Stopping-sight-distance simulation using a new probabilistic approach

Abstract

Geometric design guidelines provide deterministic analytical formulas to facilitate the calculation of the distance required to perform emergency stops on highways. Such expressions include several pre-fixed parameters that may produce an oversized or even an uneconomical outcome. Moreover, this deterministic approach provides little knowledge about the effects of deviating from the standards. Furthermore, the estimation of required stopping sight distances (SSD) considers the grade in an imprecise manner since it may change along the section travelled by the vehicle during the stopping maneuver, e.g. in sag or crest. For these reasons, this paper proposes a new probabilistic approach in order to estimate SSD. First, the authors have adopted the equation provided by the German guidelines, which takes the effect of grade into account along the section travelled during the stopping maneuver. The simulation performs 100,000 trials of emergency stops on the alignment of a real highway in order to test the reliability of the method, comparing the results to the available sight distance (ASD). ASD values have been computed through software based on geographic information systems, using a digital surface model for a better three-dimensional depiction of the highway and its roadsides. The simulation has assigned random variation to several SSD and ASD factors in order to reflect the uncertainty related to this phenomenon. The results made it possible to determine the most hazardous zones in terms of visibility conditions, which corresponded in most cases to the tightest alignment elements.