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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 PERMISSION 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, FMCSA, 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 opinion and conclusions expressed or implied in the report are those of the research agency. They are not necessarily those of the TRB, the National Research Council, AASHTO, or the U.S. Government. This report has not been edited by TRB.
i PREFACE This Final Report for Phases I and II is submitted by Midwest Research Institute (MRI) in accordance with the contractual requirements of NCHRP Project 17-26, Methodology to Predict the Safety Performance of Urban and Suburban Arterials. This report was prepared by Mr. Douglas W. Harwood, Ms. Karin M. Bauer, Ms. Karen R. Richard, Mr. David K. Gilmore, Mr. Jerry L. Graham, Ms. Ingrid B. Potts, and Dr. Darren J. Torbic of Midwest Research Institute, and Dr. Ezra Hauer. This report covers the original scope of the research which addresses the preparation of a draft chapter on urban and suburban arterials for the forthcoming Highway Safety Manual (HSM). A recent addition to the project scope of work involves the development of a safety prediction methodology for collisions involving pedestrians on urban and suburban arterials. That work is currently underway in Phase III of the project and will be reported separately. The draft HSM chapter presented in Appendix B of this report will be revised when the pedestrian safety methodology is available. Sincerely, Midwest Research Institute Douglas W. Harwood Principal Traffic Engineer Approved: Robert G. Barton, Ph.D. Director Engineering Division March 2007
TABLE OF CONTENTS LIST OF FIGURES ...................................................................................................................... III LIST OF TABLES..........................................................................................................................V CHAPTER 1. INTRODUCTION ................................................................................................... 1 Background............................................................Error! Bookmark not defined. Research Objective and Scope................................................................................ 2 Organization of This Report ................................................................................... 3 CHAPTER 2. SAFETY PERFORMANCE OF URBAN AND SUBURBAN ARTERIALS 5 Literature Review.................................................................................................... 5 HSM User Survey ................................................................................................. 38 CHAPTER 3 ...................................................................................................................................... RECOMMENDED STRUCTURE FOR SAFETY PREDICTION METHODOLOGY ..............41 Potential Applications or Uses of the Methodology ............................................. 41 Candidate Input Variables..................................................................................... 42 Output Variables ................................................................................................... 46 Arterial Components to Be Modeled Separately .................................................. 49 Safety Prediction Methodology for Each Arterial Component............................. 50 Statistical Modeling Approach ............................................................................. 53 Calibration Factors for Application of the Prediction Methodology by Specific Highway Agencies......................................................................................... 57 Predictions for Accident Severity and Accident Type.......................................... 58 Considering Observed Accident History Data in the Safety Prediction Methodology.................................................................................................. 58 CHAPTER 4. DEVELOPMENT OF PROJECT DATABASE ...................................................63 Participating Highway Agencies........................................................................... 63 Site Selection for Roadway Segments .................................................................. 63 Site Selection for Intersections ............................................................................. 65 Data Collection ..................................................................................................... 66 Accident Data........................................................................................................ 69 Validation Data ..................................................................................................... 79 CHAPTER 5. BASE MODELS AND ADJUSTMENT FACTORS............................................85 Roadway Segments............................................................................................... 85 Intersections ........................................................................................................ 105 CHAPTER 6. ACCIDENT MODIFICATION FACTORS........................................................119 Roadway Segments............................................................................................. 120 Intersections ........................................................................................................ 126 CHAPTER 7. SUMMARY OF HSM METHODOLOGY.........................................................129 Overall Structure of Methodology ...................................................................... 129 Definition of Roadway Segments and Intersections........................................... 129 Roadway Segment Methodology........................................................................ 130 Intersection Methodology ................................................................................... 140 Combining Predicted and Observed Accident Counts........................................ 149 Worksheets and Precision in Computations ....................................................... 150 CHAPTER 8. VALIDATION OF THE HSM METHODOLOGY............................................151
ii Roadway Segment Validation Study .................................................................. 151 Intersection Validation Study ............................................................................. 168 CHAPTER 9. CONCLUSIONS AND RECOMMENDATIONS.......................................175 REFERENCES ............................................................................................................................177 APPENDIX AâSURVEY RESULTS APPENDIX BâDRAFT VERSION OF HSM CHAPTER 10
1 CHAPTER 1. INTRODUCTION BACKGROUND The Transportation Research Board (TRB) and the American Association of State Highway and Transportation Officials (AASHTO) have begun a major initiative to develop a Highway Safety Manual (HSM). The HSM began from recognition of the need that, for safety to receive proper consideration in the highway project development process, analysts needed tools to make quantitative statements about the safety effects of proposed projects or design alternatives. Most of the potential impacts of highway projects other than safety (e.g., traffic operations, air quality, noise, wetlands, construction cost) are addressed in quantitative fashion during the design process. However, while the accident history of the site may be documented quantitatively, most safety assessments of proposed projects are qualitative in nature and do not include explicit estimates of the likely safety effects of the project. Even when an attempt to develop quantitative estimates of safety effects is made, the accuracy of those estimates is limited by the lack of reliable tools for preparing such estimates. It is a concern that, when accurate traffic operational and environmental assessments are available, these factors may receive more weight in the decision making process, to the detriment of safety. The HSM initiative grew out of the recognition that safety analysts need methodologies to predict safety similar to the quantitative methodologies that traffic operations engineers have available to assess capacity and level of service from specific traffic operational service measures using the Highway Capacity Manual (HCM) (1). The HSM initiative began with a conference session at the TRB annual meeting in January 1999 to explore the role of safety in the HCM. The consensus of the speakers at that session was that the HCM was already complex enough without trying to address safety and that a separate stand-alone document was needed. A workshop sponsored by TRB and FHWA was then held in Irvine, California, in December 1999 to consider the need for an HSM and develop a scope for a potential first edition. The Irvine workshop led to two specific initiatives: the formation of a TRB Joint Subcommittee on the Development of the Highway Safety Manual and the conduct of NCHRP Project 17-18(4) to prepare a detailed outline, a strategic plan, and a prototype chapter for the HSM. The TRB Joint Subcommittee has now become the TRB Task Force on Development of the Highway Safety Manual and may, in the future, be recognized as a full TRB Committee. AASHTO has formed a Joint Task Force to review and approve the HSM for publication. The HSM will be targeted primarily to those on the front line of daily decision making in state highway agencies, and in local agencies including cities, counties, and metropolitan planning organizations (MPOs). These include analysts evaluating the potential effects of proposed improvement projects and those conducting planning, design, or operational studies. The TRB Task Force has reviewed and approved a draft outline for the HSM, working from recommendations presented in Development of a Highway Safety Manual (2). The key components of the HSM will be:
iii LIST OF FIGURES Figure 1. Lane width AMF for two-lane highways from IHSDM crash prediction module .......7 Figure 2. Effect of lane width on midblock crash rates (13). ......................................................8 Figure 3. Shoulder width AMF for two-lane highways from IHSDM crash prediction module.......................................................................................................15 Figure 4. Effect of shoulder width on midblock crash rates ......................................................16 Figure 5. Relationship between on-road accidents and degree of curve for four-lane undivided urban arterials ............................................................................................17 Figure 6. Density of accidents and access points on urban arterials..........................................25 Figure 7. Estimated relationship between accident rate and unsignalized intersection density on four-lane urban highways. ........................................................................25 Figure 8. Estimated relationship between accident rate and business access density on four-lane urban highways...........................................................................................26 Figure 9. Assignment of accidents to roadway segments and intersections ..............................74 Figure 10. Example of regression relationship and observed data for two-lane roadways in Michigan with no shoulder and no on-street parking.................................................88 Figure 11. Plots for base models for total multiple-vehicle nondriveway collisions on roadway segments by roadway type...........................................................................90 Figure 12. Plots for base models for fatal-and-injury multiple-vehicle nondriveway collisions on roadway segments by roadway type. ....................................................91 Figure 13. Plots for base models for property-damage-only multiple-vehicle nondriveway collisions on roadway segments by roadway type. ....................................................91 Figure 14. Plots for base models for total single-vehicle collisions on roadway segments by roadway type. .............................................................................................................97 Figure 15. Plots for base models for fatal-and-injury single-vehicle collisions on roadway segments by roadway type. ........................................................................................98 Figure 16. Plots for base models for property-damage-only single-vehicle collisions on roadway segments by roadway type...........................................................................98 Figure 17. Plots for base models for total multiple-vehicle collisions at intersections by intersection type. ......................................................................................................108 Figure 18. Plots for base models for fatal-and-injury multiple-vehicle collisions at intersections by intersection type. ............................................................................109 Figure 19. Plots for base models for property-damage-only multiple-vehicle collisions at intersections by intersection type. ............................................................................109 Figure 20. Plots for base models for total single-vehicle collisions at intersections by intersection type. ......................................................................................................114 Figure 21. Plots for base models for fatal-and-injury single-vehicle collisions at intersections by intersection type. ............................................................................114 Figure 22. Plots for base models for property-damage-only single-vehicle collisions at intersections by intersection type. ............................................................................115 Figure 23. Relationships of fixed-object collisions to fixed-object density and offset [adapted from Zegeer and Cynecki (130)]. ..............................................................123
