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NCHRP Web-Only Document 336 Proposed Guidelines for Fixed Objects in the Roadside Design Guide Ingrid B. Potts Douglas W. Harwood MRIGlobal Kansas City, MO Conduct of Research Report for NCHRP Project 17-82 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 336 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 Sheila A. Moore, Program Associate Natalie Barnes, Director of Publications Heather DiAngelis, Associate Director of Publications Linda A. Dziobek, Senior Editor Jennifer Correro, Assistant Editor NCHRP PROJECT 17-82 PANEL Field of TrafficâArea of Safety Tobey Reynolds, New Hampshire Department of Transportation, Concord, NH (Chair) David R. Bizuga, New Jersey Department of Transportation, Trenton, NJ Stephen David Hall, Connecticut Department of Transportation, Newington, CT Jane C. Lundquist, Texas Department of Transportation, Austin, TX Filiberto Sotelo, Illinois Department of Transportation, Springfield, IL Richard Storm, HDR, Minneapolis, MN Scott Zeller, Washington State Department of Transportation, Olympia, WA Richard B. Albin, FHWA Liaison Kelly K. Hardy, AASHTO Liaison Stephen F. Maher, TRB Liaison
iv Contents Summary ..........................................................................................................................................1 Chapter 1. Introduction ....................................................................................................................3 1.1 Background ...........................................................................................................3 1.2 Research Objectives and Scope .............................................................................4 1.3 Organization of This Report ..................................................................................5 Chapter 2. Summary of Literature Review ......................................................................................7 2.1 Studies of Roadway Departure and Fixed-Object Crashes ...................................7 2.2 Crash Modification Factors for Removal and Relocation of Fixed Objects .......10 2.3 RSAP Applications .............................................................................................11 2.4 AASHTO Roadside Design Guide ......................................................................13 2.5 RAP Model for Run-Off-Road Crashes ..............................................................16 2.6 NCHRP Project 17-54 Model .............................................................................21 2.7 Empirical Crash Modeling Approaches ..............................................................32 Chapter 3. Survey of Current Highway Agency Practice ..............................................................34 3.1 Survey Objectives ...............................................................................................34 3.2 Survey Response Rate and Type of Respondents ...............................................34 3.3 Summary of Survey Responses ...........................................................................35 Chapter 4. Database Development ................................................................................................41 4.1 Existing Databases and Supplementary Sites ......................................................41 4.2 Database Assembly .............................................................................................42 4.3 Extent of Available Data .....................................................................................44 Chapter 5. Empirical Model Development ....................................................................................53 5.1 Model Forms Used with Negative Binomial Regression ....................................53 5.2 Model Forms Used with Logistic Regression .....................................................55 5.3 Modeling Results for Tree Groups Using Negative Binomial Regression .........57 5.4 Modeling Results for Individual Trees Using Negative Binomial Regression ...59 5.5 Modeling Results for Individual Utility Poles Using Negative Binomial Regression ...........................................................................................................59 5.6 Modeling Results for Tree Groups Using Logistic Regression ..........................60 5.7 Modeling Results for Individual Trees Using Logistic Regression ....................61 5.8 Models for Individual Utility Poles Using Logistic Binomial Regression .........61 5.9 Discussion of Results ..........................................................................................61 Chapter 6. Application of the RAP Model for Predicting Tree- and Utility-Pole-Related Crashes .........................................................................................................................63 6.1 Summary of the RAP Model for Predicting Run-Off-Road Crashes ..................63 6.2 Strengths and Weaknesses of the RAP Model for Application to Crash Prediction for Roadside Trees and Utility Poles .................................................64 6.3 Approach to Addressing Weaknesses of the RAP Model for Application to Roadside Trees and Utility Poles ........................................................................65 Chapter 7. Modified RAP Model for Predicting Crashes Involving Collisions with Trees and Utility Poles .................................................................................................................73
v 7.1 Roadway and Roadside Characteristics Data Needed to Estimate Roadside Tree and Utility Pole Crashes ..............................................................................74 7.2 General Form of Crash Prediction Model for Run-Off-Road Crashes ...............75 7.3 Road Safety Scores for Run-Off-Road Crashes ..................................................80 7.4 Crash Likelihood Adjustment Factors .................................................................84 7.5 Crash Severity Adjustment Factors .....................................................................87 7.6 Calibration Procedure ..........................................................................................90 7.7 Computational Examples ....................................................................................91 Chapter 8. Development of Design Guidance for Mitigating Collisions with Trees and Utility Poles ...............................................................................................................107 8.1 Design Guidance ...............................................................................................107 8.2 Text Recommended for Incorporation in the AASHTO Roadside Design Guide .....................................................................................107 Chapter 9. Conclusions and Recommendations ..........................................................................108 Chapter 10. References .........................................................................................................109 Current iRAP Model for Predicting Run-Off-Road Crashes ...............................113 A.1 General Form of Crash Prediction Model for Run-Off-Road Crashes .............113 A.2 Road Safety Scores for Run-Off-Road Crashes ................................................114 A.3 Crash Likelihood Risk Factors ..........................................................................119 A.4 Crash Severity Risk Factors ..............................................................................123 Recommended Text for Potential Use in a Future Edition of the AASHTO Roadside Design Guide..............................................................................................126 NCHRP Web-Only Document 336: Proposed Guidelines for Fixed Objects in the Roadside Design Guide is associated with NCHRP Research Report 1016: Design Guidelines for Mitigating Collisions with Trees and Utility Poles. Readers can read or purchase NCHRP Research Report 1016 on the National Academies Press website (www.nap.edu). Figures Figure 1. Crash Edge Definitions by Highway Type ..................................................................22 Figure 2. Roadside Departure Envelope from RSAP ..................................................................69 Figure 3. Factor for Distance from Traveled Way to Roadside Object Expressed as a Piecewise Linear Function ...........................................................................................71 Figure 4. Operating Speed Factors Representing the Relative Risk of Injury for Motor- Vehicle Occupants Involved in Run-Off-Road Crashes as a Function of Mean Traffic Speed ................................................................................................................82 Figure 5. External Flow Influence Factors for Motor-Vehicle Crashes as a Function of AADT ..........................................................................................................................83 Figure 6. Factor for Distance from Traveled Way to Roadside Object Expressed as a Piecewise Linear Function ...........................................................................................88
vi Tables Table 1. Low-Cost Roadway Departure Safety Countermeasures ............................................10 Table 2. Proposed Slope CMF and TAF ....................................................................................11 Table 3. Suggested Clear-Zone Distances in Feet from Edge of Through Traveled Lane ........14 Table 4. Horizontal Curve Adjustment Factor ...........................................................................14 Table 5. Objectives and Strategies for Reducing Utility Pole Crashes ......................................15 Table 6. Objectives and Strategies for Reducing Crashes with Trees .......................................15 Table 7. Crash Types to which Run-Off-Road Risk Factors for Roadside Features Apply ......18 Table 8. Crash Severity Risk Factors for Distance to Roadside Objects ...................................18 Table 9. Crash Severity Risk Factors for Roadside Object Types .............................................19 Table 10. Crash Severity Risk Factors for Paved Shoulder Width ..............................................20 Table 11. Coefficient Values for βSHLD and βUNSHLD ...................................................................24 Table 12. Crash Modification Factors for Longitudinal Barrier Type (CMFLBT) .......................25 Table 13. Crash Modification Factors for Longitudinal Barrier Offset (CMFLBO) .....................26 Table 14. Crash Modification Factors for Longitudinal Barrier Offset (CMFLBO) Adjusted by a Proportional Factor for Each Roadway Type .......................................................26 Table 15. Crash Modification Factors for Narrow Fixed Object Density (CMFFO) ....................27 Table 16. Predicted Fatal-and-Injury Run-Off Crash Frequencies for Assumed Conditions on the Roadside to the Right Side of a Rural Two-Lane Undivided Highway ............28 Table 17. Crash Modification Factors for Narrow Fixed Object Offset (CMFNO) ......................29 Table 18. Crash Modification Factors for Miscellaneous Features Density ................................30 Table 19. Crash Modification Factors for Miscellaneous Features Offset ..................................31 Table 20. Crash Modification Factors for Roadside Slope (CMFS) ............................................31 Table 21. Response Rate for the Highway Agency Survey .........................................................34 Table 22. Summary Data for Tree Groups at Kentucky Sites on Rural Two-Lane Undivided Highways ...................................................................................................45 Table 23. Summary Data for Individual Trees at Kentucky Sites on Rural Two-Lane Undivided Highways ...................................................................................................47 Table 24. Summary Data for Individual Utility Poles at Kentucky Sites on Rural Two-Lane Undivided Highways ...................................................................................................49 Table 25. Summary Data for Tree Groups at Washington Sites on Rural Two-Lane Undivided Highways ...................................................................................................51 Table 26. Summary Data for Individual Trees at Washington Sites on Rural Two-Lane Undivided Highways ...................................................................................................51 Table 27. Summary Data for Individual Utility Poles at Washington Sites on Rural Two- Lane Undivided Highways ..........................................................................................52 Table 28. Negative Binomial Regression Analysis Results for Property-Damage-Only Primary Tree-Related Crashes on Roadway Segments with Roadside Tree Groups ..........................................................................................................................57 Table 29. Predicted Property-Damage-Only Primary Tree-Related Crash Frequency as a Function of AADT for a Rural Two-Lane Undivided Highway Based on the Model Shown in Equation (19) ....................................................................................58 Table 30. Predicted Property-Damage-Only Primary Tree-Related Crash Frequency as a Function of Offset Distance to Roadside Objects for a Rural Two-Lane Undivided Highway Based on the Model Shown in Equation (19) .............................59
vii Table 31. Negative Binomial Regression Analysis Results for Property-Damage-Only Primary and Secondary Utility-Pole-Related Crashes on Roadway Segments with Individual Utility Poles ................................................................................................59 Table 32. Logistic Regression Analysis Results for Property-Damage-Only Primary Tree- Related Crashes on Roadway Segments with Roadside Tree Groups .........................60 Table 33. Logistic Regression Analysis Results for Property-Damage-Only Primary and Secondary Tree-Related Crashes on Roadway Segments with Roadside Tree Groups ..........................................................................................................................60 Table 34. Crash Severity Distribution for Use as Default Values for Estimating Tree Crashes on Rural Nonfreeways with the Modified RAP Model ..................................66 Table 35. Deaths Per Fatal Crash and Persons Injured Per Crash for Use as Default Values for Estimating Tree Crashes on Rural Nonfreeway Crashes with the Modified RAP Model ..................................................................................................................66 Table 36. Crash Severity Distribution for Use as Default Values for Estimating Utility Pole Crashes on Rural Nonfreeway with the Modified RAP Model ...................................66 Table 37. Deaths Per Fatal Crash and Persons Injured Per Crash for Use as Default Values for Estimating Utility Pole Crashes on Rural Nonfreeways with the Modified RAP Model ..................................................................................................................67 Table 38. Crash Severity Proportions for Use as Default Values for Rural Nonfreeways ..........78 Table 39. Deaths and Persons Injured Per Crash for Use as Default Values for Rural Nonfreeways ................................................................................................................78 Table 40. Roadway Design Speed Factor ....................................................................................82 Table 41. External Flow Influence Factors for Motor Vehicle Crashes as a Function of AADT Ranges and Midpoints .....................................................................................83 Table 42. Crash Likelihood Adjustment Factors for Lane Width ................................................84 Table 43. Crash Likelihood Adjustment Factors for Horizontal Curvature ................................85 Table 44. Crash Likelihood Adjustment Factors for Advance Visibility of Curve .....................85 Table 45. Crash Likelihood Adjustment Factors for Percent Grade ............................................85 Table 46. Crash Likelihood Adjustment Factors for Shoulder Rumble Strips ............................86 Table 47. Crash Likelihood Adjustment Factors for Delineation ................................................86 Table 48. Crash Likelihood Adjustment Factors for Road Surface Condition ............................87 Table 49. Crash Likelihood Adjustment Factors for Pavement Skid Resistance ........................87 Table 50. Crash Severity Adjustment Factors for Paved Shoulder Width...................................89 Table 51. Summary of Crash Predictions for Example #1 ..........................................................94 Table 52. Summary of Crash Predictions for Example #2 ..........................................................97 Table 53. Summary of Crash Predictions for Example #3 ........................................................100 Table 54. Summary of Crash Predictions for Example #4 ........................................................106
