Modeling the Maximum Lateral Extent of Encroachment and Probability of Passenger-Vehicle Rollover on Slopes

Abstract

The 1967 “Yellow Book” points out that “[f]or adequate safety it is desirable to provide an unencumbered recovery area up to 30 feet from the edge of the traveled way.” The 1974 Yellow Book points out that a 30-ft recovery area is not a one-size-fits-all guideline. This paper examines the probability of an errant vehicle trajectory extending laterally to a location of interest (e.g., across the median, to the barrier, to a fixed object, etc.) and the likelihood the vehicle will rollover prior to reaching that point as a function of the foreslope. The influence of encroachment speed and angle, vehicle type, and slope on the probable lateral extent was examined using survival analysis of simulated vehicle trajectories. Field collected data were used to scale the simulated data to better represent field collected encroachment speeds and angles. The work presented in this paper represents a significant enrichment to the understanding of the lateral extent of errant vehicle trajectories during an encroachment and the probability of rollover by slope and distance traveled on the slope. These models can be used for guideline development and extended in the future to include ditches and other complex terrain features. The two figures presented in this paper which represent the conclusions of this research are a significant step toward reducing the computational burden of analyzing roadside safety alternatives thereby significantly reducing the reliance on computer programs such as Roadside Analysis Program.