The National Academies

NCHRP 17-55 [Completed]

Guidelines for Slope Traversability

  Project Data
Funds: $500,000
Research Agency: Texas Transportation Institute
Principal Investigator: Nauman Sheikh
Effective Date: 5/2/2012
Completion Date: 11/15/2018
Comments: Publication decision pending.

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 AASHTO Roadside Design Guide (RDG) considers foreslope ratios ranging from 3:1 up to 4:1 to be traversable but non-recoverable. Slopes steeper than 3:1 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.  A recent study suggests 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 constitutes recoverable, traversable, and critical sideslope conditions for today’s vehicle fleet. Proper assessment of slope traversability will help reduce the number of rollover crashes and associated fatalities.
The objective of this research is to develop guidelines for what constitutes recoverable, traversable, and critical sideslope conditions considering the characteristics of today’s passenger vehicle fleet.
Accomplishment of the project objective will require at least the following tasks.
Phase I
(1). Conduct a literature review on slope traversibility, including crash studies of various degrees of severity and safety risk associated with different slope and vehicle type characteristics.(2). Review and analyze data from Fatality Analysis Reporting System (FARS) and other existing databases to identify vehicle types most likely to rollover on slopes. (3). Compare characteristics, including stability, of the vehicle types identified in Task 2 to the vehicle types in the Manual for Assessing Safety Hardware (MASH). (4). Develop computer simulation for MASH test vehicles to identify the limits of recoverable, traversable, and critical slopes. Define conditions where the foreslope is the main factor in the test outcome. Quantify vehicle performance, influenced by conditions such as departure angle, tracking conditions, steepness of slope, height of fill, soil friction, speed, and other pertinent factors.(5). Design a test matrix to determine the critical performance conditions for recoverable, traversable, and critical sideslope conditions.(6). Prepare an interim report providing the results of Tasks 1 through 5.  The interim report should include critical inputs into crash outcomes, vehicle type comparison, situations where the foreslopes are the critical roadway feature, how the test matrix was developed, and an updated work plan for Phase II. The research plan shall provide a 1-month period for review and approval of the interim report. An interim meeting of the project panel to discuss the report with the research agency will be required. The research agency shall not begin work on the remaining tasks without NCHRP approval.
Phase II
(7). Conduct crash testing based on the approved Phase II work plan to validate the models.(8). Calibrate the Task 4 computer model based on results of crash testing. Apply verification and validation techniques in accordance with NCHRP Project 22-24, Procedures for Verification and Validation of Computer Simulations Used for Roadside Safety Applications, to these results which produced NCHRP Web-Only Doc 179, by the same name(9). Use the computer simulation for MASH vehicles to identify the limits of recoverable, traversable, and critical slopes. Re-quantify vehicle performance, influenced by conditions such as departure angle, tracking conditions, steepness of slope, height of fill, soil friction, speed, and other pertinent factors.(10). Determine the expected performance of the vehicle types identified in Task 2.(11). Develop the guidelines using the results of the computer simulations from Task 9 and expected vehicle performance determined in Task 10.(12). Submit a final report documenting the entire research effort. Include the proposed guidelines in a stand-alone appendix, in a format suitable for consideration by AASHTO.

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