Cost-Effectiveness of Guardrail-Bridgerail Transition Improvements: Double W-Beam versus Decreased Post Spacing
Edward Post, Richard Ruby, Patrick McCoy, Terry Wipf
This study was initiated at the request of the Nebraska Department of Roads (NDR) to gain more insight into the performance characterisitics of two guardrail-bridgerail transitions systems; the AASHTO stiff-post system and the NDR "double" beam system. The stiff-post system provides larger size posts on reduced spacings; whereas, the NDR system installs another length of guardrail alongside the face of the existing guardrail with uniform 6 ft-3 in. post spacings. The NDR system eliminates the difficulty of increasing the stiffness of existing systems because of the concrete bridge abutments and/or wing walls restricting the placement of additional posts on reduced spacings. The NDR system has been questioned by some engineers because its performance has not been verified by full-scale testing. Therefore, the objective of this study was to connect a study of "limited" scope using the Barrier VII computer program to ascertain the cost-effectiveness of the stiff-post system in comparison to the NDR "double" beam system. The study took into consideration the effects of two size automobiles impacting the guardrail transitions under all possible combinations of impact speed and angle. The study showed that (1) the stiff-post system was not cost-effictive because it produced more injury type accidents, (2) the stiff-post system resulted in larger exit angles thereby creating increased concern of secondary collisions with other vehicles, and (3) the structural adequacy of the guard-rail-bridgerail connection in both systems was the single most important design element. The findings of this study show that a reasonable doubt exists as to the cost-effectiveness of the AASHTO stiff-post system under a wide range of traffic impact conditions. Further research should be conducted to compare the performance characteristics of the two systems by means of full-scale testing and computer model simulations.
Guardrail, Roadside Safety, BARRIER VII