Long Down-Grade Highway Segment's Safety Investigation and Causation Inference in China

In Chinese highway system, especially within mountainous areas, there arose plenty of excessive long down-hill highway segments. Accident documents from many provinces revealed many of those long down-hill segments burdened substantially with high traffic crash frequency. Preliminary analysis showed that it was truck to dominate the majority of crash involved vehicles in these down-hill segments due to malfunction of braking system. This paper concentrates on the traffic safety performance of down-grade segments of mountainous highways to seek how the traffic and design factors aggregate the risks of vehicle, and most importantly, to glean how the primary index, length of grade, influence the safety performance of road segments. Specially highlighted among this paper's findings is the causation model between length of grade and crash counts under the interaction with other confounding factors such as grade and curvature of horizontal alignments. This outcome would hopefully be employed as the meaningful reference to highway geometry design, especially for the restriction of plausible length of down-hill highway grades.