Critical Impact Points for Transitions and Terminals

Guidelines for evaluating the safety performance of roadside safety features generally recommend that a worst case or critical impact point (CIP) be selected for crash testing. NCHRP Report 350 presents families of curves that can be used to determine the CIP for a transition section. However, these curves have been observed to overestimate the stiffness of a transition system and provide CIP values closer to the more rigid system of the transition (e.g., bridge rail end) than appropriate. New CIP selection curves for transitions are presented. A procedure is provided to aid in determining the CIP for transition sections with multiple rail elements or variations in post strength and post spacing. Various existing and theoretical transitions systems with wide-ranging combinations of beam and post strengths were used to validate the curves. The newly developed CIP relationships for transitions are recommended in lieu of the existing relationships contained in NCHRP Report 350. To facilitate the development of guidelines for the selection of a CIP for terminals, a new definition is proposed. The proposed definition for the CIP is the point along the terminal at which vehicle behavior transitions from gating to redirection. A methodology for determining the CIP using computer simulation techniques is investigated. The data clearly demonstrate that the selection of a single default impact location for all terminal configurations may not provide the CIP for many designs.