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NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool v CONTENTS Summary ........................................................................................................................................ 1Â Chapter 1. Background ................................................................................................................ 4Â Objective and Scope ................................................................................................................... 4Â Organization of Web-Only Document ........................................................................................ 4Â Chapter 2. Research Approach ................................................................................................... 6Â Research Methodology for NCHRP Project 17-76 ..................................................................... 6Â Phase I Efforts ............................................................................................................................. 6Â Phase II Efforts ........................................................................................................................... 7Â Chapter 3. Findings ...................................................................................................................... 8Â Phase I Efforts ............................................................................................................................. 8Â Phase I. Conduct Project Management ............................................................................... 8Â Phase I. Identify Existing Procedures for Setting Posted Speed Limit ............................... 8Â Phase I. Review Literature .................................................................................................. 8Â Phase I. Identify Potential Operating Speed Datasets ......................................................... 9Â Phase I. Obtain Samples of Speed Data .............................................................................. 9Â Phase I. Review Other Techniques for Selecting Posted Speed Limits .............................. 9Â Phase II Efforts ......................................................................................................................... 10Â Phase II. Build and Analyze Austin, Texas, Database ...................................................... 10Â Phase II. Build and Analyze Washtenaw County, Michigan, Database ........................... 10Â Phase II. Develop SLS-Procedure ..................................................................................... 10Â Phase II. Develop SLS-Tool ............................................................................................. 13Â Phase II. Conduct Technology Transfer Activities ........................................................... 13Â Phase II. Document Research ........................................................................................... 14Â Chapter 4. Conclusions, Options, and Suggested Research .................................................... 15Â Conclusions ............................................................................................................................... 15Â Options ...................................................................................................................................... 15Â Suggested Research .................................................................................................................. 16Â Appendix A. Relationship among Roadway Characteristics, Speed, and Safety for Urban/Suburban Streets ................................................................................................ 19Â TrafficâVehicles ..................................................................................................................... 19Â Annual Average Daily Traffic Relationship with Crashes ............................................... 19Â
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool vi Congestion Relationship with Crashes ............................................................................. 19Â Percent Trucks Relationship with Crashes ....................................................................... 19Â Operating Speed Relationship with Crashes ..................................................................... 19Â Percentage of Trucks Relationship with Operating Speed ............................................... 20Â Vehicle Characteristics Relationship with Operating Speed ............................................ 20Â Traffic Control Device .............................................................................................................. 20Â Posted Speed Limit Relationship with Crashes ................................................................ 20Â Posted Speed Limit Relationship with Operating Speed .................................................. 21Â Signalized Intersection Presence Relationship with Crashes ............................................ 22Â Roadway Geometry .................................................................................................................. 22Â Horizontal Alignment Relationship with Crashes ............................................................ 22Â Horizontal or Vertical Alignment Relationship with Operating Speed ............................ 22Â Vertical Alignment Relationship with Crashes ................................................................. 22Â Median Relationship with Crashes ................................................................................... 22Â Median Relationship with Operating Speed ..................................................................... 23Â Median Width Relationship with Crashes ........................................................................ 23Â Median Width Relationship with Operating Speed .......................................................... 23Â Number of Lanes Relationship with Crashes .................................................................... 23Â Number of Lanes Relationship with Operating Speed ..................................................... 24Â Lane Width Relationship with Crashes............................................................................. 24Â Lane Width Relationship with Operating Speed .............................................................. 24Â Shoulder Width Relationship with Crashes ...................................................................... 24Â Curb or Shoulder Relationship with Operating Speed ...................................................... 24Â Bicycle Lanes Relationship with Crashes ......................................................................... 24Â Bicycle Lane Presence Relationship with Operating Speed ............................................. 25Â Segment Length Relationship with Crashes ..................................................................... 25Â Segment Limit Relationship with Operating Speed .......................................................... 25Â Intersection Angle Relationship with Crashes .................................................................. 25Â Intersection Lighting Relationship with Crashes .............................................................. 26Â On-Street Parking Relationship with Operating Speed .................................................... 26Â Surroundings/Roadside ............................................................................................................. 26Â Access Density (Driveways and Intersections) Relationship with Crashes ...................... 26Â Access Density (Driveways and Intersections) Relationship with Operating Speed ....... 26Â
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool vii School/School Zone Relationship with Crashes ............................................................... 26Â School/School Zone Relationship with Operating Speed ................................................. 26Â Parking Relationship with Crashes ................................................................................... 27Â Liquor Store Relationship with Crashes ........................................................................... 27Â Sidewalk Presence Relationship with Operating Speed ................................................... 27Â Roadside Hazard Rating or Pole/Tree Density Relationship with Operating Speed ........ 27Â Development (Surrounding Land Use) Relationship with Operating Speed .................... 27Â Other Variables ......................................................................................................................... 28Â One-Way or Two-Way Relationship with Operating Speed ............................................ 28Â Functional Classification Relationship with Operating Speed ......................................... 28Â Weather Relationship with Operating Speed .................................................................... 28Â Time of the Day Relationship with Operating Speed ....................................................... 28Â Overview of Variable Relationship with Speed and Crashes ................................................... 29Â Appendix B. Relationship among Roadway Characteristics, Speed, and Safety for High-Speed Highways ..................................................................................................... 31Â TrafficâVehicles ..................................................................................................................... 31Â Average Daily Traffic Relationship with Crashes ............................................................ 31Â Average Daily Traffic Relationship with Operating Speed .............................................. 31Â Percent Trucks Relationship with Crashes ....................................................................... 31Â Percent Trucks Relationship with Operating Speed ......................................................... 31Â Operating Speed Relationship with Crashes ..................................................................... 32Â Traffic Control Devices ............................................................................................................ 32Â Posted Speed Limit Relationship with Crashes ................................................................ 32Â Posted Speed Limit Relationship with Operating Speed .................................................. 33Â No-Passing Zone Relationship with Crashes .................................................................... 33Â Passing Lane Relationship with Crashes .......................................................................... 33Â Passing Lane Relationship with Operating Speed ............................................................ 34Â Roadway Geometry .................................................................................................................. 34Â Horizontal Alignment Relationship with Crashes ............................................................ 34Â Horizontal Alignment Relationship with Operating Speed .............................................. 34Â Vertical Alignment Relationship with Crashes ................................................................. 35Â Vertical Alignment Relationship with Operating Speed .................................................. 35Â Terrain Relationship with Crashes .................................................................................... 35Â
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool viii Median Relationship with Crashes ................................................................................... 35Â Median Width Relationship with Crashes ........................................................................ 35Â Median Relationship with Operating Speed ..................................................................... 36Â Median Width Relationship with Operating Speed .......................................................... 36Â Number of Lanes Relationship with Crashes .................................................................... 36Â Lane Width Relationship with Crashes............................................................................. 36Â Lane Width Relationship with Operating Speed .............................................................. 36Â Shoulder Width Relationship with Crashes ...................................................................... 36Â Shoulder Width Relationship with Operating Speed ........................................................ 36Â Lane Position Relationship with Operating Speed ........................................................... 37Â Surroundings ............................................................................................................................. 37Â Access Density (Driveways and Intersections) Relationship with Crashes ...................... 37Â Access Density (Driveways and Intersections) Relationship with Operating Speed ....... 37Â Urban or Rural Relationship with Crashes ....................................................................... 38Â School Relationship with Crashes .................................................................................... 38Â Development (Surrounding Land Use) Relationship with Operating Speed .................... 38Â Other Variables ......................................................................................................................... 38Â Time of the Day Relationship with Operating Speed ....................................................... 38Â Overview of Variable Relationship with Speed and Crashes ................................................... 38Â Appendix C. Speed Limit Setting Approaches ......................................................................... 41Â Recent Key Publications Calling for Changes to How Speed Limits Are Set ......................... 42Â Overview on Approaches to Setting Speed Limits ................................................................... 42Â Engineering (Operating Speed) ......................................................................................... 42Â Engineering (Road Risk) ................................................................................................... 42Â Expert System Example .................................................................................................... 45Â Citywide or Default Speed Limits .................................................................................... 48Â Slow Zones........................................................................................................................ 50Â Manual on Uniform Traffic Control Devices ........................................................................... 50Â Posted Speed Limit Compared to Operating Speed .................................................................. 54Â City Streets Using Donated Spot Speed Studies ............................................................... 54Â Freeways and Two-Lane Highways Using Naturalistic Driving Study Data ................... 58Â Online Review of State Guidance for Setting Speed Limits .................................................... 59Â Factors Considered, Especially 85th Percentile Speed ..................................................... 60Â
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool ix Rounding of Speed Value ................................................................................................. 62Â Comparison between Existing Speed Limit Setting Approaches ............................................. 62Â Discussion of Speed Limit Approaches .................................................................................... 67Â Appendix D. Relationship among Urban/Suburban Roadway Characteristics, Operating Speed, and Crashes in Austin, Texas .......................................................... 68Â Overview ................................................................................................................................... 68Â Database Developmemt ............................................................................................................ 68Â Operating Speed Data from On-Road Tubes .................................................................... 68Â Crash Records Information System .................................................................................. 71Â Average Annual Daily Traffic .......................................................................................... 71Â Roadway Geometry and Traffic Control Device Characteristics Data ............................. 72Â Data Integration Overview ................................................................................................ 73Â Descriptive Statistics ......................................................................................................... 76Â Round 1: Data Analysis Using Negative Binomial Regression ............................................... 80Â Overview ........................................................................................................................... 80Â Variable Relationships with Crashes ................................................................................ 81Â Variable Relationships with Posted Speed Limit.............................................................. 84Â Round 2: Data Analysis Using Path Analysis Approach .......................................................... 85Â Overview ........................................................................................................................... 85Â Results and Discussion ..................................................................................................... 87Â Round 3: Data Analysis Using Path Analysis Approach and Segments with Posted Speed Limits of 45 mph and Lower .............................................................................. 91Â Overview ........................................................................................................................... 91Â Results and Discussion ..................................................................................................... 93Â Findingsâ Impact on SLS-Tool ................................................................................................. 94Â Appendix E. Relationship among Urban/Suburban Roadway Characteristics, Posted Speed Limit, and Crashes in Washtenaw County, Michigan ......................... 95Â Overview ................................................................................................................................... 95Â Database Development ............................................................................................................. 95Â Roadway Inventory Data .................................................................................................. 96Â Identification of Study Roadway Segments ...................................................................... 96Â Historical Traffic Crash Data ............................................................................................ 99Â Additional Geometric and Roadway Characteristics ...................................................... 100Â
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool x Data AnalysisâMethodology ................................................................................................ 101 Regression Model Overview........................................................................................... 101 Results and Discussion ........................................................................................................... 102 Variable Relationships with Crashes .............................................................................. 102 Variable Relationships with Posted Speed Limit............................................................ 105 Findingsâ 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 contractorâs 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. Â
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. ............................................. 29Â Table 2. Factors of urban/suburban streets that affect operating speed. ....................................... 30Â Table 3. Factors of high-speed highways that affect crashes. ....................................................... 39Â Table 4. Factors of high-speed highways that affect operating speed. ......................................... 40Â Table 5. Example of factors that could influence a driverâs speed choice. .................................. 41Â Table 6. Approaches to setting speed limits. ................................................................................ 43Â Table 7. Advantages and disadvantages for approaches to set speed limits. ................................ 44Â Table 8. Input variables required for USLIMITS2. ...................................................................... 48Â Table 9. USLIMITS2 suggested minimum length per speed limit. .............................................. 48Â Table 10. Speeds and proportion of vehicles exceeding a speed from Boston citywide speed limit change study. .................................................................................................. 49Â Table 11. NCHRP Research Report 855 suggested target speed for context/roadway. ............... 53Â Table 12. Characteristics of roadway contexts. ............................................................................ 54Â Table 13. Number of speed studies by roadway type and state. ................................................... 55Â Table 14. Factors typically considered by U.S. states in setting posted speed limits based on material found online for 31 states............................................................................... 61Â Table 15. Comparison of input variables used in speed prediction or suggested posted speed limit procedures for urban/suburban (or developed) areas (variables AâC). ......... 64Â Table 16. Comparison of input variables used in speed prediction or suggested posted speed limit procedures for urban/suburban (or developed) areas (variables LâO). .......... 65Â Table 17. Comparison of input variables used in speed prediction or suggested posted speed limit procedures for urban/suburban (or developed) areas (variables PâS). ........... 66Â Table 18. Comparison of input variables used in speed prediction or suggested posted speed limit procedures for urban/suburban (or developed) areas (variable T). ................ 67Â Table 19. Austin on-road binned tube data file format. ................................................................ 69Â Table 20. Number of segments by functional classification. ........................................................ 71Â Table 21. Number of segments by posted speed limit. ................................................................. 71Â Table 22. Number of segments by number of lanes. .................................................................... 71Â Table 23. Roadway and traffic control device variables for City of Austin, Texas. .................... 72Â Table 24. Initial speed measures developed. ................................................................................ 76Â Table 25. Descriptive statistics of variables for City of Austin, Texas. ....................................... 77Â Table 26. Score statistics for model shown in Table 27. .............................................................. 82Â Table 27. Variables with significant effects on KABC segment (KABC_NID) crashes. ............ 82Â Table 28. Score statistics for model shown in Table 29. .............................................................. 83Â Table 29. Variables with significant effects on KABCO segment (KABCO_NID) crashes. ....... 84Â Table 30. Variables with significant effects on posted speed limit. ............................................. 85Â Table 31. Round 2 speed measures considered. ........................................................................... 86Â Table 32. Roadway characteristic variables considered in path analysis. .................................... 86Â Table 33. Estimated regression coefficients for KABC_NID crashes by path analysis (mediator variable: PSL, CoefVar, PerOvPSL, StdSpd, Pace, or SpdAve). ..................... 88Â Table 34. Estimated regression coefficients for KABC_NID crashes by path analysis (mediator variable: Abs(PSLâAvg), PSLâAvg, PSLâS85, or S85âAvg). ...................... 89Â
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). ..................... 90Â Table 36. Estimated regression coefficients for KABCO_NID crashes by path analysis (mediator variable: Abs(PSLâAvg), PSLâAvg, PSLâS85, or S85âAvg). ...................... 91Â Table 37. Estimated regression coefficients for KABC_NID crashes by path analysis with mediator variable Abs(PSLâAvg) based on 649 sites. ..................................................... 93Â Table 38. Summary of criteria for inclusion as potential study roadway. .................................... 97Â Table 39. Roadway and traffic control device variables for Washtenaw County, Michigan. ........................................................................................................................ 100Â Table 40. Descriptive statistics of variables for Washtenaw County, Michigan. ....................... 101Â Table 41. Score statistics for model shown in Table 42. ............................................................ 102Â Table 42. Variables with significant effects on KABC segment crashes. .................................. 103Â Table 43. Score statistics for model shown in Table 46. ............................................................ 104Â Table 44. Variables with significant effects on KABCO segment crashes. ............................... 104Â Table 45. Models exploring the significance of variables for posted speed limit. ..................... 106Â Table 46. Overview of decision rules for SLSG = Limited Access............................................ 108Â Table 47. Average KABCO crash rate per 100 MVM for Limited Access SLSG. .................... 110Â Table 48. Average KABC crash rate per 100 MVM for Limited Access SLSG. ....................... 111Â Table 49. Minimum section length for a particular speed limit. ................................................. 113Â Table 50. Overview of decision rules for SLSG = Undeveloped. .............................................. 115Â 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). ................. 119Â 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). ........ 120Â Table 53. RHR for undeveloped roadways. ................................................................................ 122Â Table 54. Overview of decision rules for SLSG = Developed. .................................................. 124Â Table 55. Decision matrix for sidewalk presence/width, sidewalk buffer, and pedestrian activity combinations for Developed SLSG. .................................................................. 125Â 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). ............................................................... 130Â 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). ............................................................... 130Â Table 58. Overview of decision rules for SLSG = Full Access. ................................................. 133Â Table 59. Decision matrix for sidewalk presence/width, sidewalk buffer, and pedestrian activity combinations for Full-Access SLSG. ................................................................ 134Â Â Â
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. .............................. 2Â Figure 2. Examples of speed limits. ................................................................................................ 4Â Figure 3. USLIMITS2 flow chart for speed limit calculation without crash data for roadway section in developed areas. ................................................................................. 47Â Figure 4. Operating speed by posted speed limit for urban arterials. ........................................... 56Â Figure 5. Operating speed by posted speed limit for urban collectors.......................................... 56Â Figure 6. Operating speed by posted speed limit for urban locals. ............................................... 57Â Figure 7. Operating speed by posted speed limit for city streets. ................................................. 57Â Figure 8. Using box plots to illustrate average operating speed by posted speed limit for city streets. ........................................................................................................................ 58Â Figure 9. Austin road tube traffic counter locations. .................................................................... 70Â Figure 10. Example of crashes (dots) and road tube locations (squares with labels starting with ST) for a road in Austin. ........................................................................................... 74Â Figure 11. Example of crash assignment using 30-ft buffer along a sample road segment. ........ 75Â Figure 12. Operating speed versus posted speed limit for city streets in Austin. ......................... 78Â Figure 13. Violin plots for average speed (SpdAve) for different geometric features. ................ 79Â Figure 14. Violin plots for percent of vehicles over the posted speed limit (PerOverPSL) for different geometric features. ....................................................................................... 80Â Figure 15. Comparison of crash rate to the difference between posted speed limit and average speed. ................................................................................................................... 92Â Figure 16. Path analysis findings for segments with posted speed limits of 20 to 45 mph. ......... 94Â Figure 17. Location of study roadway segments in Washtenaw County, Michigan. ................... 95Â Figure 18. Washtenaw County roadway network and urban boundaries. .................................... 96Â Figure 19. Segmentation of study road segments in Washtenaw County, Michigan. .................. 97Â Figure 20. Map of study road segments in Washtenaw County, Michigan. ................................. 98Â Figure 21. Study road segments by posted speed limit. ................................................................ 98Â Figure 22. Study road segments by AADT estimate. ................................................................... 99Â Figure 23. Example of traffic crash locations along study road segments (2013â2017). ............. 99Â Figure 24. Crash prediction using HSM equations for four-lane undivided and divided roadways. ........................................................................................................................ 116Â