Vertical Alignments and Safety Performance: A Combinational Effect of Load, Vertical Grade, and Length

Vertical alignments of grades and lengths are critical aspects of expressway geometric profile design which evidently influence segment safety performance. On vertical grades, rear end collision of cars to trucks is a frequent form of accidents. To alleviate this highly spotted safety issue and to reveal the relationship between loads, vertical grades, lengths, and segment safety performance, vehicle dynamic simulation method was employed in this stduy, which is effective in evaluating highway safety performance. Semi-trailer was set as the typical unfavorable vehicle on vertical grades and its model was constructed based on online-extracted parameters, which can more precisely describe semi-trailers currently. Vehicle dynamic response of semi-trailers with different loads was subsequently simulated on different vertical grades and lengths. Speed difference between semi-trailers and cars, a critical indicator of roadway safety performance, was calculated based on the simulation results. The result was testified by field data collected at a vertical grade segment on an expressway in China. Based on the simulation result, highway agencies can actively limit speed of cars to reduce speed difference between semi-trailers and cars and to enhance segment safety performance.

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