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Suggested Citation:"CONTENTS." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"CONTENTS." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"CONTENTS." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"CONTENTS." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"CONTENTS." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Page 8
Page 9
Suggested Citation:"CONTENTS." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"CONTENTS." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"CONTENTS." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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Suggested Citation:"CONTENTS." National Academies of Sciences, Engineering, and Medicine. 2021. Development of a Posted Speed Limit Setting Procedure and Tool. Washington, DC: The National Academies Press. doi: 10.17226/26200.
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NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool v CONTENTS Summary ........................................................................................................................................ 1  Chapter 1. Background ................................................................................................................ 4  Objective and Scope ................................................................................................................... 4  Organization of Web-Only Document ........................................................................................ 4  Chapter 2. Research Approach ................................................................................................... 6  Research Methodology for NCHRP Project 17-76 ..................................................................... 6  Phase I Efforts ............................................................................................................................. 6  Phase II Efforts ........................................................................................................................... 7  Chapter 3. Findings ...................................................................................................................... 8  Phase I Efforts ............................................................................................................................. 8  Phase I. Conduct Project Management ............................................................................... 8  Phase I. Identify Existing Procedures for Setting Posted Speed Limit ............................... 8  Phase I. Review Literature .................................................................................................. 8  Phase I. Identify Potential Operating Speed Datasets ......................................................... 9  Phase I. Obtain Samples of Speed Data .............................................................................. 9  Phase I. Review Other Techniques for Selecting Posted Speed Limits .............................. 9  Phase II Efforts ......................................................................................................................... 10  Phase II. Build and Analyze Austin, Texas, Database ...................................................... 10  Phase II. Build and Analyze Washtenaw County, Michigan, Database ........................... 10  Phase II. Develop SLS-Procedure ..................................................................................... 10  Phase II. Develop SLS-Tool ............................................................................................. 13  Phase II. Conduct Technology Transfer Activities ........................................................... 13  Phase II. Document Research ........................................................................................... 14  Chapter 4. Conclusions, Options, and Suggested Research .................................................... 15  Conclusions ............................................................................................................................... 15  Options ...................................................................................................................................... 15  Suggested Research .................................................................................................................. 16  Appendix A. Relationship among Roadway Characteristics, Speed, and Safety for Urban/Suburban Streets ................................................................................................ 19  Traffic—Vehicles ..................................................................................................................... 19  Annual Average Daily Traffic Relationship with Crashes ............................................... 19 

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool vi Congestion Relationship with Crashes ............................................................................. 19  Percent Trucks Relationship with Crashes ....................................................................... 19  Operating Speed Relationship with Crashes ..................................................................... 19  Percentage of Trucks Relationship with Operating Speed ............................................... 20  Vehicle Characteristics Relationship with Operating Speed ............................................ 20  Traffic Control Device .............................................................................................................. 20  Posted Speed Limit Relationship with Crashes ................................................................ 20  Posted Speed Limit Relationship with Operating Speed .................................................. 21  Signalized Intersection Presence Relationship with Crashes ............................................ 22  Roadway Geometry .................................................................................................................. 22  Horizontal Alignment Relationship with Crashes ............................................................ 22  Horizontal or Vertical Alignment Relationship with Operating Speed ............................ 22  Vertical Alignment Relationship with Crashes ................................................................. 22  Median Relationship with Crashes ................................................................................... 22  Median Relationship with Operating Speed ..................................................................... 23  Median Width Relationship with Crashes ........................................................................ 23  Median Width Relationship with Operating Speed .......................................................... 23  Number of Lanes Relationship with Crashes .................................................................... 23  Number of Lanes Relationship with Operating Speed ..................................................... 24  Lane Width Relationship with Crashes............................................................................. 24  Lane Width Relationship with Operating Speed .............................................................. 24  Shoulder Width Relationship with Crashes ...................................................................... 24  Curb or Shoulder Relationship with Operating Speed ...................................................... 24  Bicycle Lanes Relationship with Crashes ......................................................................... 24  Bicycle Lane Presence Relationship with Operating Speed ............................................. 25  Segment Length Relationship with Crashes ..................................................................... 25  Segment Limit Relationship with Operating Speed .......................................................... 25  Intersection Angle Relationship with Crashes .................................................................. 25  Intersection Lighting Relationship with Crashes .............................................................. 26  On-Street Parking Relationship with Operating Speed .................................................... 26  Surroundings/Roadside ............................................................................................................. 26  Access Density (Driveways and Intersections) Relationship with Crashes ...................... 26  Access Density (Driveways and Intersections) Relationship with Operating Speed ....... 26 

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool vii School/School Zone Relationship with Crashes ............................................................... 26  School/School Zone Relationship with Operating Speed ................................................. 26  Parking Relationship with Crashes ................................................................................... 27  Liquor Store Relationship with Crashes ........................................................................... 27  Sidewalk Presence Relationship with Operating Speed ................................................... 27  Roadside Hazard Rating or Pole/Tree Density Relationship with Operating Speed ........ 27  Development (Surrounding Land Use) Relationship with Operating Speed .................... 27  Other Variables ......................................................................................................................... 28  One-Way or Two-Way Relationship with Operating Speed ............................................ 28  Functional Classification Relationship with Operating Speed ......................................... 28  Weather Relationship with Operating Speed .................................................................... 28  Time of the Day Relationship with Operating Speed ....................................................... 28  Overview of Variable Relationship with Speed and Crashes ................................................... 29  Appendix B. Relationship among Roadway Characteristics, Speed, and Safety for High-Speed Highways ..................................................................................................... 31  Traffic—Vehicles ..................................................................................................................... 31  Average Daily Traffic Relationship with Crashes ............................................................ 31  Average Daily Traffic Relationship with Operating Speed .............................................. 31  Percent Trucks Relationship with Crashes ....................................................................... 31  Percent Trucks Relationship with Operating Speed ......................................................... 31  Operating Speed Relationship with Crashes ..................................................................... 32  Traffic Control Devices ............................................................................................................ 32  Posted Speed Limit Relationship with Crashes ................................................................ 32  Posted Speed Limit Relationship with Operating Speed .................................................. 33  No-Passing Zone Relationship with Crashes .................................................................... 33  Passing Lane Relationship with Crashes .......................................................................... 33  Passing Lane Relationship with Operating Speed ............................................................ 34  Roadway Geometry .................................................................................................................. 34  Horizontal Alignment Relationship with Crashes ............................................................ 34  Horizontal Alignment Relationship with Operating Speed .............................................. 34  Vertical Alignment Relationship with Crashes ................................................................. 35  Vertical Alignment Relationship with Operating Speed .................................................. 35  Terrain Relationship with Crashes .................................................................................... 35 

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool viii Median Relationship with Crashes ................................................................................... 35  Median Width Relationship with Crashes ........................................................................ 35  Median Relationship with Operating Speed ..................................................................... 36  Median Width Relationship with Operating Speed .......................................................... 36  Number of Lanes Relationship with Crashes .................................................................... 36  Lane Width Relationship with Crashes............................................................................. 36  Lane Width Relationship with Operating Speed .............................................................. 36  Shoulder Width Relationship with Crashes ...................................................................... 36  Shoulder Width Relationship with Operating Speed ........................................................ 36  Lane Position Relationship with Operating Speed ........................................................... 37  Surroundings ............................................................................................................................. 37  Access Density (Driveways and Intersections) Relationship with Crashes ...................... 37  Access Density (Driveways and Intersections) Relationship with Operating Speed ....... 37  Urban or Rural Relationship with Crashes ....................................................................... 38  School Relationship with Crashes .................................................................................... 38  Development (Surrounding Land Use) Relationship with Operating Speed .................... 38  Other Variables ......................................................................................................................... 38  Time of the Day Relationship with Operating Speed ....................................................... 38  Overview of Variable Relationship with Speed and Crashes ................................................... 38  Appendix C. Speed Limit Setting Approaches ......................................................................... 41  Recent Key Publications Calling for Changes to How Speed Limits Are Set ......................... 42  Overview on Approaches to Setting Speed Limits ................................................................... 42  Engineering (Operating Speed) ......................................................................................... 42  Engineering (Road Risk) ................................................................................................... 42  Expert System Example .................................................................................................... 45  Citywide or Default Speed Limits .................................................................................... 48  Slow Zones........................................................................................................................ 50  Manual on Uniform Traffic Control Devices ........................................................................... 50  Posted Speed Limit Compared to Operating Speed .................................................................. 54  City Streets Using Donated Spot Speed Studies ............................................................... 54  Freeways and Two-Lane Highways Using Naturalistic Driving Study Data ................... 58  Online Review of State Guidance for Setting Speed Limits .................................................... 59  Factors Considered, Especially 85th Percentile Speed ..................................................... 60 

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool ix Rounding of Speed Value ................................................................................................. 62  Comparison between Existing Speed Limit Setting Approaches ............................................. 62  Discussion of Speed Limit Approaches .................................................................................... 67  Appendix D. Relationship among Urban/Suburban Roadway Characteristics, Operating Speed, and Crashes in Austin, Texas .......................................................... 68  Overview ................................................................................................................................... 68  Database Developmemt ............................................................................................................ 68  Operating Speed Data from On-Road Tubes .................................................................... 68  Crash Records Information System .................................................................................. 71  Average Annual Daily Traffic .......................................................................................... 71  Roadway Geometry and Traffic Control Device Characteristics Data ............................. 72  Data Integration Overview ................................................................................................ 73  Descriptive Statistics ......................................................................................................... 76  Round 1: Data Analysis Using Negative Binomial Regression ............................................... 80  Overview ........................................................................................................................... 80  Variable Relationships with Crashes ................................................................................ 81  Variable Relationships with Posted Speed Limit.............................................................. 84  Round 2: Data Analysis Using Path Analysis Approach .......................................................... 85  Overview ........................................................................................................................... 85  Results and Discussion ..................................................................................................... 87  Round 3: Data Analysis Using Path Analysis Approach and Segments with Posted Speed Limits of 45 mph and Lower .............................................................................. 91  Overview ........................................................................................................................... 91  Results and Discussion ..................................................................................................... 93  Findings’ Impact on SLS-Tool ................................................................................................. 94  Appendix E. Relationship among Urban/Suburban Roadway Characteristics, Posted Speed Limit, and Crashes in Washtenaw County, Michigan ......................... 95  Overview ................................................................................................................................... 95  Database Development ............................................................................................................. 95  Roadway Inventory Data .................................................................................................. 96  Identification of Study Roadway Segments ...................................................................... 96  Historical Traffic Crash Data ............................................................................................ 99  Additional Geometric and Roadway Characteristics ...................................................... 100 

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool x Data Analysis—Methodology ................................................................................................ 101  Regression Model Overview........................................................................................... 101  Results and Discussion ........................................................................................................... 102  Variable Relationships with Crashes .............................................................................. 102  Variable Relationships with Posted Speed Limit............................................................ 105  Findings’ Impact on SLS-Tool ............................................................................................... 105  Appendix F. Decision Rule Development................................................................................ 107  SLSG = Limited Access ......................................................................................................... 107  Overview ......................................................................................................................... 107  Decision Rule Development ........................................................................................... 107  Input Variables ................................................................................................................ 112  Warning Messages .......................................................................................................... 112  SLSG = Undeveloped ............................................................................................................. 113  Overview ......................................................................................................................... 113  Decision Rule Development ........................................................................................... 113  Input Variables ................................................................................................................ 121  Warning Messages .......................................................................................................... 122  SLSG = Developed ................................................................................................................. 123  Overview ......................................................................................................................... 123  Decision Rule Development ........................................................................................... 123  Input Variables ................................................................................................................ 131  Warning Messages .......................................................................................................... 132  SLSG = Full Access ................................................................................................................ 132  Overview ......................................................................................................................... 132  Decision Rule Development ........................................................................................... 132  Input Variables ................................................................................................................ 135  Warning Messages .......................................................................................................... 136  Appendix G. Workshop Slides ................................................................................................. 137  Abbreviations ............................................................................................................................ 159  References .................................................................................................................................. 163   Note: The contractor’s report published herein is associated with NCHRP Research Report 966: Posted  Speed Limit Setting Procedure and Tool: User Guide. Readers can read or purchase NCHRP Research  Report 966 at www.trb.org.  

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool xi LIST OF TABLES Page Table 1. Factors of urban/suburban city streets that affect crashes. ............................................. 29  Table 2. Factors of urban/suburban streets that affect operating speed. ....................................... 30  Table 3. Factors of high-speed highways that affect crashes. ....................................................... 39  Table 4. Factors of high-speed highways that affect operating speed. ......................................... 40  Table 5. Example of factors that could influence a driver’s speed choice. .................................. 41  Table 6. Approaches to setting speed limits. ................................................................................ 43  Table 7. Advantages and disadvantages for approaches to set speed limits. ................................ 44  Table 8. Input variables required for USLIMITS2. ...................................................................... 48  Table 9. USLIMITS2 suggested minimum length per speed limit. .............................................. 48  Table 10. Speeds and proportion of vehicles exceeding a speed from Boston citywide speed limit change study. .................................................................................................. 49  Table 11. NCHRP Research Report 855 suggested target speed for context/roadway. ............... 53  Table 12. Characteristics of roadway contexts. ............................................................................ 54  Table 13. Number of speed studies by roadway type and state. ................................................... 55  Table 14. Factors typically considered by U.S. states in setting posted speed limits based on material found online for 31 states............................................................................... 61  Table 15. Comparison of input variables used in speed prediction or suggested posted speed limit procedures for urban/suburban (or developed) areas (variables A–C). ......... 64  Table 16. Comparison of input variables used in speed prediction or suggested posted speed limit procedures for urban/suburban (or developed) areas (variables L–O). .......... 65  Table 17. Comparison of input variables used in speed prediction or suggested posted speed limit procedures for urban/suburban (or developed) areas (variables P–S). ........... 66  Table 18. Comparison of input variables used in speed prediction or suggested posted speed limit procedures for urban/suburban (or developed) areas (variable T). ................ 67  Table 19. Austin on-road binned tube data file format. ................................................................ 69  Table 20. Number of segments by functional classification. ........................................................ 71  Table 21. Number of segments by posted speed limit. ................................................................. 71  Table 22. Number of segments by number of lanes. .................................................................... 71  Table 23. Roadway and traffic control device variables for City of Austin, Texas. .................... 72  Table 24. Initial speed measures developed. ................................................................................ 76  Table 25. Descriptive statistics of variables for City of Austin, Texas. ....................................... 77  Table 26. Score statistics for model shown in Table 27. .............................................................. 82  Table 27. Variables with significant effects on KABC segment (KABC_NID) crashes. ............ 82  Table 28. Score statistics for model shown in Table 29. .............................................................. 83  Table 29. Variables with significant effects on KABCO segment (KABCO_NID) crashes. ....... 84  Table 30. Variables with significant effects on posted speed limit. ............................................. 85  Table 31. Round 2 speed measures considered. ........................................................................... 86  Table 32. Roadway characteristic variables considered in path analysis. .................................... 86  Table 33. Estimated regression coefficients for KABC_NID crashes by path analysis (mediator variable: PSL, CoefVar, PerOvPSL, StdSpd, Pace, or SpdAve). ..................... 88  Table 34. Estimated regression coefficients for KABC_NID crashes by path analysis (mediator variable: Abs(PSL−Avg), PSL−Avg, PSL−S85, or S85−Avg). ...................... 89 

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool xii Table 35. Estimated regression coefficients for KABCO_NID crashes by path analysis (mediator variable: PSL, CoefVar, PerOvPSL, StdSpd, Pace, or SpdAve). ..................... 90  Table 36. Estimated regression coefficients for KABCO_NID crashes by path analysis (mediator variable: Abs(PSL−Avg), PSL−Avg, PSL−S85, or S85−Avg). ...................... 91  Table 37. Estimated regression coefficients for KABC_NID crashes by path analysis with mediator variable Abs(PSL−Avg) based on 649 sites. ..................................................... 93  Table 38. Summary of criteria for inclusion as potential study roadway. .................................... 97  Table 39. Roadway and traffic control device variables for Washtenaw County, Michigan. ........................................................................................................................ 100  Table 40. Descriptive statistics of variables for Washtenaw County, Michigan. ....................... 101  Table 41. Score statistics for model shown in Table 42. ............................................................ 102  Table 42. Variables with significant effects on KABC segment crashes. .................................. 103  Table 43. Score statistics for model shown in Table 46. ............................................................ 104  Table 44. Variables with significant effects on KABCO segment crashes. ............................... 104  Table 45. Models exploring the significance of variables for posted speed limit. ..................... 106  Table 46. Overview of decision rules for SLSG = Limited Access............................................ 108  Table 47. Average KABCO crash rate per 100 MVM for Limited Access SLSG. .................... 110  Table 48. Average KABC crash rate per 100 MVM for Limited Access SLSG. ....................... 111  Table 49. Minimum section length for a particular speed limit. ................................................. 113  Table 50. Overview of decision rules for SLSG = Undeveloped. .............................................. 115  Table 51. Average crash rate per 100 MVM for Undeveloped SLSG (i.e., context = rural with roadway types of principal arterial, minor arterial, collector, or local). ................. 119  Table 52. Average F&I crash rate per 100 MVM for Undeveloped SLSG (i.e., context = rural with roadway types of principal arterial, minor arterial, collector, or local). ........ 120  Table 53. RHR for undeveloped roadways. ................................................................................ 122  Table 54. Overview of decision rules for SLSG = Developed. .................................................. 124  Table 55. Decision matrix for sidewalk presence/width, sidewalk buffer, and pedestrian activity combinations for Developed SLSG. .................................................................. 125  Table 56. Average KABCO crash rate per 100 MVM from USLIMITS2 for Developed SLSG (i.e., context = urban, suburban, or rural town with roadway types of principal arterial, minor arterial, or collector). ............................................................... 130  Table 57. Average KABC crash rate per 100 MVM from USLIMITS2 for Developed SLSG (i.e., context = urban, suburban, or rural town with roadway types of principal arterial, minor arterial, or collector). ............................................................... 130  Table 58. Overview of decision rules for SLSG = Full Access. ................................................. 133  Table 59. Decision matrix for sidewalk presence/width, sidewalk buffer, and pedestrian activity combinations for Full-Access SLSG. ................................................................ 134     

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool xiii LIST OF FIGURES Page Figure 1. Overview of SLS-Procedure to calculate the suggested speed limit. .............................. 2  Figure 2. Examples of speed limits. ................................................................................................ 4  Figure 3. USLIMITS2 flow chart for speed limit calculation without crash data for roadway section in developed areas. ................................................................................. 47  Figure 4. Operating speed by posted speed limit for urban arterials. ........................................... 56  Figure 5. Operating speed by posted speed limit for urban collectors.......................................... 56  Figure 6. Operating speed by posted speed limit for urban locals. ............................................... 57  Figure 7. Operating speed by posted speed limit for city streets. ................................................. 57  Figure 8. Using box plots to illustrate average operating speed by posted speed limit for city streets. ........................................................................................................................ 58  Figure 9. Austin road tube traffic counter locations. .................................................................... 70  Figure 10. Example of crashes (dots) and road tube locations (squares with labels starting with ST) for a road in Austin. ........................................................................................... 74  Figure 11. Example of crash assignment using 30-ft buffer along a sample road segment. ........ 75  Figure 12. Operating speed versus posted speed limit for city streets in Austin. ......................... 78  Figure 13. Violin plots for average speed (SpdAve) for different geometric features. ................ 79  Figure 14. Violin plots for percent of vehicles over the posted speed limit (PerOverPSL) for different geometric features. ....................................................................................... 80  Figure 15. Comparison of crash rate to the difference between posted speed limit and average speed. ................................................................................................................... 92  Figure 16. Path analysis findings for segments with posted speed limits of 20 to 45 mph. ......... 94  Figure 17. Location of study roadway segments in Washtenaw County, Michigan. ................... 95  Figure 18. Washtenaw County roadway network and urban boundaries. .................................... 96  Figure 19. Segmentation of study road segments in Washtenaw County, Michigan. .................. 97  Figure 20. Map of study road segments in Washtenaw County, Michigan. ................................. 98  Figure 21. Study road segments by posted speed limit. ................................................................ 98  Figure 22. Study road segments by AADT estimate. ................................................................... 99  Figure 23. Example of traffic crash locations along study road segments (2013–2017). ............. 99  Figure 24. Crash prediction using HSM equations for four-lane undivided and divided roadways. ........................................................................................................................ 116 

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Several types of speed limits exist, including statutory speed limit, posted speed limit, school zone speed limit, work zone speed limit, variable speed limit, and advisory speed.

The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool documents the research efforts and findings from an NCHRP Project 17-76 to identify factors that influence a driver’s operating speed and the development of a Speed Limit Setting Procedure and Tool.

The document is supplemental to NCHRP Research Report 966: Posted Speed Limit Setting Procedure and Tool: User Guide.

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