W-Beam Strong Post Guardrail Minimum Height TL-2 Safety Analyses

Background: A constant challenge is faced when addressing roadside safety issues to meet maintenance requirements when roads are repaved, and also for a changing fleet of vehicles. It has been shown through simulation and confirmed by crash testing under National Cooperative Highway Research Program (NCHRP) 350 TL 3 that barriers below the nominal top-of-rail heights increase the propensity for the 2000p vehicle to vault the barrier [1,2]. Consequently, there has been considerable effort to make agencies aware of the need to maintain barrier heights to avoid potential vaulting problems and to develop modifications for barriers to cost effectively increase their height. Under the Manual for Assessing Safety Hardware (MASH) Test Level 2 (TL2), barriers have the same crash test requirements except that a lower impact speed is used. Research has not, however, considered guardrail height needs for TL 2 criteria for NCHRP 350 nor the new MASH criteria. Agencies need to understand the risks, if any, of allowing barriers to exist at lower heights and be able to evaluate options to strategically remedy possible problems. The question of whether to raise a guardrail or not comes up on many occasions during pavement construction projects. Guidelines have been developed for high-speed (TL-3) facilities to address when barriers should be raised. With fiscal constraints on all agencies, it is desirable to know if it would be satisfactory for a strong-post W-beam barrier to remain in place at a lower than standard height in a TL-2 type of condition (45 mph). Computerized crash simulation has been used many times over the past 20+ years to design and/or evaluate w-beam guardrail systems. The FHWA study cited earlier is an excellent example of such and effort. Models have become more detailed and subsequent analyses have demonstrated that barrier design details can be effectively replicated (e.g., the crash performance of raising block-outs up to 3 inches [3]. Other guardrail performance conditions have also been evaluated [4 -12]. Thus, the use of models and simulation can be expected to provide viable results for evaluating the heights of barriers for TL-2 conditions. A simulation approach is also viable for analyzing changes for factors that can vary in the field relative to the actual conditions of the barrier or the nature of the impact conditions. Since validated models exist for the small car (1100kg) and pick-up truck (2270kg) issues of both snagging and vaulting can both be evaluated under MASH criteria. It is also expected that the simulations will be able to address differences in vehicle-to-barrier interfaces that will occur for varying degrees of shoulder slope or drop-off and lateral placement of the barrier. Therefore there is a need for acceptable TL-2 safety limits for the minimum heights of W-Beam Strong Post Guardrails on lower-speed, lower-volume roadways. This can easily be done by first performing computer simulations and then physically crash testing the most promising configuration for verification. Although most construction projects installing barrier would specify standard height strong-post W-beam at the standard height, the primary question we need to answer is "At what height can a strong-post W-beam remain in place in a TL-2 condition when we are repaving a roadway?" Objectives: The goal of this project is to determine the minimum guardrail height for W-Beam Strong Post barrier that meets safety performance criteria set forth under MASH Test Level 2 conditions.