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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2021. Safety Performance of Part-Time Shoulder Use on Freeways, Volume 2: Conduct of Research Report. Washington, DC: The National Academies Press. doi: 10.17226/26393.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

    NCHRP Web-Only Document 309: Safety Performance of Part-Time Shoulder Use on Freeways Volume 2: Conduct of Research Report Pete Jenior Kittelson & Associates, Inc. Baltimore, MD James Bonneson Laura Zhao Kittelson & Associates, Inc. Reston, VA Wayne Kittelson Kittelson & Associates, Inc. Portland, OR In association with: Eric Donnell Vikash Gayah Pennsylvania State University University Park, PA Conduct of Research Report for NCHRP Project 17-89 Submitted June 2021 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 309, Volume 2 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 Edward T. Harrigan, Senior Program Officer Anthony P. Avery, Senior Program Assistant Natalie Barnes, Director of Publications Heather DiAngelis, Associate Director of Publications Kathleen Mion, Senior Editorial Assistant NCHRP PROJECT 17-89 PANEL Field of Traffic—Area of Safety Ida van Schalkwyk, Washington State Department of Transportation, Olympia, WA (Chair) Steven Buckley, JEO Consulting Group, Topeka, KS Casey Emoto, Santa Clara Valley Transportation Authority, San Jose, CA Bryan Katz, ToXcel, LLC, Blacksburg, VA Peter Martin, CDM Smith, San Francisco, CA Bonnie Polin, Massachusetts Department of Transportation, Boston, MA Julius Rinosa, Ferrovial Services, Middleburg, FL Guohui Zhang, University of Hawai'i, Manoa, Honolulu, HI George Merritt, FHWA Liaison Kelly Hardy, AASHTO Liaison AUTHOR ACKNOWLEDGMENTS The research described in this report was performed under NCHRP Project 17-89. The report was written by Pete Jenior, Dr. James Bonneson, Laura Zhao, and Wayne Kittelson of Kittelson & Associates, Inc. and by Dr. Eric Donnell and Dr. Vikash Gayah of The Pennsylvania State University. Dr. Justin Geistefeldt provided strategic direction and information from Europe for the literature review portion of the project and Dr. Fred Mannering provide strategic direction and senior review of random parameter and latent class modeling efforts. Dr. Anusha Musunuru of Kittelson & Associates assisted the research team with data collection. Renato Guadamuz and Houjun Tang of The Pennsylvania State University assisted research team with modeling. The authors wish to thank the Highway Safety Information Service for providing crash data for the states of Minnesota and Ohio, and the Departments of Transportation in Colorado, Georgia, Hawaii, and Virginia for providing data from their respective states.

v Contents Summary ........................................................................................................................................................... 1  CHAPTER 1‐ INTRODUCTION ............................................................................................................................. 3  Report Organization ........................................................................................................................................... 4  Related Documents ............................................................................................................................................ 4  CHAPTER 2‐LITERATURE REVIEW .................................................................................................................. 5  Introduction ....................................................................................................................................................... 5  Background ........................................................................................................................................................ 5  Safety and Operational Effects of PTSU and BOS................................................................................................. 7  Design Elements and Traffic Characteristics Affecting Freeway Safety ................................................................ 11  Operational Strategies Affecting Freeway Safety ............................................................................................... 13  Knowledge Gaps ............................................................................................................................................... 15  CHAPTER 3‐MODELING APPROACHES ........................................................................................................ 17  Site Types ......................................................................................................................................................... 17  Predictive Model ............................................................................................................................................... 18  CHAPTER 4‐PROJECT DATABASE .................................................................................................................. 24  Data Collection Activities .................................................................................................................................. 24  Database Summary ........................................................................................................................................... 37  Exploratory Analysis .......................................................................................................................................... 53  CHAPTER 5‐HSM PREDICTIVE MODEL ........................................................................................................ 56  Predictive Model Form ...................................................................................................................................... 57  Modeling Approach .......................................................................................................................................... 60  Statistical Analysis Methods .............................................................................................................................. 63  Fatal‐and‐Injury Crash Frequency Prediction Model ........................................................................................... 64  Property‐Damage‐Only Crash Frequency Prediction Model ................................................................................ 95  CHAPTER 6‐ CRASH SEVERITYAND CRASH TYPE DISTRIBUTIONS ................................................. 115  Severity Distribution ....................................................................................................................................... 116  Crash Type Distribution ................................................................................................................................... 139  CHAPTER 7‐ADVANCED CRASH PREDICTION MODELS ....................................................................... 148  Methodological Background ............................................................................................................................ 148  Highway Safety Database ................................................................................................................................ 152  Model Estimation ............................................................................................................................................ 153  Model Comparisons ........................................................................................................................................ 170  Validation ....................................................................................................................................................... 172  Empirical Fixed Parameters Model .................................................................................................................. 176 

vi CHAPTER 8‐SUPPLEMENTAL SAFETY FINDINGS ................................................................................... 179  Introduction .................................................................................................................................................... 179  Overview of Statistical Modeling Approach ..................................................................................................... 180  Question 1: Safety Effect of BOS and PTSU ...................................................................................................... 183  Questions 2a and 2b: Safety Effect of PTSU Presence ....................................................................................... 194  Question 3: Safety Effect of Converting BOS to PTSU ....................................................................................... 198  Question 4: Safety Effect of Left versus Right Shoulder .................................................................................... 198  Question 5a: Safety Effect of Dynamic Signage ................................................................................................ 202  Question 5b: Safety Effect of Dynamic PTSU .................................................................................................... 203  Question 6: Safety Effect of Shoulder Width .................................................................................................... 205  Question 7: Safety Effect of PTSU on Severity .................................................................................................. 208  Summary of Findings ....................................................................................................................................... 217  CHAPTER 9‐CONCLUSIONS ............................................................................................................................ 218  REFERENCES ....................................................................................................................................................... 222  ACRONYMS .......................................................................................................................................................... 227  APPENDIX A‐DATA COLLECTION PROCEDURES GUIDE ...................................................................... 228  Purpose of Document ..................................................................................................................................... 228  Definition and Measurement of Geometric Data ............................................................................................. 228  Quality Assurance/Quality Control .................................................................................................................. 241  APPENDIX B‐DATABASE DICTIONARY ...................................................................................................... 243  Introduction .................................................................................................................................................... 243  Site Data ......................................................................................................................................................... 243  Crash Data ...................................................................................................................................................... 267  PTSU Open/Close Status ................................................................................................................................. 270  Master Dataset Examples ................................................................................................................................ 270  APPENDIX C‐OHIO CRASH ASSIGNMENT RULES .................................................................................... 272  I‐71 ................................................................................................................................................................. 272  I‐275 ............................................................................................................................................................... 273  I‐70 ................................................................................................................................................................. 274 

vii List of Figures Figure 1. Components of a freeway with PTSU operation. ..................................................................................... 7  Figure 2. Illustrative sites for one-directional freeway facility evaluation. .................................................. 18  Figure 3. Relationship between turnout spacing and FI crash rate ratio. ...................................................... 54  Figure 4. Relationship between proportion of time PTSU operating and FI crash rate. .......................... 55  Figure 5. Example calculation of the length of transition zone within a site. ............................................... 59  Figure 6. Predicted versus reported FI crash frequency using Georgia validation data. ......................... 73  Figure 7. Predicted versus reported FI crashes on freeway segments. .......................................................... 76  Figure 8. Predicted versus reported FI crashes at ramp entrance speed-change lanes. ......................... 77  Figure 9. Predicted versus reported FI crashes at ramp exit speed-change lanes. .................................... 78  Figure 10. Estimated horizontal curve AF for FI crashes. ..................................................................................... 79  Figure 11. Estimated lane width AF for FI crashes. ................................................................................................. 80  Figure 12. Estimated inside shoulder width AF for FI crashes. .......................................................................... 81  Figure 13. Estimated inside shoulder rumble strip AF for FI crashes. ............................................................ 82  Figure 14. Estimated median width AF for FI crashes. .......................................................................................... 83  Figure 15. Lane change AF as a function of distance from ramp gore – FI crashes. .................................. 84  Figure 16. Lane change AF for segments between a pair of interchanges – FI crashes. .......................... 85  Figure 17. Estimated outside shoulder width AF for FI crashes. ....................................................................... 86  Figure 18. Estimated outside clearance AF for FI crashes. ................................................................................... 87  Figure 19. Estimated ramp entrance AF for FI crashes. ........................................................................................ 89  Figure 20. Estimated ramp exit AF for FI crashes. ................................................................................................... 90  Figure 21. Estimated turnout presence AF for FI crashes. ................................................................................... 91  Figure 22. Estimated freeway segment model for FI crashes. ............................................................................ 94  Figure 23. Estimated ramp entrance model for FI crashes. ................................................................................. 94  Figure 24. Estimated ramp exit model for FI crashes............................................................................................. 95  Figure 25. Predicted vs. reported PDO crashes on freeway segments. ........................................................... 99  Figure 26. Predicted versus reported PDO crashes at ramp entrance speed-change lanes. ............... 100  Figure 27. Predicted versus reported PDO crashes at ramp exit speed-change lanes. ......................... 101  Figure 28. Estimated horizontal curve AF for PDO crashes. ............................................................................. 102  Figure 29. Estimated lane width AF for PDO crashes. ......................................................................................... 102  Figure 30. Estimated inside shoulder width AF for PDO crashes. .................................................................. 103  Figure 31. Estimated median width AF for PDO crashes. .................................................................................. 104  Figure 32. Estimated outside shoulder width AF for PDO crashes. ............................................................... 106  Figure 33. Estimated outside clearance AF for PDO crashes. ........................................................................... 107  Figure 34. Estimated ramp entrance AF for PDO crashes. ................................................................................ 108  Figure 35. Estimated ramp exit AF for PDO crashes. ........................................................................................... 109  Figure 36. Estimated turnout presence AF for PDO crashes. ........................................................................... 110  Figure 37. Estimated freeway segment model for PDO crashes. .................................................................... 113  Figure 38. Estimated ramp entrance model for PDO crashes. ......................................................................... 113  Figure 39. Estimated ramp exit model for PDO crashes. .................................................................................... 114  Figure 40. Examination of proportion of AADT during hours with high volume. ................................... 123  Figure 41. Examination of proportion of site adjacent to barrier. ................................................................. 124  Figure 42. Comparison of HSM SDF with proposed SDFs. ................................................................................. 135  Figure 43. Estimated freeway segment model for FI crashes, RP and HSM Supplement models. .... 158  Figure 44. Estimated median width AF for FI crashes, RP model. ................................................................. 159  Figure 45. Estimated outside shoulder rumble strip AF for FI crashes, RP model. ................................ 160  Figure 46. Fatal and injury class distributions. ...................................................................................................... 163  Figure 47. Estimated freeway segment model for FI crashes, LC and HSM Supplement models. .... 164 

viii Figure 48. Estimated lane width AF for FI crashes. .............................................................................................. 165  Figure 49. Estimated median width AF for FI crashes. ....................................................................................... 165  Figure 50. Estimated inside shoulder width AF for FI crashes. ....................................................................... 166  Figure 51. Estimated outside shoulder width AF for FI crashes. .................................................................... 166  Figure 52. Estimated inside shoulder rumble strip AF for FI crashes. ......................................................... 167  Figure 53. Estimated outside shoulder rumble strip AF for FI crashes. ...................................................... 167  Figure 54. Scatterplot of predicted versus observed FI crash frequency. .................................................. 171  Figure 55. Scatterplot of predicted versus observed PDO crash frequency............................................... 171 

ix List of Tables Table 1. Part-time shoulder use design and operations options. ........................................................................ 6  Table 2. Safety experience with BOS projects. ........................................................................................................... 10  Table 3. Adjustment to free-flow speed for lane width. ........................................................................................ 13  Table 4. Knowledge gaps for PTSU- and BOS-related conditions potentially affecting safety. ............. 16  Table 5. Study facilities. ...................................................................................................................................................... 26  Table 6. Example one-way AADT volume calculation (GDOT 2020). .............................................................. 28  Table 7. Study period dates by state and facility. ..................................................................................................... 35  Table 8. Police-reporting thresholds for PDO crashes by state. ......................................................................... 36  Table 9. Percent of crashes without hour of day by state. .................................................................................... 36  Table 10. Database sample size. ...................................................................................................................................... 38  Table 11. Summary geometric and traffic characteristics. ................................................................................... 39  Table 12. Summary of turnout characteristics. ......................................................................................................... 40  Table 13. Freeway segment data summary for PTSU facilities. ......................................................................... 41  Table 14. Freeway segment data summary for comparison and supplemental facilities. ...................... 41  Table 15. Ramp entrance speed-change lane site data summary for PTSU facilities. ............................... 42  Table 16. Ramp entrance speed-change lane site data summary for comparison and supplemental facilities. ..................................................................................................................................................................................... 42  Table 17. Ramp exit speed-change lane site data summary for PTSU facilities. ......................................... 43  Table 18. Ramp exit speed-change lane site data summary for comparison and supplemental facilities. ..................................................................................................................................................................................... 43  Table 19. Freeway segment barrier and PTSU characteristics for PTSU facilities. .................................... 44  Table 20. Freeway segment barrier characteristics for comparison and supplemental facilities. ...... 45  Table 21. Ramp entrance speed-change lane site barrier and PTSU characteristics for PTSU facilities. ..................................................................................................................................................................................... 46  Table 22. Ramp entrance speed-change lane site barrier characteristics for comparison and supplemental facilities. ....................................................................................................................................................... 46  Table 23. Ramp exit speed-change lane site barrier and PTSU characteristics for PTSU facilities. .... 47  Table 24. Ramp exit speed-change lane site barrier characteristics for comparison and supplemental facilities. ....................................................................................................................................................... 47  Table 25. Summary of crash data by state and site type. ...................................................................................... 48  Table 26. Summary of FI crash rates for PTSU and comparison facilities. .................................................... 49  Table 27. Summary of crash rates for various PTSU features. ............................................................................ 50  Table 28. Proportion of each hour PTSU operation is open, except GA 400. ................................................ 51  Table 29. Proportion of each hour PTSU operation is open, GA 400. .............................................................. 52  Table 30. Proportion of a typical day PTSU facility is open. ................................................................................ 53  Table 31. Examination of FI crash rate ratios associated with two conditions. .......................................... 54  Table 32. Predictive model estimation statistics, FI crashes, all site types, four states. .......................... 71  Table 33. Predictive model validation statistics, FI crashes, all site types, four states. ........................... 73  Table 34. Final predictive model estimation statistics, FI crashes, all site types, five states. ................ 75  Table 35. Final predictive model estimation statistics, FI crashes, freeway segment, five states. ...... 76  Table 36. Final predictive model estimation statistics, FI crashes, ramp entrance speed-change lane, five states. ................................................................................................................................................................................. 77  Table 37. Final predictive model estimation statistics, FI crashes, ramp exit speed-change lane, five states. .......................................................................................................................................................................................... 78  Table 38. Estimated PTSU operation AF for FI crashes. ........................................................................................ 93  Table 39. Final predictive model estimation statistics, PDO crashes, all site types, five states. ........... 97  Table 40. Final predictive model estimation statistics, PDO crashes, freeway segment, five states. . 98 

x Table 41. Final predictive model estimation statistics, PDO crashes, ramp entrance speed-change lane, five states. ...................................................................................................................................................................... 99  Table 42. Final predictive model estimation statistics, PDO crashes, ramp exit speed-change lane, five states. .............................................................................................................................................................................. 100  Table 43. Estimated PTSU operation AF for PDO crashes. ................................................................................ 112  Table 44. Summary geometric and traffic characteristics. ................................................................................ 117  Table 45. Crash count distribution by state, PTSU operation, and severity. .............................................. 119  Table 46. FI crash count distribution by state, PTSU operation, and severity. ......................................... 120  Table 47. FI crash count distribution by PTSU operation, site type, and severity. .................................. 121  Table 48. FI crash proportions by PTSU operation, site type, and severity. .............................................. 122  Table 49. Model development process and decision criteria. .......................................................................... 127  Table 50. Predictive model estimation statistics – severity distribution. ................................................... 131  Table 51. Comparison of predicted severity distribution for freeway segments. ................................... 137  Table 52. Comparison of predicted severity distribution for ramp entrance speed-change lanes. . 138  Table 53. Comparison of predicted severity distribution for ramp exit speed-change lanes............. 139  Table 54. FI crash type distribution by state, site type, and PTSU operation. ........................................... 140  Table 55. FI crash type distribution by site type and PTSU operation. ........................................................ 141  Table 56. PDO crash type distribution by state, site type, and PTSU operation. ...................................... 142  Table 57. PDO crash type distribution by site type and PTSU operation. ................................................... 142  Table 58. Crash distribution example. ....................................................................................................................... 144  Table 59. Proposed FI crash type distribution. ...................................................................................................... 146  Table 60. Proposed PDO crash type distribution. ................................................................................................. 147  Table 61. Fixed parameters model for FI crashes. ................................................................................................ 154  Table 62. Random parameters model for FI crashes. .......................................................................................... 156  Table 63. PTSU adjustment factors in random parameters model for FI crashes. .................................. 161  Table 64. Latent class model for FI crashes. ............................................................................................................ 162  Table 65. Fixed parameters model for PDO crashes. ........................................................................................... 168  Table 66. Random parameters model for PDO crashes. ..................................................................................... 169  Table 67. Fixed parameters model for FI crashes (no Georgia data). ........................................................... 173  Table 68. Random parameters model for FI crashes (no Georgia data). ..................................................... 174  Table 69. Model validation comparisons. ................................................................................................................. 176  Table 70. Predictive model re-estimation statistics for FI crashes based on empirical approach. .. 177  Table 71. Predictive model re-estimation statistics for PDO crashes based on empirical approach. .................................................................................................................................................................................................... 177  Table 72. AFs included in models based on two modeling approaches. ..................................................... 178  Table 73. Descriptive statistics of BOS sites. ........................................................................................................... 184  Table 74. Fixed parameters model of total crashes for BOS sites. ................................................................. 186  Table 75. Fixed parameters model of FI crashes for BOS sites. ....................................................................... 187  Table 76. Descriptive statistics of PTSU sites. ........................................................................................................ 189  Table 77. Summary of crash data by site type. ....................................................................................................... 189  Table 78. Estimated PTSU operation AF for FI crashes. ..................................................................................... 191  Table 79. Comparison of predicted severity distribution for freeway segments. ................................... 192  Table 80. Change in crash frequency, severity, and cost associated with PTSU operation. ................ 193  Table 81. Descriptive statistics of PTSU sites and comparison sites. ........................................................... 195  Table 82. Fixed parameters model of total crashes per hour for PTSU sites. ............................................ 197  Table 83. Descriptive statistics of PTSU sites and PTSU comparison sites. ............................................... 199  Table 84. Fixed parameters model of total crashes for PTSU sites. ............................................................... 201  Table 85. Descriptive statistics of sites with static PTSU. .................................................................................. 204  Table 86. Fixed parameters model of total crashes for sites with static PTSU. ........................................ 205  Table 87. Total crash frequency CMF for converting PTSU site from static to dynamic operation. 205 

xi Table 88. Descriptive statistics of sites with PTSU. .............................................................................................. 206  Table 89. Fixed parameters model of total crashes for PTSU sites. ............................................................... 207  Table 90. Summary of PTSU crash severity data. .................................................................................................. 210  Table 91. Summary of BOS crash severity data. .................................................................................................... 211  Table 92. Fatal-and-severe-injury crash severity model for PTSU freeway segments. ......................... 212  Table 93. Fatal-and-severe-injury crash severity model for comparison freeway segments. ........... 212  Table 94. Fatal-and-severe-injury crash severity model for matched PTSU freeway segments. ...... 213  Table 95. Fatal-and-severe-injury crash severity model for BOS freeway segments. ........................... 214  Table 96. Fatal-and-severe-injury crash severity model for comparison freeway segments. ........... 215  Table 97. Fatal-and-severe-injury crash severity model for matched BOS freeway segments. ........ 215  Table 98. Summary of Findings .................................................................................................................................... 217 

1 Summary Part-time shoulder use is a congestion relief strategy that allows use of the left or right shoulders as travel lanes during some, but not all, hours of the day. PTSU operation in the United States (US) has been exclusively on freeways, and findings presented in this document are from studies of freeways. This project prepared models that predict (1) the frequency of fatal-and-injury (FI) crashes and (2) the frequency of property-damage-only (PDO) crashes. These models were developed and applied to assess the overall effect of PTSU on total and severe crash frequency. Models were developed for freeway segments, ramp entrance speed-change lane sites, and ramp exit speed-change lane sites. Models consider crashes on all lanes of the freeway, not just the shoulder used for PTSU. A severity distribution function was also developed to predict the distribution of crashes among different severity levels. Each model includes variables that describe the traffic demand characteristics, geometric elements, and PTSU operational features. The models are recommended for inclusion in the next edition of the Highway Safety Manual (HSM). The variables related to PTSU design and operation that result in different crash frequency predictions are provided in the following list:  Proportion of time that PTSU operates.  PTSU lane width.  Proportion of segment length with PTSU transition zone present.  Number of through lanes on the freeway.  Proportion of segment length with turnout present.  Turnout spacing. Transition zones are locations upstream, downstream, or between portions of a freeway with a PTSU typical section. Turnouts are paved areas adjacent to a shoulder used for PTSU that function as refuge areas for disabled vehicles. When applied, the models indicate the following characteristics:  Urban freeway segments with PTSU operation are typically associated with a larger FI crash frequency than those segments without PTSU operation.  FI crash frequency was found to be higher on segments where the proportion of time that PTSU operates is larger.  Segments with a turnout present were associated with a smaller FI crash frequency than those sites without a turnout present.  Segments with an average lane width of 11 feet were found to have a larger FI crash frequency than those segments with an average lane width of 12 feet. This project initially explored the safety performance of sites with bus-on-shoulder (BOS) operation as well. The analysis found that the presence of BOS operation did not have a statistically significant effect on total crash frequency or fatal-and-injury crash frequency. Additional modeling of crash data was conducted to examine differences in safety performance of various PTSU features and operating strategies, and findings include the following.  Hours in which the shoulder is open are associated with a 137% higher total crash frequency than hours in which the shoulder is closed, all other factors being the same (e.g., volume, lane width, etc.).  During the hours in which the shoulder is closed, there is no practical difference in the safety performance between sites without PTSU and sites with PTSU that is closed to traffic.

2  The difference in safety performance between sites with left-side PTSU and sites with right-side PTSU was not statistically significant, but the sample of left-side PTSU sites was small and should be an area of future research.  There is no practical difference in the safety performance between sites with dynamic signs and sites with statis signs.  Facilities converted from static operation to dynamic operation during the study period has a 7.3 percent decrease in total crashes following conversion.

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Part-time shoulder use is a congestion relief strategy that allows use of the left or right shoulders as travel lanes during some, but not all, hours of the day.

The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 309: Safety Performance of Part-Time Shoulder Use on Freeways, Volume 2: Conduct of Research Report describes the development of crash prediction models for freeways with PTSU operation.

Supplemental to the document is a Freeway Analysis Tool, which includes BOS data, S D PTSU Data, and a Prediction Tool, as well as NCHRP Web-Only Document 309: Safety Performance of Part-Time Shoulder Use on Freeways, Volume 1: Informational Guide and Safety Evaluation Guidelines.

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