Improved Prediction Models for Crash Types and Crash Severities (2021) / Chapter Skim
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V CONTENTS Summary ..................................................................................................................................................... xvi 1 Background ........................................................................................................................................... 1 1.1 Project Background ....................................................................................................................... 1 1.2 Project Objectives ......................................................................................................................... 1 2 Analysis Approaches ............................................................................................................................. 3 2.1 Scope of Report ............................................................................................................................. 3 2.2 Crash Type Definitions .................................................................................................................. 3 2.2.1 Crash Types ........................................................................................................................... 3 2.2.2 Delineation of Intersection versus Segment Crashes ........................................................... 6 2.3 Model Estimation Approach ......................................................................................................... 6 2.3.1 Crash Count Models .............................................................................................................. 6 2.3.2 Alternatives for Model Form ................................................................................................. 7 2.3.3 Model Estimation and Fit Statistics ....................................................................................... 8 2.3.4 Crash Severity Modeling ....................................................................................................... 9 2.4 Estimation and Validation Data .................................................................................................. 10 3 Models for Two‐Lane Rural Highways ................................................................................................ 12 3.1 Roadway Segments ..................................................................................................................... 12 3.1.1 Estimation and Validation Data .......................................................................................... 12 3.1.2 Estimated Models ............................................................................................................... 14 3.1.3 Validation of Models ........................................................................................................... 15 3.2 Intersections ............................................................................................................................... 18 3.2.1 Estimation and Validation Data .......................................................................................... 18 3.2.2 Estimated Models ............................................................................................................... 25 3.2.3 Validation of Models ........................................................................................................... 31 4 Models for Multilane Rural Highways ................................................................................................. 35 4.1 Roadway Segments ..................................................................................................................... 35 4.1.1 Estimation and Validation Data .......................................................................................... 35 4.1.2 Estimated Models ............................................................................................................... 40 4.1.3 Validation of Models ........................................................................................................... 43
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vi 4.2 Intersections ............................................................................................................................... 47 4.2.1 Estimation and Validation Data .......................................................................................... 47 4.2.2 Estimated Models ............................................................................................................... 54 4.2.3 Validation of Models ........................................................................................................... 54 5 Models for Urban and Suburban Arterials .......................................................................................... 61 5.1 Roadway Segments ..................................................................................................................... 61 5.1.1 Estimation Data ................................................................................................................... 61 5.1.2 Estimated Models ............................................................................................................... 67 5.1.3 Validation of Models ........................................................................................................... 68 5.2 Intersections ............................................................................................................................... 83 5.2.1 Estimation Data ................................................................................................................... 83 5.2.2 Estimated Models ............................................................................................................... 89 5.2.3 Validation of Models ........................................................................................................... 95 6 Revisiting the HSM Calibration Approach ......................................................................................... 102 6.1 Approaches Considered ............................................................................................................ 102 6.1.1 Background on HSM Approach ......................................................................................... 102 6.1.2 Approach 1 ........................................................................................................................ 103 6.1.3 Approach 2 ........................................................................................................................ 103 6.2 Approach 1 Results ................................................................................................................... 103 6.2.1 Urban Four‐Lane Divided Segments ................................................................................. 103 6.2.2 Urban Two‐Lane Undivided Segments .............................................................................. 104 6.2.3 Rural Two‐Lane, Three‐Leg Stop‐Controlled Intersections ............................................... 104 6.3 Approach 2 Results ................................................................................................................... 105 6.4 Conclusions on Calibration Exercise ......................................................................................... 108 6.4.1 Summary of Findings ......................................................................................................... 108 6.4.2 Recommended Calibration Procedure Update ................................................................. 108 7 Findings and Conclusions .................................................................................................................. 111 7.1 Proposed Models and Procedures for Manual ......................................................................... 111 7.1.1 Two‐Lane Rural Highway Models ...................................................................................... 111 7.1.2 Multilane Rural Highway Models ...................................................................................... 114 7.1.3 Urban/Suburban Arterial Models ..................................................................................... 115 7.1.4 Revisit of the Calibration Procedure ................................................................................. 118
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vii 7.2 Conclusions ............................................................................................................................... 118 8 References ........................................................................................................................................ 120 Appendix A Average Condition Models A‐1 Appendix B Crash Severities – Ordered Probit Fractional Split Modeling Approach B‐1 Appendix C Draft Content for Highway Safety Manual, 2nd Edition C‐1 Appendices A, B, and C are not published here, but are available on the TRB website (www.TRB.org) by searching for "NCHRP Web‐Only Document 295".
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viii LIST OF TABLES Table 2‐1: Base Condition Crash Type Models Estimated for Each Facility Type ......................................... 5 Table 2‐2: Data Used for Estimation and Validation ................................................................................... 11 Table 3‐1: HSM Base Conditions and Data Availability, Two‐Lane Undivided (2U) Segments ................... 12 Table 3‐2: Descriptive Statistics for Base Condition SPF Estimation, Two‐Lane Undivided (2U)
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ix Table 4‐4: Descriptive Statistics for Base Condition Validation Data, Four‐Lane Undivided (4U) Segments .................................................................................................................................................................... 37 Table 4‐5: HSM Base Conditions and Data Availability, Four‐Lane Divided (4D)
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x Table 5‐2: Ohio Segment Crash Type Statistics for Five‐Year Period for Base Condition Sites (Urban/Suburban Arterial Segments) ......................................................................................................... 64 Table 5‐3: OH Segment Continuous Variable Statistics for Base Condition Sites (Urban/Suburban Arterial Segments)
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xi Table 5‐37: Descriptive statistics for base condition SPFs (3ST: 2082 intersections) ................................. 87 Table 5‐38: Descriptive statistics for base condition SPFs (4SG: 589 intersections)
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xii LIST OF FIGURES Figure 2‐1: General Taxonomy of Crash Types ............................................................................................. 4 Figure 6‐1: Suggested Calibration Process ................................................................................................ 110 Figure 7‐1: Crash Types Estimated for Segments on Two‐Lane Rural Highways ...................................... 112 Figure 7‐2: Specific Crash Types Included in the Estimated Crash Types (Rural 2U) . ............................... 113 Figure 7‐3: Crash Types Estimated for Intersections on Two‐Lane Rural Highways. ................................ 113 Figure 7‐4: Specific Crash Types included in the Estimated Crash Types (Rural 3ST, 4ST and 4SG)
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From page 12... ...
xiii LIST OF ABBREVIATIONS Crash types ANG Right angle ANIMAL Animal related BIKE Bicycle–vehicle FO Fixed object HO Head‐on HO+SOD Head‐on plus sideswipe opposite direction ID Intersecting direction (all severity levels) LEFT Left turn MO Moving object MV Multiple vehicle (all severity levels)
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xiv Facility types 2U Two‐lane undivided 3SG Three‐leg signal‐controlled 3ST Three‐leg stop‐controlled 3T Two‐lane plus two‐way left‐turn lane 4D Four‐lane divided 4SG Four‐leg signal‐controlled 4ST Four‐leg stop‐controlled 4U Four‐lane undivided 5T Four‐lane plus two‐way left‐turn lane Variables (with definitions) AADT Average annual daily traffic AADTmaj AADT on the major road (higher volume)
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xv ParkingProp Proportion of curb length with on‐street parking Parking Type Indicates angled or parallel on‐street parking Speed Limit Posted speed limit in miles per hour Other Abbreviations AASHTO American Association of State Highway and Transportation Officials AIC Akaike's Information Criterion BIC Bayesian Information Criterion CMF Crash modification factor DOT Department of Transportation FHWA Federal Highway Administration GLM Generalized linear model GOF Goodness of fit HSIS Highway Safety Information System HSM Highway Safety Manual MAD Mean absolute deviation MSPE Mean squared prediction error NB Negative binomial OH Ohio SPF Safety performance function
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