Safety Evaluation for Freeway Exit Ramp Based on Speed Consistency

Speed differential has direct relationship with road safety. Conventional speed consistency measures draw sample data from independent population that follows a normal distribution. These methods may fall into ecological fallacy and overestimate the safety level of road elements. After deducing the relationship between the difference in operating speed ΔV85 and85(ΔV), the 85th percentile value of individual speed reduction, this paper recommends 85(ΔV) and speed reduction rate 85(ΔV)R is used to evaluate ramp safety. Point speeds of individual vehicle at freeway diverge area, upper ramp, and lower ramp are collected by radar guns. Safety performance of 37 exit ramps are appraised using three different measures, namely, ΔV85, 85(ΔV), and 85(ΔV)R. The results show that 85(ΔV) is 1.42 to 2.02 times of ΔV85, and that the evaluation result of 85(ΔV)R is between the other two. The measure 85(ΔV)R considers not only individual speed but also the base speed on upstream element, which avoid the pitfalls of ecological fallacy and overestimation possessed by conventional measures. It is a safer and reasonable measure that should be adopted in practice.

摘 要速度差与安全性有着直接的关系。传统方法以集计的运行速度差ΔV85 作为速度一致性指标，容易陷入生态学谬论的误区，并且会高估道路的安全性能。本文比较了几种常用速度一致性评价方法的优劣，采用85位单车速度差85ΔV作为出口匝道速度一致性指标，解析了85ΔV≥ΔV85 的关系。通过雷达对沪宁高速和南京绕城高速出口减速车道、匝道上半部以及匝道下半部单车速度的调查，得出连续路段的运行速度差ΔV85 、85位单车速度差85ΔV以及85位速度变化率85ΔV R，验证了85ΔV约为ΔV85 的1.42 ∼ 2.02倍，表明使用85ΔV或者85ΔV R作为速度一致性评价指标是更加可靠合理的。最后通过速度差以及速度差变化率大小分级，用三种方法综合判定了37个出口匝道的安全等级。