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ACKNOWLEDGMENT This work was sponsored by the American Association of State Highway and Transportation Officials (AASHTO), in cooperation with the Federal Highway Administration, and was conducted in the National Cooperative Highway Research Program (NCHRP), which is administered by the Transportation Research Board (TRB) of the National Academies. COPYRIGHT INFORMATION Authors herein are responsible for the authenticity of their materials and for obtaining written permissions from publishers or persons who own the copyright to any previously published or copyrighted material used herein. Cooperative Research Programs (CRP) grants permission to reproduce material in this publication for classroom and not-for-profit purposes. Permission is given with the understanding that none of the material will be used to imply TRB, AASHTO, FAA, FHWA, FRA, FTA, Transit Development Corporation, or AOC 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 Research Council, or the program sponsors. The information contained in this document was taken directly from the submission of the author(s). This material has not been edited by TRB.
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TABLE OF CONTENTS List of Figures ........................................................................................................................................................ iii List of Tables ......................................................................................................................................................... iii Acknowledgments ................................................................................................................................................. v Abstract ................................................................................................................................................................ vi Executive Summary ............................................................................................................................................. vii CHAPTER 1: INTRODUCTION ............................................................................................................ 1 Research Objective and Scope ............................................................................................................................... 1 Research Approach ................................................................................................................................................ 1 Organization of This Report ................................................................................................................................... 2 CHAPTER 2: LITERATURE REVIEW ................................................................................................. 3 Introduction ........................................................................................................................................................... 3 Background ............................................................................................................................................................ 3 Procedures for Quantifying the Safety Effect of Signal Installation ........................................................................ 6 Factors Influencing Intersection Safety ................................................................................................................ 12 Safety Effect of Signal Installation........................................................................................................................ 13 Safety Predictive Methods ................................................................................................................................... 17 Alternatives to Signalization ................................................................................................................................ 27 Summary ............................................................................................................................................................. 28 CHAPTER 3: PRACTITIONER SURVEY SUMMARY ..................................................................... 30 Introduction ......................................................................................................................................................... 30 Engineering Study for Signal Installation .............................................................................................................. 30 Experience with the Crash Experience Warrant ................................................................................................... 34
Potential Improvements to the Crash Experience Warrant .................................................................................. 35 Summary ............................................................................................................................................................. 36 CHAPTER 4: SAFETY EVALUATION PROCEDURE ...................................................................... 37 Introduction ......................................................................................................................................................... 37 Development of a Safety Evaluation Procedure ................................................................................................... 37 Development of Selected Models and Parameters .............................................................................................. 43 CHAPTER 5: PROPOSED WARRANT DEVELOPMENT .............................................................. 62 Introduction ......................................................................................................................................................... 62 Purpose and Scope of Proposed Warrant ............................................................................................................ 62 Development Process .......................................................................................................................................... 64 Proposed Warrant Content .................................................................................................................................. 75 CHAPTER 6: CONCLUSIONS AND RECOMMENDATIONS ......................................................... 78 Conclusions .......................................................................................................................................................... 78 Recommendations ............................................................................................................................................... 79 CHAPTER 7: REFERENCES ................................................................................................................. 80 APPENDIX A: SURVEY OF PRACTITIONERS ................................................................................ 82 Introduction......................................................................................................................................................... 82 Part I â Engineering Study .................................................................................................................................... 83 Part II â Experience with Existing Warrant ........................................................................................................... 86 Part III â Suggested Improvements to Warrant .................................................................................................... 87 Part IV â General Information .............................................................................................................................. 89 ii
List of Figures Figure 1. Relationship between number of lanes and total crash frequency. ............................... 19 Figure 2. Variation in average crash frequency among intersections. .......................................... 19 Figure 3. Examination of the effect of signal installation. ............................................................ 21 Figure 4. Comparison of fatal-and-injury crashes at urban three-leg intersections. ..................... 23 Figure 5. Comparison of fatal-and-injury crashes at urban four-leg intersections. ...................... 24 Figure 6. Comparison of all crashes at rural four-leg intersections â two lanes on major road. .. 25 Figure 7. Comparison of all crashes at rural four-leg intersections â four lanes on major road. .. 26 Figure 8. Comparison of predicted and measured overdispersion parameters for FI and PDO crashes. .......................................................................................................................................... 52 Figure 9. Relationship between overdispersion parameters for FI and PDO crashes. .................. 52 Figure 10. Comparison of predicted and measured overdispersion parameters for angle and rear- end crashes. ................................................................................................................................... 54 Figure 11. Relationship between overdispersion parameters for angle and rear-end crashes. ..... 54 Figure 12. Total-crash thresholds as a function of major-road and minor-road AADT. .............. 70 List of Tables Table 1. Findings from a survey of practitioners. ........................................................................... 6 Table 2. Geometric design elements and traffic control features that affect intersection safety. . 13 Table 3. Crash modification factors for conversion from stop control to signal control. ............. 15 Table 4. HSM predictive methods. ............................................................................................... 18 Table 5. Alternatives to signalization. .......................................................................................... 27 Table 6. Crashes susceptible to correction by signal installation. ................................................ 28 Table 7. Alternatives to signalization responses. .......................................................................... 33 Table 8. Additional factors to consider in the crash experience warrant. ..................................... 35 Table 9. Unit crash costs. .............................................................................................................. 39 Table 10. Scope elements for the safety evaluation procedure. .................................................... 40 Table 11. Coefficients for SPFs that predict FI crash frequency at rural three-leg intersections. 45 Table 12. Coefficients for SPFs that predict FI crash frequency at rural four-leg intersections. . 46 Table 13. Coefficients for SPFs that predict FI crash frequency at urban intersections. .............. 47 Table 14. Coefficients for SPFs that predict PDO crash frequency. ............................................ 49 Table 15. Overdispersion parameters for FI, PDO, and total crash SPFs. .................................... 50 Table 16. Overdispersion parameters for angle and rear-end crash SPFs. ................................... 53 Table 17. Estimated overdispersion parameters for rural two-lane highway intersections. ......... 55 Table 18. Estimated overdispersion parameters for rural multilane highway intersections. ........ 56 Table 19. Estimated overdispersion parameters for urban and suburban arterial intersections. ... 57 Table 20. Unit crash costs. ............................................................................................................ 59 Table 21. Crash cost calculations for rural intersections. ............................................................. 60 Table 22. Crash cost calculations for urban intersections............................................................. 61 iii
Table 23. Candidate scope elements for the proposed crash experience warrant. ........................ 63 Table 24. Decision rules to define warrant criterion..................................................................... 64 Table 25. Input variable values. .................................................................................................... 66 Table 26. Crash distribution for intersections with stop control on the minor road. .................... 67 Table 27. Equivalent average daily traffic volumes based on Warrant 1. .................................... 68 Table 28. Equivalent average daily traffic volume based on Criterion C of Warrant 7. .............. 68 Table 29. Crash count estimation procedure for rural two-lane highway intersections. .............. 69 Table 30. Reported crash threshold for use with Criterion B of Warrant 7 based on one-year crash history and considering all crash types. ............................................................................... 72 Table 31. Reported crash threshold for use with Criterion B of Warrant 7 based on three-year crash history and considering all crash types. ............................................................................... 72 Table 32. Reported crash threshold for use with Criterion B of Warrant 7 based on one-year crash history and considering angle crashes. ................................................................................ 73 Table 33. Reported crash threshold for use with Criterion B of Warrant 7 based on three-year crash history and considering angle crashes. ................................................................................ 73 iv
Acknowledgments The research reported herein was performed under NCHRP Project 07-18 by Kittelson & Associates, Inc. Dr. James Bonneson served as the Principal Investigator for the project and supervised the research conducted. The other authors of this report are Ms. Kelly Laustsen, Mr. Lee Rodegerdts, and Mr. Scott Beaird. They are all affiliated with Kittelson & Associates, Inc. v
Abstract The crash experience warrant in the Manual on Uniform Traffic Control Devices (MUTCD) specifies three criteria that must be met before a traffic signal can be considered for installation at an intersection. One of these criteria requires that five or more crashes of a type susceptible to correction by a signal must have occurred during a one-year period. Research was undertaken to evaluate the validity of this criterion and, if needed, to develop a crash experience warrant that is based on safety science and is consistent with the Highway Safety Manual (HSM). A procedure was developed for quantifying the safety effect of signal installation. It is based on the predictive methods in the HSM. The procedure considers the effect of signal installation on intersection crash frequency and crash severity. The procedure was used to develop revised content for the crash experience warrant. The minimum number of crashes was found to vary by area type, intersection legs, and number of lanes on each intersection approach. A revised crash experience warrant was prepared and recommended for inclusion in the MUTCD. vi
