Traffic Speed Variability as an Indicator of the Provided Road Safety Level in Two-Lane Rural Highways

Abstract

The influence of traffic speed in the provided road safety level is an indisputable fact, especially on rural two-lane highways. This paper aims to highlight through actual measurements the critical traffic speed characteristics that can be used as indicators to highlight the provided road safety level. In this framework, traffic speed measurements were conducted at several locations of a two-lane rural highway, mainly on horizontal curves, during day and night. Among these measurements, only cases with a headway greater than 6 seconds were utilized, in order to ensure free flow conditions, while a separation into passenger cars was held. At the same time, parameters such as mean speed, standard deviation, traffic flow and composition and 85-percentile speed for passenger cars were determined, while road's geometric elements were utilized. These data were used to determine the homogeneity of the geometric design (using Criterion II), the expected number of road accidents (using the HSM method) and the risk indicator (using the FM19 software). Utilizing all the above data, a correlation between the critical traffic speed parameters and the corresponding results obtained from methodologies evaluating the provided road safety level were made, as well as, with geometric elements related to road infrastructure. Through the analysis, an indicator related to speed dispersion and to road safety is indicated. The results shows that this indicator seems to be strongly correlated with the provided road safety level in horizontal curves. In addition, the limited variability in traffic speed leads to an increased road safety level regarding to passenger cars and time periods examined, highlighting the importance of homogeneity in geometric design.