National Academies Press: OpenBook

Guidelines for Slope Traversability (2019)

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Suggested Citation:"Contents." National Academies of Sciences, Engineering, and Medicine. 2019. Guidelines for Slope Traversability. Washington, DC: The National Academies Press. doi: 10.17226/25415.
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Suggested Citation:"Contents." National Academies of Sciences, Engineering, and Medicine. 2019. Guidelines for Slope Traversability. Washington, DC: The National Academies Press. doi: 10.17226/25415.
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Suggested Citation:"Contents." National Academies of Sciences, Engineering, and Medicine. 2019. Guidelines for Slope Traversability. Washington, DC: The National Academies Press. doi: 10.17226/25415.
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Suggested Citation:"Contents." National Academies of Sciences, Engineering, and Medicine. 2019. Guidelines for Slope Traversability. Washington, DC: The National Academies Press. doi: 10.17226/25415.
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Suggested Citation:"Contents." National Academies of Sciences, Engineering, and Medicine. 2019. Guidelines for Slope Traversability. Washington, DC: The National Academies Press. doi: 10.17226/25415.
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Suggested Citation:"Contents." National Academies of Sciences, Engineering, and Medicine. 2019. Guidelines for Slope Traversability. Washington, DC: The National Academies Press. doi: 10.17226/25415.
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iii CONTENTS CHAPTER 1. INTRODUCTION ................................................................................................ 1  1.1 RESEARCH PROBLEM STATEMENT ............................................................................. 1  1.2 RESEARCH OBJECTIVE ................................................................................................... 1  1.3 RESEARCH APPROACH AND REPORT LAYOUT ........................................................ 2  CHAPTER 2. LITERATURE REVIEW .................................................................................... 3  2.1 ROADSIDE DESIGN GUIDE (RDG), AASHTO ............................................................... 8  2.2 ENCROACHMENT SIMULATION STUDIES ................................................................ 11  2.2 VEHICLE TESTING WITH REMOTE CONTROL DRIVER INPUT ............................ 19  CHAPTER 3. CRASH DATA ANALYSIS .............................................................................. 23  3.1 INTRODUCTION .............................................................................................................. 23  3.2 CRASH DATABASES ....................................................................................................... 24  3.2.1 FARS Database ........................................................................................................ 25  3.2.2 GES Database .......................................................................................................... 25  3.2.3 CDS and Associated Databases ............................................................................... 27  3.3 CLASSIFICATION OF VEHICLES .................................................................................. 29  3.4 RISK MEASURES AND STATISTICAL METHODS ..................................................... 33  3.4.1 Composition Ratio (CR) .......................................................................................... 33  3.4.2 Relative Risk (RR) ................................................................................................... 34  3.5 ANALYSIS PROCEDURES AND RESULTS .................................................................. 35  3.5.1 Composition Ratio ................................................................................................... 36  3.6 CONCLUSION ................................................................................................................... 45  CHAPTER 4. COMPARISON OF VEHICLE CHARACTERISTICS ................................ 51  CHAPTER 5. SIMULATION ANALYSIS............................................................................... 55  5.1 INTRODUCTION .............................................................................................................. 55  5.2 SIMULATION TOOLS ...................................................................................................... 55  5.2.1 Vehicle Characteristics ............................................................................................ 55  5.2.2 Terrain Modeling ..................................................................................................... 56  5.2.3 Vehicle and Terrain Contact .................................................................................... 56  5.2.4 Multi-rigid Body Vehicle Dynamics Codes vs. Finite Element Codes ................... 56  5.3 VEHICLE MODELS .......................................................................................................... 61  5.4 WRAPPER PROGRAM ..................................................................................................... 64  5.4.1 Wrapper Program Structure and Overview .............................................................. 65  5.4.2 Inputs Generation Module ....................................................................................... 69  5.4.3 Simulation Stopping Conditions .............................................................................. 72  5.4.4 Simulation Outputs .................................................................................................. 72  5.5 SOIL FURROWING FORCES .......................................................................................... 74  5.6 SENSITIVITY STUDIES ................................................................................................... 77  5.6.1 Perception-Reaction Time ....................................................................................... 78  5.6.2 Encroachment Yaw Rate .......................................................................................... 78  5.7 EVALUATION OF FRICTION MODEL AND LATERAL COEFFICIENT .................. 79  5.7.1 Effectiveness of the Friction Ellipse Model ............................................................. 81  5.7.2 Selection of Appropriate Lateral Friction Coefficient ............................................. 84  5.8 SIMULATION MATRIX ................................................................................................... 85 

iv 5.9 SIMULATION RESULTS ................................................................................................. 87  CHAPTER 6. VEHICLE TESTING ON SLOPE .................................................................... 97  6.1 TEST MATRIX DESIGN .................................................................................................. 97  6.2 EXPECTED TEST RESULT VARIATIONS .................................................................... 98  6.3 VEHICLE TRAVERSAL TESTS .................................................................................... 105  6.3.1 Testing Procedure .................................................................................................. 106  6.3.2 Test Vehicles .......................................................................................................... 107  6.3.3 Test Results ............................................................................................................ 110  6.3.4 Terrain Deterioration and Its Effects ..................................................................... 124  6.4 TEST AND SIMULATION COMPARISON .................................................................. 124  CHAPTER 7. GUIDELINE DEVELOPMENT ..................................................................... 135  7.1 PROBABILITY DISTRIBUTIONS ................................................................................. 135  7.1.1 Encroachment Angle and Speed Probability Distributions ................................... 135  7.1.2 Driver Input Probability Distribution..................................................................... 136  7.1.3 Vehicle Type Probability Distribution ................................................................... 138  7.2 EVALUATION OF WEIGHTED SIMULATION RESULTS ........................................ 141  7.2.1 Shoulder Width ...................................................................................................... 141  7.2.2 Foreslope Width ..................................................................................................... 142  7.2.3 Foreslope ................................................................................................................ 142  7.2.4 Generalized Traversability ..................................................................................... 143  7.3 TRAVERSABILITY GUIDELINES ............................................................................... 143  CHAPTER 8. SLOPE TRAVERSABILITY GUIDELINES ................................................ 151  8.1 EXAMPLE OF USE ......................................................................................................... 154  CHAPTER 9. SUMMARY AND CONCLUSIONS ............................................................... 155  REFERENCES .......................................................................................................................... 157  APPENDIX A - CLASSIFICATION OF VEHICLE BODY TYPES BY NHTSA ............ 161  APPENDIX B - RISK MEASURES AND STATISTICAL METHODS ............................. 165  APPENDIX C - CARSIM WRAPPER PROGRAM USERS’ GUIDE ................................ 175 

v LIST OF FIGURES Figure 2.1. Rollover occupant fatalities by vehicle type. (1) .......................................................... 5  Figure 2.2. Designation of roadside regions. (7) ............................................................................. 9  Figure 2.3. Survey results of typical roadside foreslopes. ........................................................... 18  Figure 2.4. Block diagram of automatic vehicle controller. ........................................................ 20  Figure 3.1. Distributions of slope-related rollover and crashes by vehicle body types - FARS data. ........................................................................................................................ 37  Figure 3.2. Vehicle composition ratios by vehicle body types - FARS data. .............................. 38  Figure 3.3. Relative risks of slope-related rollovers for one particular vehicle body type to that of all the rest of the body types and associated confidence intervals: FARS data. ................................................................................................................................... 41  Figure 3.4. Relative risks of slope-related rollovers for one particular vehicle body type to that of 4-door sedans and associated confidence intervals - FARS data. ..................... 42  Figure 3.5. Relative risks of slope-related rollovers for one particular vehicle body type to that of the rest of the body types and associated confidence intervals - GES data. ................................................................................................................................... 44  Figure 4.1. Static stability factor comparison of vehicles most likely to rollover on slopes................................................................................................................................. 53  Figure 4.2. Roll moment of intertia comparison of vehicles most likely to rollover on slopes................................................................................................................................. 54  Figure 4.3. Relative risks of slope-related rollovers for one particular vehicle body type to that of the rest of the body types and associated confidence intervals. ........................ 54  Figure 5.1. Comparisons Carsim and LS-DYNA in simulating vehicle encroachment events on slope (a) pickup truck encroachment, (b) passenger car encroachment. .......... 57  Figure 5.2. Body to terrain contact forces. ................................................................................... 60  Figure 5.3. Comparison of vehicle properties between 2001 Ford Taurus and 2004 Honda Accord. .................................................................................................................. 62  Figure 5.4. Carsim’s Class-B vehicle model was used as base model to making changes. ........ 63  Figure 5.5. Locations of hard points underneath Kia Rio (in millimeters). ................................. 63  Figure 5.6. TTI’s Carsim Wrapper Program main flowchart. ..................................................... 67  Figure 5.7. Flowchart of the inputs generation module of TTI’s wrapper program. ................... 68  Figure 5.8. Flowcharts of the subroutines for generating Carsim (a) terrain/road inputs, and (b) events inputs (steering, braking, encroachment speeds, angles, rates, etc.). ........ 70  Figure 5.9. Flowchart of TTI’s code for applying body-to-terrain contact with Carsim. ............ 71  Figure 5.10. Properties of tires in Carsim. Lateral force is plotted as function of slip angle for different vertical tire loads. ................................................................................ 75  Figure 5.11. Friction ellipse model for modeling tire forces due to soil furrowing. ................... 76  Figure 5.12. Roadside ditch profile used for sensitivity analysis. ............................................... 77  Figure 5.13. Results of the sensitivity analyses for determining perception-reaction time. ........ 79  Figure 5.14. Results of the sensitivity analyses for determining encroachment yaw rate. .......... 79  Figure 5.15. Simulation matrix for evaluation of the friction ellipse model and determination of the lateral friction coefficient. ............................................................... 81  Figure 5.16. Lateral tire forces for small car and pickup truck with 25 mi/h initial speed, tracking initial conditions, and panic return-to-road steer after 1 second P/R time. ........ 82 

vi Figure 5.17. Lateral tire forces for small car and pickup truck with 35 mi/h initial speed, non-tracking initial conditions, constant steer angle, and full ABS brakes. ..................... 83  Figure 5.18. Lateral tire forces for small car and pickup truck with 55 mi/h initial speed, non-tracking initial conditions, constant steer angle, and full ABS brakes. ..................... 83  Figure 5.19. Percentages and numbers of rollovers for different values of maximum lateral coefficient of friction. ............................................................................................ 85  Figure 5.20. Simulation matrix. ................................................................................................... 86  Figure 5.21. Driver inputs for the encroachment simulations. .................................................... 87  Figure 5.22. Influence of roadside slope. ..................................................................................... 90  Figure 5.23. Influence of shoulder width. .................................................................................... 91  Figure 5.24. Influence of slope width. ......................................................................................... 92  Figure 5.25. Influence of encroachment speed. ........................................................................... 93  Figure 5.26. Influence of encroachment angle. ............................................................................ 94  Figure 5.27. Influence of vehicle type. ........................................................................................ 95  Figure 6.1. Full-scale slope traversability test matrix and test site cross-section. ....................... 98  Figure 6.2. Variation in vehicle kinematics due to different terrain friction. Four simulations are shown overlaid with same vehicle model but varying maximum lateral friction coefficient. ............................................................................................... 100  Figure 6.3. Vehicle kinematics of pickup trucks encroaching at 55 mi/h and 5-degree angle with different terrain coefficients. ......................................................................... 101  Figure 6.4. Vehicle kinematics of pickup trucks encroaching at 35 mi/h and 5-degree angle with different terrain coefficients. ......................................................................... 102  Figure 6.5. Vehicle kinematics of small cars encroaching at 45 mi/h and 10-degree angle with different terrain coefficients. ................................................................................... 103  Figure 6.6. Vehicle kinematics of small cars encroaching at 55 mi/h and 5-degree angle with different terrain coefficients. ................................................................................... 104  Figure 6.7. Design and photo of the ditch with 1V:3H foreslope for vehicle traversal testing. ............................................................................................................................. 106  Figure 6.8. Steering and braking control installed on small car (left) and pickup (right). ........ 107  Figure 6.9. Pickup truck test vehicle properties. ........................................................................ 108  Figure 6.10. Small car test vehicle properties. ........................................................................... 109  Figure 6.11. Test target and actual encroachment conditions. ................................................... 110  Figure 6.12. Sequential photographs for Test No. 479340-1-1 (rear view). .............................. 112  Figure 6.12. Sequential photographs for Test No. 479340-1-1 (rear view) (continued). .......... 113  Figure 6.13. Sequential photographs for Test No. 479340-1-2 (rear view). .............................. 114  Figure 6.13. Sequential photographs for Test No. 479340-1-2 (rear view) (continued). .......... 115  Figure 6.14. Sequential photographs for Test No. 479340-1-3 (rear view). .............................. 116  Figure 6.14. Sequential photographs for Test No. 479340-1-3 (rear view) (continued). .......... 117  Figure 6.15. Sequential photographs for Test No. 479340-1-4 (rear view). .............................. 118  Figure 6.15. Sequential photographs for Test No. 479340-1-4 (rear view) (continued). .......... 119  Figure 6.16. Sequential photographs for Test No. 479340-1-5 (rear view). .............................. 120  Figure 6.16. Sequential photographs for Test No. 479340-1-5 (rear view) (continued). .......... 121  Figure 6.17. Sequential photographs for Test No. 479340-1-6 (rear view). .............................. 122  Figure 6.17. Sequential photographs for Test No. 479340-1-6 (rear view) (continued). .......... 123  Figure 6.18. Sequential photographs for Test No. 479340-1-1 (rear view). .............................. 126  Figure 6.19. Sequential photographs for Test No. 479340-1-2 (rear view). .............................. 127 

vii Figure 6.20. Sequential photographs for Test No. 479340-1-3 (rear view). .............................. 128  Figure 6.21. Sequential photographs for Test No. 479340-1-4 (rear view). .............................. 129  Figure 6.22. Sequential photographs for Test No. 479340-1-5 (rear view). .............................. 130  Figure 6.23. Sequential photographs for Test No. 479340-1-6 (rear view). .............................. 131  Figure 6.241. Simulation versus test comparison of vehicle path for tests with pickup. ........... 132  Figure 6.25. Simulation versus test comparison of vehicle path for tests with small car. ......... 133  Figure 7.1. Encroachment angle distribution for weighting simulation results. ........................ 136  Figure 7.2. Encroachment speed distribution for weighting simulation results. ....................... 136  Figure 7.3. Tracking and non-tracking conditions of the vehicle. ............................................. 137  Figure 7.4. Probability distribution for driver inputs used in the simulation analyses. ............. 138  Figure 7.5. Passenger car classification method by HLDI. ........................................................ 138  Figure 7.6. Probability distribution for vehicle types used in simulation. ................................. 139  Figure 7.7. Probability distributions for weighting simulation results. ..................................... 140  Figure 7.8. Influence of shoulder width on rollover probability for various foreslopes. ........... 144  Figure 7.9. Influence of foreslope width on rollover probability. ............................................. 145  Figure 7.10. Influence of foreslope on rollover probability. ...................................................... 146  Figure 7.11. Probability of various outcomes for vehicle encroaching on slopes ..................... 147  Figure 7.12. Slope traversability guidelines showing vehicle rollover probability. .................. 148  Figure 7.13. Slope traversability guidelines showing probability of vehicle return to roadway. .......................................................................................................................... 149  Figure 8.1. Slope traversability guidelines. ............................................................................... 152  Figure 8.2. Slope traversability guidelines showing probability of vehicle return to roadway. .......................................................................................................................... 153 

viii LIST OF TABLES Table 2.1. Top five most harmful events after vehicles involved certain roadside features as the first harmful event (FARS: 2004 to 2009) ................................................................ 4  Table 3.1. Slope-related rollover crash frequencies and distribution by NHTSA’s vehicle body type: FARS data ....................................................................................................... 31  Table 3.2. Slope-related rollover crash frequencies and distribution by NHTSA’s vehicle body type: GES data ......................................................................................................... 32  Table 3.3. Distributions of slope-related rollover crashes and slope-related crashes by vehicle body type and associated vehicle composition ratios: FARS data ....................... 37  Table 3.4. Distributions of slope-related rollover crashes and slope-related crashes by vehicle body type and associated vehicle composition ratios: GES data ......................... 38  Table 3.5. Relative risks of slope-related rollovers for one particular vehicle type to that of all the rest of the vehicle types and associated confidence interval: FARS data .......... 39  Table 3.6. Relative risks of slope-related rollovers for one particular vehicle type to that of 4-door sedans and associated confidence interval: FARS data .................................... 40  Table 3.7. Relative risks of slope-related rollovers for one particular vehicle type to that of the rest of the vehicle types and associated confidence interval: GES data ................. 43  Table 3.8. Top ranked vehicle models and model-years based on slope-related rollover crash frequencies: FARS data ........................................................................................... 46  Table 3.9. Risk of rollover for a number of vehicle models by drive system .............................. 48  Table 5.1. Simulation outcomes logged in the aggregate simulation results table. ..................... 73  Table 5.2. Simulation output data saved for each simulation case. ............................................. 74  Table B.1. Number of slope-related crashes by vehicle type and rollover status ..................... 167  Table B.2. Number of slope-related rollover crashes and a surrogate measure for slope- related crashes ................................................................................................................. 170 

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TRB’s National Cooperative Highway Research Program (NCHRP) has released a pre-publication version of Research Report 911: Guidelines for Slope Traversability, which includes guidelines for determining the traversability of roadside slopes considering the characteristics of the current passenger vehicle fleet.

As part of development of this report, researchers performed full-scale traversability tests and compared the performance of the vehicles with the simulations performed for the same test conditions.

Rollovers are the leading cause of fatalities in single vehicle ran-off-road (SVROR) crashes. Analysis of six years of data from the National Automotive Sampling System Crashworthiness Data System indicates that 31% of SVROR crashes result in a rollover. Approximately 75% of these rollover crashes are initiated by vehicles digging into the ground on embankments or in ditches after encroaching onto the roadside.

Development of NCHRP Research Report 911 was prompted by concern that some roadside slope conditions that have for many years been considered traversable for passenger cars may not be traversable for light trucks. With the steadily increasing percentage of light trucks in the vehicle fleet, further research was needed to determine what should be considered as safe sideslope conditions for today’s vehicle fleet. Proper assessment of slope traversability may help reduce the number of rollover crashes and associated fatalities.

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