway FI model statistical description–multiple-vehicle model–three states ..... 133 TABLE 35. Freeway FI model statistical description–single-vehicle model–three states ......... 135 TABLE 36. Freeway FI model statistical description–ramp entrance model–three states ......... 136 TABLE 37. Freeway FI model statistical description–ramp exit model–three states ................ 137 TABLE 38. Distribution of FI crashes on freeways ................................................................... 139 TABLE 39. Freeway PDO model statistical description–combined model –three states .......... 158 TABLE 40. Freeway PDO model statistical description–multiple-vehicle model–three states . 159 TABLE 41. Freeway PDO model statistical description–single-vehicle model–three states ..... 161 TABLE 42. Freeway PDO model statistical description–ramp entrance model–three states .... 162 TABLE 43. Freeway PDO model statistical description–ramp exit model–three states ............ 163 TABLE 44. Distribution of PDO crashes on freeways ............................................................... 165 TABLE 45. Ramp variables from HSIS database ...................................................................... 184 TABLE 46. Ramp variables from supplemental data sources .................................................... 185 TABLE 47. Input data for ramp curve speed prediction ............................................................ 189 TABLE 48. Input data for example application .......................................................................... 193 TABLE 49. Results for example application .............................................................................. 194 TABLE 50. Ramp FI model statistical description–combined model–two states ...................... 203 TABLE 51. Ramp model validation statistics ............................................................................ 205 TABLE 52. Ramp FI model statistical description–combined model–three states .................... 206

x TABLE 53. Ramp FI model statistical description–multiple-vehicle model–three states .......... 207 TABLE 54. Ramp FI model statistical description–single-vehicle model–three states ............. 208 TABLE 55. Distribution of FI crashes on ramps ........................................................................ 210 TABLE 56. Ramp PDO model statistical description–combined model–three states ................ 222 TABLE 57. Ramp PDO model statistical description–multiple-vehicle model–three states ..... 222 TABLE 58. Ramp PDO model statistical description–single-vehicle model–three states ......... 224 TABLE 59. Distribution of PDO crashes on ramps ................................................................... 225 TABLE 60. Terminal variables from HSIS database ................................................................. 239 TABLE 61. Variables from supplemental data sources .............................................................. 240 TABLE 62. Turn lane CMF model statistical description - total crashes ................................... 244 TABLE 63. Leg-specific turn lane CMFs - total crashes ........................................................... 245 TABLE 64. Leg-specific turn lane CMFs - FI crashes ............................................................... 246 TABLE 65. Leg-specific turn lane CMFs - PDO crashes .......................................................... 247 TABLE 66. Indicator variable values for typical ramp terminal configurations ........................ 262 TABLE 67. Terminal FI model statistical description–combined model–two states–signalized263 TABLE 68. Terminal model validation statistics–signalized ..................................................... 264 TABLE 69. Terminal FI model statistical description–combined model–three states–signalized ..................................................................................................................................................... 265 TABLE 70. Terminal FI model statistical description–A2 and B2 configuration–signalized ... 266 TABLE 71. Terminal FI model statistical description–A4 and D3ex configuration–signalized 267 TABLE 72. Terminal FI model statistical description–B4 and D3en configuration–signalized 268 TABLE 73. Terminal FI model statistical description–D4 configuration–signalized ................ 270 TABLE 74. Calibrated protected left-turn operation CMF for FI crashes ................................. 272 TABLE 75. Calibrated right-turn channelization CMF for FI crashes–signalized ..................... 273 TABLE 76. Calibrated access point frequency CMF for FI crashes–signalized ........................ 274 TABLE 77. Terminal FI model statistical description–combined model–two states–unsignalized ..................................................................................................................................................... 281 TABLE 78. Terminal model validation statistics–unsignalized ................................................. 282 TABLE 79. Terminal FI model statistical description–combined model–three states–unsignalized ..................................................................................................................................................... 283 TABLE 80. Terminal FI model statistical description–A2 and B2 configuration–unsignalized 284 TABLE 81. Terminal FI model statistical description–A4 and D3ex configuration–unsignalized ..................................................................................................................................................... 285 TABLE 82. Terminal FI model statistical description–B4 and D3en configuration–unsignalized ..................................................................................................................................................... 286 TABLE 83. Terminal FI model statistical description–D4 configuration–unsignalized ............ 287 TABLE 84. Calibrated access point frequency CMF for FI crashes–unsignalized .................... 290 TABLE 85. Terminal PDO model statistical description–combined model–three states– signalized .................................................................................................................................... 294 TABLE 86. Terminal PDO model statistical description–A2 and B2 configuration–signalized 295 TABLE 87. Terminal PDO model statistical description–A4 and D3ex configuration–signalized ..................................................................................................................................................... 296 TABLE 88. Terminal PDO model statistical description–B4 and D3en configuration–signalized ..................................................................................................................................................... 297 TABLE 89. Terminal PDO model statistical description–D4 configuration–signalized ........... 299 TABLE 90. Calibrated left-turn CMF for PDO crashes–signalized ........................................... 300

xi TABLE 91. Calibrated right-turn CMF for PDO crashes–signalized ......................................... 301 TABLE 92. Calibrated protected left-turn operation CMF for PDO crashes ............................. 301 TABLE 93. Calibrated right-turn channelization CMF for PDO crashes–signalized ................ 302 TABLE 94. Calibrated access point frequency CMF for PDO crashes–signalized ................... 303 TABLE 95. Terminal PDO model statistical description–combined model–three states– unsignalized ................................................................................................................................ 307 TABLE 96. Terminal PDO model statistical description–A2 and B2 configuration–unsignalized ..................................................................................................................................................... 309 TABLE 97. Terminal PDO model statistical description–A4 and D3ex configuration– unsignalized ................................................................................................................................ 310 TABLE 98. Terminal PDO model statistical description–B4 and D3en configuration– unsignalized ................................................................................................................................ 311 TABLE 99. Terminal PDO model statistical description–D4 configuration–unsignalized ....... 312 TABLE 100. Calibrated left-turn CMF for PDO crashes–unsignalized ..................................... 313 TABLE 101. Calibrated right-turn CMF for PDO crashes–unsignalized ................................... 314 TABLE 102. Summary statistics for freeway SDF development ............................................... 324 TABLE 103. Parameter estimation for freeway SDF ................................................................. 326 TABLE 104. Freeway severity distribution based on area type and state .................................. 332 TABLE 105. Example application of SDF local calibration procedure ..................................... 334 TABLE 106. Summary statistics for ramp SDF development ................................................... 336 TABLE 107. Parameter estimation for ramp SDF ...................................................................... 337 TABLE 108. Ramp severity distribution based on lanes, area type, and ramp type .................. 340 TABLE 109. Summary statistics for crossroad ramp terminal SDF development ..................... 343 TABLE 110. Parameter estimation for crossroad ramp terminal SDF ....................................... 344 TABLE 111. Crossroad ramp terminal severity distribution–signalized .................................... 346 TABLE 112. Crossroad ramp terminal severity distribution–unsignalized ................................ 348

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Safety Prediction Methodology and Analysis Tool for Freeways and Interchanges Get This Book
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Prior to this research project, state highway agencies did not have tools for reflecting safety in their decisions concerning freeway and interchange projects.

The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 306: Safety Prediction Methodology and Analysis Tool for Freeways and Interchanges documents a safety prediction method for freeways that is suitable for incorporation in the Highway Safety Manual. Within the document are Appendices A through F: Practitioner Interviews, Database Enhancement, Proposed HSM Freeways Chapter, Proposed HSM Ramps Chapter, Proposed HSM Appendix B for Part C, and Algorithm Description.

Supplemental to the document are an Enhanced Safety Analysis Tool, a User Manual for the Tool, a Workshop Agenda, an Instructor Guide, and a PowerPoint Presentation.

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