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NCHRP Web-Only Document 328 Safety Effects of Raising Speed Limits to 75 mph and Higher Karen Dixon Kay Fitzpatrick Dominique Lord Chiara Silvestri Dobrovolny Raul Avelar Bahar Dadashova Texas A&M Transportation Institute College Station, TX Fred Mannering Nawaf Alnawmasi University of Southern Florida Tampa, FL Jennifer Atkinson Joseph Jones Adil Cheema K.C. Matthews Leidos Reston, VA Conduct of Research Report for NCHRP Project 17-79 Submitted March 2022 © 2022 by the National Academy of Sciences. National Academies of Sciences, Engineering, and Medicine and the graphical logo are trademarks of the National Academy of Sciences. All rights reserved. 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 techniquesâthe 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, APTA, FAA, FHWA, FTA, GHSA, or NHTSA 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 Transportation Research Board does not develop, issue, or publish standards or specifications. The Transportation Research Board manages applied research projects which provide the scientific foundation that may be used by Transportation Research Board sponsors, industry associations, or other organizations as the basis for revised practices, procedures, or specifications. The Transportation Research Board, the National Academies, and the sponsors of the National Cooperative Highway Research Program do not endorse products or manufacturers. Trade or manufacturersâ names appear herein solely because they are considered essential to the object of the report. The information contained in this document was taken directly from the submission of the author(s). This material has not been edited by TRB.
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C O O P E R A T I V E R E S E A R C H P R O G R A M S CRP STAFF FOR NCHRP WEB-ONLY DOCUMENT 328 Christopher J. Hedges, Director, Cooperative Research Programs Waseem Dekelbab, Deputy Director, Cooperative Research Programs, and Manager, National Cooperative Highway Research Program Sid Mohan, Associate Program Manager, Implementation and Technology Transfer, National Cooperative Highway Research Program David M. Jared, Senior Program Officer Traci Caldwell, Senior Program Assistant Natalie Barnes, Director of Publications Heather DiAngelis, Associate Director of Publications Kami Cabral, Editor Jennifer Correro, Assistant Editor NCHRP PROJECT 17-79 PANEL Field of TrafficâArea of Safety Steven Buckley, JEO Consulting Group, Topeka, KS (Chair) Kelly L. Becker, North Carolina Department of Transportation, Garner, NC Christina Bennett, South Dakota Department of Transportation, Pierre, SD Joseph V. Gorman, Michigan Department of Transportation, Lansing, MI Beth Ann Larson, Maryland State Highway Administration, Baltimore, MD Chris Lindsey, Texas Department of Transportation, Austin, TX Jonathan Michael Lloyd, Illinois Department of Transportation, Schaumburg, IL Xiaoduan Sun, University of Louisiana, Lafayette, Lafayette, LA Jonathan Upchurch, Ivins, UT Guan Xu, FHWA Liaison Kelly K. Hardy, AASHTO Liaison Bernardo B. Kleiner, TRB Liaison
iv TABLE OF CONTENTS LIST OF TABLES ....................................................................................................................... viii LIST OF FIGURES ....................................................................................................................... xi LIST OF ABBREVIATIONS ...................................................................................................... xiii CHAPTER 1. Background .............................................................................................................. 1 Problem Statement and Research Objective ............................................................................... 1 Report Organization .................................................................................................................... 2 CHAPTER 2. Literature Review .................................................................................................... 3 Literature Review Introduction ................................................................................................... 3 Historical Speed Limit Changes and Their Safety Implications ................................................. 4 NMSL Period, 1974-1987 ....................................................................................................... 4 NMSL Relaxation, 1987-1994 ................................................................................................ 5 Repeal of NMSL, 1995-Present ............................................................................................ 11 Indirect Safety Implications of Speed Limit Changes .............................................................. 15 Speed Concepts ..................................................................................................................... 15 Traffic Flow .......................................................................................................................... 20 Vehicle Characteristics ......................................................................................................... 20 Economic Factors.................................................................................................................. 21 Roadway Characteristics ....................................................................................................... 21 Roadside Characteristics ....................................................................................................... 23 Chapter Summary ..................................................................................................................... 32 CHAPTER 3. State of Practice ..................................................................................................... 34 Background ............................................................................................................................... 35 Implementation ......................................................................................................................... 35 Results ....................................................................................................................................... 36 Wrap Up .................................................................................................................................... 37 CHAPTER 4. Roadside Hardware Suitability for Elevated Speed Limits ................................... 39
v Descriptive Statistics for NASS-CDS and NCHRP Project 17-43 Crash Data ........................ 39 Merging NCHRP Project 17-43 and NASS-CDS Crash Data .................................................. 40 Results ....................................................................................................................................... 42 Impact Severity Analysis .......................................................................................................... 46 Chapter Summary ..................................................................................................................... 48 Discussion ................................................................................................................................. 49 Guardrail ............................................................................................................................... 49 Median Barrier ...................................................................................................................... 50 Bridge Rails .......................................................................................................................... 51 Crash Cushions ..................................................................................................................... 52 End Treatments ..................................................................................................................... 52 Transitions............................................................................................................................. 53 Breakaway Supports ............................................................................................................. 53 Roadside Hardware Research Needs ........................................................................................ 54 CHAPTER 5. Spillover Study ...................................................................................................... 56 Site Selection and Database Development for Spillover Study ................................................ 56 Spillover Analysis ..................................................................................................................... 57 Summary of Spillover Assessment Findings ............................................................................ 58 CHAPTER 6. Texas Safety Assessment ....................................................................................... 59 Before-After Texas Speed Limit Study .................................................................................... 59 Study Design ......................................................................................................................... 59 Statistical Methodology ........................................................................................................ 60 Texas Before-After Analysis Database Development .......................................................... 64 Texas Exploratory Analysis of Before-After Data ............................................................... 74 Texas Before-After Statistical Model Development and Analysis ....................................... 77 Before-After Analysis Conclusions â Texas Safety Analysis .............................................. 79 Cross-Sectional Texas Speed Limit Study ................................................................................ 80 Texas Cross-Sectional Analysis Database Development ..................................................... 80 Texas Cross-Sectional Analysis of Cross-Sectional Data .................................................... 87
vi Cross-Sectional Analysis Conclusions â Texas Safety Analysis .......................................... 93 Summary â Texas Safety Analysis ........................................................................................... 93 CHAPTER 7. Kansas Safety Assessment ..................................................................................... 96 Kansas Study Introduction ........................................................................................................ 96 Methodological Approach â Crash Frequency Analysis .......................................................... 99 Estimation Results â Crash Frequency Analysis .................................................................... 101 Random Parameters Negative Binomial Estimation Results .............................................. 102 Comparison of Predicted Mean Crash Frequencies by Year .............................................. 104 Methodological Approach â Injury-Severity Analysis ....................................................... 106 Estimation Results â Injury-Severity Analysis ....................................................................... 109 Driver-injury severities in single-vehicle crashes on road segments with raised speed limits ........................................................................................................... 110 Driver-injury severities in two-vehicle crashes on road segments with raised speed limits ........................................................................................................... 122 Summary of Kansas Safety Study Findings ............................................................................ 136 CHAPTER 8. Transferability Assessment via Socioeconomic Analysis ................................... 138 Methodological Approach and Modeling ............................................................................... 138 Methodology ....................................................................................................................... 138 Time Series Regression Models.......................................................................................... 138 Study Sites and Data Overview .............................................................................................. 139 Selection of Study Sites ...................................................................................................... 139 Data Collection ................................................................................................................... 140 Analysis Results for Socio-Economic Comparison ................................................................ 141 Assessment Analysis ........................................................................................................... 142 Evaluation Analysis ............................................................................................................ 145 Summary of Socioeconomic Transferability Assessment Findings ....................................... 158 CHAPTER 9. Operating Speed Associated with Rural Freeway Maximum Speed Limit Changes from 70 to 75 mph (Kansas) ................................................................. 159 Database Development ........................................................................................................... 159 Site Selection ...................................................................................................................... 159
vii Study Period ........................................................................................................................ 161 Data Analysis .......................................................................................................................... 161 Summary of Operating Speed Findings for Kansas ................................................................ 164 CHAPTER 10. Conclusions........................................................................................................ 166 Final Comments ...................................................................................................................... 168 REFERENCES ........................................................................................................................... 170 APPENDIX â Summary Tables for Socioeconomic Comparison Study .................................. 183
viii LIST OF TABLES Table 1. Summary of the 1997 Post-NMSL 10 mph Speed Limit Increase Studies .................... 10 Table 2. States with 75 mph and Higher Speed Limits ................................................................. 11 Table 3. Exponent of Power Model for Nine Safety Measures .................................................... 18 Table 4. Percentile Speeds given the Posted Speed Limit ............................................................ 19 Table 5. CMF for lane width, rural highways with AADT > 2000. ............................................. 22 Table 6. CMF for conversion from 10 feet to wider medians. ..................................................... 24 Table 7. Changes from NCHRP Report 350 Standard to MASH Standard â Test Matrices ........ 27 Table 8. Key Changes -- NCHRP Report 350 to MASH Standard â Test Vehicles .................... 27 Table 9. Descriptive Statistics for Segregated Data by Speed Limit ........................................... 29 Table 10. Descriptive Statistics for Segregated Data by Speed Limit ......................................... 29 Table 11. Key-Variables from the Linked Databases ................................................................... 41 Table 12. Descriptive Statistics of Object Impact Speed for Speed Limit Categories. ................ 42 Table 13. Descriptive Statistics for Impact Angle for Speed Limit Categories. ........................... 44 Table 14. Descriptive Statistics for Object Impact Angle for Speed Limit Categories. ............... 45 Table 15. MASH Test 3-11 Predicted IS Comparison for Speed Limit Categories. .................... 47 Table 16. MASH Test 3-10 Predicted IS Comparison for Speed Limit Categories. .................... 48 Table 17. Merged Road Characteristic Data for Segments with Changes to Posted Speed Limits ................................................................................................................... 66 Table 18. Mileage by Level of Urbanization ................................................................................ 67 Table 19. Matched Crashes by Year Using Control, Section, and Distance from Origin ........... 70 Table 20. Number of Matched Crashes by Route and Geo-location ............................................ 71 Table 21. Model Parameter, Covariates, Ranges, and Functional Forms ..................................... 77 Table 22. Before-After Model Parameters .................................................................................... 77 Table 23. Before-After Model Parameters by Crash Type ........................................................... 78 Table 24. Safety Effectiveness Estimates for Texas Before-After Assessment ........................... 79 Table 25. Number of Crashes per Year ........................................................................................ 84 Table 26. Number of Segments and Total Miles per Facility Type .............................................. 85 Table 27. Descriptive Statistics for Texas Rural Freeways .......................................................... 85
ix Table 28. Descriptive Statistics for Texas Rural Multi-Lane Divided Highways ........................ 86 Table 29. Descriptive Statistics for Texas Rural Multi-Lane Undivided Highways .................... 86 Table 30. Descriptive Statistics for Texas Rural Two-Lane Highways ........................................ 87 Table 31. Estimation Results for Total Crashes on Rural Texas Freeways .................................. 88 Table 32. Estimation Results for KAB Crashes on Texas Rural Freeways .................................. 89 Table 33. Estimation Results for Texas Rural Multi-Lane Divided Highways ............................ 90 Table 34. Estimation Results, Rural Multi-Lane Undivided Highway Total Crashes .................. 91 Table 35. Estimation Results for Rural Two-Lane Highway Total Crashes ................................ 92 Table 36. Summary of Texas Speed Limit Analysis .................................................................... 95 Table 37. Summary Statistics of Variables included in the Negative Binomial with Random Parameter Model for Crash Frequency in Kansas ................................................................... 101 Table 38. Negative Binomial with Random Parameters Model Results for Annual Crash Frequency in Kansas Crash Data (all random parameters normally distributed) .................... 103 Table 39. Summary Statistics of Variables included in the KS Single Vehicle Models ............ 111 Table 40. Random Parameters Approach with Heterogeneity in Means and Variances Results for Single-Vehicle Crash-Injury Severity (KS 2009) .................................................................... 112 Table 41. Random Parameters with Heterogeneity in Means and Variances Results for Single- Vehicle Crash-Injury Severity (KS 2010)................................................................................ 113 Table 42. Random Parameters Approach with Heterogeneity in Means and Variances for Single- Vehicle Crash-Injury Severity (KS 2012)................................................................................ 114 Table 43. Random Parameters Approach with Heterogeneity in Means and Variances for Single- Vehicle Crash-Injury Severity (KS 2013)................................................................................ 115 Table 44. Likelihood Ratio Test Results based on Random Parameters approaches with Heterogeneity in means and variances of Kansas single-vehicle crash data (Ï2 values with degrees of freedom in parenthesis and confidence level in brackets). ..................................... 116 Table 45. Marginal Effects of the Explanatory Variables by Year, Driver Injury Severity in Single-Vehicle Crashes ............................................................................................................ 117 Table 46. Summary Statistics of Variables included in the Binary Logit Model of Single-Vehicle Rollover Crashes in Kansas Data (single-vehicle crashes) ...................................................... 118 Table 47. Binary Logit Model Results for Single-Vehicle Rollover Crashes in Kansas ........... 119 Table 48. Summary Statistics of Variables included in the Two-Vehicle KS Crash Models .... 124 Table 49. Random Parameters Approach with Heterogeneity in Means and Variance Results for Two-Vehicle Crash-Injury Severity (KS 2009) ....................................................................... 126
x Table 50. Random Parameters Approach with Heterogeneity in Means and Variance Results for Two-Vehicle Crash-Injury Severity (KS 2010) ....................................................................... 127 Table 51. Random Parameters Approach with Heterogeneity in Means and Variance Results for Two-Vehicle Crash-Injury Severity (KS 2012) ....................................................................... 128 Table 52. Random Parameters Approach with Heterogeneity in Means and Variance Results for Two-Vehicle Crash-Injury Severity (KS 2013) ....................................................................... 129 Table 53. Likelihood Ratio Test Results for Kansas Two-Vehicle Crashes .............................. 130 Table 54. Marginal Effects of the Explanatory Variables by Year, Driver Injury Severity in Two-Vehicle Crashes ...................................................................................................................................... 131 Table 55. Summary of Kansas Speed Limit Analysis ................................................................ 137 Table 56. Summary Statistics of Key Indicators in Kansas and Iowa ........................................ 140 Table 57. Descriptive Statistics of Monthly Crash and VMT Data for Kansas and Iowa .......... 141 Table 58. Annual Total and Rural Interstate Crashes for Kansas and Iowa ............................... 142 Table 59. Results of Two-Sample t-test ...................................................................................... 145 Table 60. F-test for Structural Change ........................................................................................ 150 Table 61. Granger Causality Test ............................................................................................... 153 Table 62. Dynamic Regression Model Estimation Results ........................................................ 156 Table 63. Rural Freeway Speed Sensor Sites with Speed Limit Change from 70 to 75 mph .... 160 Table 64. Weeks for Comparison Data Used in Analysis .......................................................... 161 Table 65. Mean Operating Speeds for One-Week Study Time Periods â Rightmost Lane ........ 162 Table 66. Mean Operating Speeds for One-Week Study Time Periods â Other Lane ............... 163 Table 67. Null Hypotheses of Equal Speeds to Test for Before-After Scenarios ....................... 164 Table 68. Descriptive Statistics of Socioeconomic Variables .................................................... 188
xi LIST OF FIGURES Figure 1. Maximum Speed Limits for Individual US States as of September 30, 2020. ................ 3 Figure 2. Causal Links. ................................................................................................................. 17 Figure 3. State of Practice Questions for Individual States .......................................................... 34 Figure 4. Tables and Relationships in NCHRP Project 17-43 Database ...................................... 40 Figure 5. Linear Regression for Evaluation of the 85th Percentile of Impact Speed with NCHRP Project 17-43 Crash Data Set. .................................................................................................... 43 Figure 6. Linear Regression for Evaluation of the 85th Percentile of Impact Angle with NCHRP Project 17-43 Crash Data Set. .................................................................................................... 45 Figure 7. Linear Regression for Evaluation of the 85th Percentile of Object Impact Angle with NCHRP Project 17-43 Crash Data Set. ..................................................................................... 46 Figure 8. Study Locations for the Spillover Preliminary Assessment .......................................... 57 Figure 9. Exploratory Histogram for Crossroad Crash Frequency and Distance ......................... 58 Figure 10. Crashes Coded as Speeding with Distance from Study Freeway ................................ 58 Figure 11. Texas Segments with Speed Limit Changes to 75 or Higher. ..................................... 65 Figure 12. AADT Records Available from Texas Road Characteristic and Crash Databases. ... 72 Figure 13. Imputed AADT values from Random Parameter Model. .......................................... 73 Figure 14. Final Set of AADTs Including Imputed Values. ........................................................ 73 Figure 15. Total Crashes at Sites with Maximum Speed Limit Changes from 70 to 75 mph. ..... 74 Figure 16. Total Crash Rate at Sites with Maximum Speed Limit change from 70 to 75 mph. ... 75 Figure 17. Texas KAB Crashes with Maximum Speed Limit Change from 70 to 75 mph. ........ 76 Figure 18. Texas KAB Crash Rates with Posted Speed Limit Change from 70 to 75 mph. ....... 76 Figure 19. Links per Interstate Segment ....................................................................................... 81 Figure 20. Inconsistencies in Rural Multi-Lane Highways .......................................................... 82 Figure 21. Annual Number of Crash-Involved Persons and Injury Level. ................................... 96 Figure 22. Annual Crash Frequency by Number of Involved Vehicles on Roadway Segments with Raised Speed Limits. ......................................................................................................... 97 Figure 23. Difference between 2010 model-predicted mean segment crash frequencies in 2012 and 2012 model-predicted mean segment crash frequencies in 2012. ..................................... 105 Figure 24. Difference between 2009 model-predicted mean segment crash frequencies in 2012 and 2012 model-predicted mean segment crash frequencies in 2012. ..................................... 106
xii Figure 25. Difference Between 2010 Model Predicted Probabilities in 2012 and 2012 âObservedâ Probabilities (driver injuries in single-vehicle crashes). ...................................... 121 Figure 26. Difference Between 2009 Model Predicted Probabilities in 2010 and 2010 Observed Probabilities (driver injuries in single-vehicle crashes). .......................................................... 122 Figure 27. Difference between 2010 model predicted probabilities in 2012 and 2012 observed probabilities (driver injuries in two-vehicle crashes). .............................................................. 134 Figure 28. Difference between 2009 model predicted probabilities in 2012 and 2012 observed probabilities (driver injuries in two-vehicle crashes). .............................................................. 135 Figure 29. Annual Crash and Traffic Volume Trends for Kansas and Iowa .............................. 143 Figure 30. Annual Socioeconomic Trends (Seasonally Adjusted) ............................................. 144 Figure 31. Box Plots of Crashes and Traffic Volumes, Before-After. ....................................... 146 Figure 32. Box Plots of Socioeconomic Factors, Before-After .................................................. 147 Figure 33. Rural Interstate Crashes, Seasonally Adjusted .......................................................... 148 Figure 34. Structural Change in Rural Interstate Crash Time Series .......................................... 149 Figure 35. Autocorrelation Function and Partial Autocorrelation Function of Rural Interstate Crash Time Series. ................................................................................................................... 151 Figure 36. Autocorrelation Function and Partial Autocorrelation Function of ARIMA Model Residuals. ................................................................................................................................. 152 Figure 37. Autocorrelation Function and Partial Autocorrelation Function of Dynamic Regression Models. .................................................................................................................. 154 Figure 38. Dynamic Residual Model Prediction Results ............................................................ 157
xiii LIST OF ABBREVIATIONS Abbreviation / Acronym Description A Incapacitating Injury AASHTO American Association of State Highway and Transportation Officials AR Autoregressive ARIMA Autoregressive integrated moving average ARIMAX ARIMA with exogenous factors ARMA Autoregressive moving average B Non-incapacitating Injury C Possible Injury CDS Crashworthiness Data System CMF Crash Modification Factor CMV Commercial Motor Vehicles DOT Department of Transportation FHWA Federal Highway Administration FM Farm to Market GDP Gross Domestic Product GES General Estimate System HSIS Highway Safety Information System HSM Highway Safety Manual IDOT Iowa Department of Transportation IS Impact Severity K Fatal Injury KS Kansas KSDOT Kansas Department of Transportation MAPE Mean absolute prediction error MASH Manual for Assessing Safety Hardware mph Miles per Hour NASS National Automotive Sampling System NCHRP National Cooperative Highway Research Program NHS National Highway System NHTSA National Highway Traffic Safety Administration NMSL National Maximum Speed Limit O Property Damage Only Crash PDO Property Damage Only Crash PSW Propensity Score Weighting PSU Primary Sampling Units SEATS Signal Extraction with ARIMA Time Series TL-3, TL-4, TL-5, and TL-6 Testing Levels for Roadside Hardware TRAMO Signal Extraction with ARIMA Time Series TRB Transportation Research Board TxDOT Texas Department of Transportation USDOT United States Department of Transportation VMT Vehicle Miles Traveled
