National Academies Press: OpenBook

Guidelines for Slope Traversability (2019)

Chapter: Chapter 1. Introduction

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Suggested Citation:"Chapter 1. Introduction." 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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Page 8
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Suggested Citation:"Chapter 1. Introduction." 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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Page 9

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1 CHAPTER 1. INTRODUCTION 1.1 RESEARCH PROBLEM STATEMENT 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 (NASS) Crashworthiness Data System (CDS) 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. Higher centers of gravity make light trucks (e.g., pickup trucks, sport utility vehicles, vans) inherently less stable than passenger cars. Numerous crash data studies have documented that light trucks are overrepresented in rollover crashes. For example, the above referenced analysis of the NASS CDS indicates the risk of a utility vehicle rolling over in a SVROR crash on a high-speed roadway is 2.2 times that of a passenger car. Sales of light trucks continue to increase each year. As a group, light truck sales currently outpace sales of passenger cars, accounting for over 50% of all new passenger vehicles sold. Thus, it is important to update roadside safety guidelines and practices to accommodate the current vehicle fleet. Due in part to a lack of roadside data in most crash databases, little information is available regarding the percentage of overturns versus total vehicle encroachments for different sideslope ratios. The 2006 American Association of State Highway and Transportation Officials (AASHTO) Roadside Design Guide (RDG) considers foreslope ratios ranging from 1V:3H up to 1V:4H to be traversable but non-recoverable. Slopes steeper than 1V:3H have historically been considered critical foreslopes. The RDG states that such slopes “will cause most vehicles to overturn and should be treated (i.e., flattened or shielded with a barrier) if they begin within the clear-zone distance of a particular highway….” This guidance is based largely on studies that were conducted in the late 1960s and early 1970s and included only a very limited number of full-scale embankment tests and computer simulations with passenger cars. There is also a 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 is needed to determine what should be considered as safe sideslope conditions for today’s vehicle fleet. Proper assessment of slope traversability will help reduce the number of rollover crashes and associated fatalities. 1.2 RESEARCH OBJECTIVE The objective of this research was to develop guidelines for determining the traversability of roadside slopes considering the characteristics of the current passenger vehicle fleet.

2 1.3 RESEARCH APPROACH AND REPORT LAYOUT The objectives of this research were achieved by completing various tasks that are described in this report. Chapter 2 presents a detailed literature review that was conducted on the topic of slope traversability. It includes research studies related to the severity of slope crashes and safety risks associated with different slopes and vehicle characteristics. It also includes past and ongoing research that involves use of simulation for identifying or mitigating slope related hazards. In Chapter 3, a review and analysis of Fatality Analysis Reporting System (FARS) and other existing databases is presented. These analyses were conducted to identify vehicle types most likely to rollover on slopes, which are also presented in this chapter. In Chapter 4, the researchers have presented a comparison of characteristics of the vehicle types identified after the analysis of the FARS database. A comparison of the design vehicles specified in AASHTO Manual for Assessing Safety Hardware (MASH) is also presented in this chapter. In Chapter 5, the researchers have presented details of the simulation analyses performed to identify the traversability of various roadside slope configurations. This chapter presents the various tools that were used for the simulation analyses, details of the models used, some of the sensitivity studies performed for various parameters used in the analyses, and the results of the simulation analyses. One aspect of this project was to perform full-scale traversability tests and to compare the performance of the vehicles with the simulations performed for the same test conditions. Details of these tests and their comparison to the simulation analyses are presented in Chapter 6. In Chapter 7, the process of using the results of the simulations to arrive at the traversability guidelines is explained in detail. This chapter describes how simulation results were weighted according to the probability of their occurrence in the real world. Development of the final guidelines using the weighted simulation results is also presented in this chapter. The proposed traversability guidelines emanating from this research are presented in Chapter 8 as a standalone chapter. This chapter can be included in other documents to assist with the implementation of this research.

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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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