Abstract
Determining the optimal length of need (LON) to prevent vehicles from striking roadside hazards is an important part of roadside design. Although barriers can provide significant safety benefits by preventing run-off-road (ROR) crashes, the barrier itself presents a small but nonzero risk of serious injury or fatality. The purpose of this study is to present a method for quantifying the protection of barrier LON using departure corridors developed with NCHRP 17-43 data. Based on real-world ROR crash trajectories from the NCHRP 17-43 database, departure corridors were constructed. This study focused on a subset of 171 crashes on roads with a speed limit between 60 and 80 km/h with a reconstructed impact speed. In these crashes, the vehicle trajectory was interrupted by either a collision or a rollover. Our approach was to extrapolate the trajectory that could have been followed by the vehicle had the collision not occurred. The original trajectory data were retained before impact and extrapolated after impact using a linear fit. These corridors incorporated the effects of hazard location, departure side, and barrier location on the runout length. Left-side departures required a longer LON to intercept the same proportion of crashes. The runout length needed for objects close to the road based on the corridor method was much smaller than the length recommended by the 2011 Roadside Design Guide. The benefit of this corridor-based approach to barrier placement is that each agency can balance barrier length and the proportion of encroachments intercepted depending on their priorities.
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