Further Considerations for Effective Median Barrier Lateral Placement for Varying Highway Cross Sections

Abstract

The primary purpose of longitudinal median barriers is to redirect errant vehicles that depart the roadway and thus prevent collisions with opposing traffic, terrain features, or roadside objects, which could lead to severe impacts. In the study reported in this paper, vehicle dynamics analysis was used to identify the influences of varying median configurations on interface effectiveness of barrier lateral placement options. The same approach taken in previous efforts to consider five vehicle types for a range of impact conditions was taken here, except that only uniform median configurations were considered. The variations in median shape, side slopes, and roadway elevations in the analyses were reflected in nonsymmetrical override and underride limits. Data for a road section under consideration for median improvement provided the basis for the assessment of the effects of variations in median configuration. Placement effectiveness plots were generated with vehicle dynamics tools for two barrier designs. The results showed effects on lateral placement locations resulted from atypical median configurations and in situations in which there was variation in the relative elevations of the roadways on a divided highway. The results suggested a need to consider the effects of transitions in the median configuration when the barrier type and the lateral position are selected for the full length of a project. The results enhanced understanding of the effectiveness of median barrier placement with various median configurations and barrier options.Over the past decade, cross-median crashes have been recognized as a serious safety problem and departments of transportation have endeavored to mitigate them in various ways. One approach has been to deploy cable barriers to redirect or capture errant vehicles before a cross-median crash can occur. Cable median barriers are considered attractive because of their low cost, short implementation time, ease of installation, and adaptability for sloped conditions. The general consensus is that cable barriers can be highly effective, but cases of underride or override have unexpectedly occurred with catastrophic results.FHWA and NCHRP research on the problem has found that vehicle-to-barrier interface effectiveness is related to lateral placement (1-4). FHWA research has shown that cable barrier effectiveness is related to its design (e.g., number and height of cables, tensioning), configuration of the median (e.g., shape, width, slopes, depth), and lateral position of the barrier within the median. Insights were revealed through vehicle dynamics analysis (VDA) that focused on the vehicleto-barrier interface for various medians and lateral placement. The fundamental premise was that, unless there was a good interface, no barrier could be effective. NCHRP Report 711: Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems provided a wealth of guidance for the effective lateral placement after an exhaustive VDA of various conditions (4). In these efforts a broad ran...

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