Cost-Effectiveness of Guardrail Improvements for Protecting Bridge Piers in Depressed Medians on Horizontal Curves
Edward Post, Patrick McCoy, Walter Witt, Terry Wipf, Patrick Chastain
During the past two decades, many full-scale crash tests have been conducted on guardrail located on level and flat terrain. However, little attention has been given to the testing and placement of guardrail on embankment slopes, and as a result, errant vehicles have vaulted over guardrail and snagged on guardrail posts. To investigate this problem, a typical site was selected on Interstate 80 near Lincoln, Nebraska on which a guardrail of current design, G4(2W)*, was protecting bridge piers in a depressed median on curved horizontal alignment. The existing guardrail was 200 ft long, offset 15 ft from the roadway and 4 ft in front of the bridge piers, and its upstream anchored end was not flared or safety treated. Seven improvement alternatives were evaluated. BARRIER VII simulations of a standard size automobile were conducted to obtain guardrail impact severities. Severity adjustments were made to account for probable causes of vaulting and snagging as predicted from HVOSM bumper trajectories. Impact condition probabilities were established to properly weight the severities under all possible combinations of encroachment speed and angle. The results of this study indicated that the most attractive improvement alternative was guardrail and over a 20 yr life at 9% compound interest it would result in (1) a reduction of 1.12 injury type accidents, and (2) a net present worth injury accident savings of $60,500. The improvement consisted of relocating the existing guardrail 2 ft closer to the bridge piers, reducing its length from 200 to 95 ft, installing a rub rail, flaring the upstream turned-down end, and providing vertical slip joints on the first 5 posts so that the guardrail can breakaway under head-on type impacts. The feasibility of providing downstream end anchorage is uncertain in this study because the computer model simulations were made at one point of impact in the vicinity of the upstream bridge pier.
Guardrail, Roadside Safety, HVOSM, BARRIER VII