A disaggregate speed consistency measure for safety evaluation at freeway diverge areas

The commonly used conventional speed consistency measures (e.g. ΔV₈₅) may fall into ecological fallacy and overestimate safety performance. To weaken these pitfalls, this paper presents a disaggregate measure, the 85th percentile individual speed difference 85(ΔV), to assess the safety performance of freeway diverging areas. A diverging area is divided into four elements, namely the upstream mainline, diverging area, downstream mainline and exit ramp. Individual speeds at the four elements of each site are shot using radar guns. The last three digits are recorded for tracing individual vehicles. More than 30,000 traceable individual speeds, together with geometric and volume information, are collected at 21 diverging areas along the freeway G42 and G2501 in Nanjing, China. The evaluation results indicate that ΔV₈₅ is overestimation-prone, which may lead decision-makers to leave potential unsafe sites unattended. The findings in this paper show that the disaggregate measure is more reasonable in safety evaluation.