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Vehicle Dynamics Investigations to Develop Guidelines for Crash Testing Cable Barriers on Sloped Surfaces

Crashworthiness evaluations of roadside barrier testing have traditionally been conducted on level terrain to allow a convenient basis for comparison of alternative designs. The emergence of a new generation of cable median barriers to address cross-median crash problems has led to increased placement of these barriers on slopes. Recent research using vehicle dynamics analysis has shown that barrier effectiveness is influenced by slope conditions, barrier design, lateral placement, and median configuration. While there has been some testing of cable median barriers on slopes, there is not a prescribed procedure for testing on slopes, nor a means to evaluate crashworthiness across a range of slope conditions. Research by the FHWA has demonstrated the usefulness of vehicle dynamics analysis to understand barrier effectiveness on slopes. This paper describes an effort to apply vehicle dynamics analysis to determine basic considerations for the development of slope testing procedures. The analyses considered the need to test for both override and underride potentials. The analysis considered various median widths for basic v-shaped medians with 4:1 side slopes (the normal side slope limit). The analysis considered the dynamics for a pick-up truck, mid-sized sedan, and small car. This study focused on impacts under the Test Level 3 (TL3) condition for cable barrier systems that are designed to work when placed anywhere in 4:1 or shallower medians. It was noted from previous research that the pick-up truck was prone to override while the small car more likely to underride a cable barrier. Since the mid-sized sedan was found to be over-represented in underride crashes, it was also considered in this analysis. The results suggest that slope testing procedures need to include an override test with a barrier placed at 10-12 ft from the nearside edge of the median and an underride test on the farside of a 40-48 ft wide median. It was noted that the mid-sized sedan had an underride potential similar to the small car, but that because of its higher mass, suspension and wheel forces were more than 20% greater than those forces of the small car. The results presented are expected to be validated with full-scale tests, but it is believed that the information generated provides a robust basis for the development of improved procedures for slope testing. Working Paper NCAC 2010-W-009 August 2010