IMPACT OF HORIZONTAL ALIGNMENT ON TRAFFIC PERFORMANCE AT RURAL TWO-LANE HIGHWAYS

Abstract

Two-lane, two-way highway facilities represent the majority of the total length of highway network at many countries. The geometric design of such facilities is considered as one of the most important factors affecting the traffic performance and safety issues, especially the horizontal alignment. However, the interaction between the horizontal curve design elements and the traffic performance still unclear and needs more investigations. This paper aims to explore the relationship between the characteristics of horizontal alignment and the traffic performance. The follower density, a new and promising performance measure, is herein used as a measure of effectiveness (MOE), since the currently used MOE (in HCM-2000), percent-time-spent-following (PTSF), is hardly observed in the field. In this study, the traffic data are collected from mid-tangent and mid-curve points at nine different sites with various horizontal alignment characteristics. Then, several relationships between the follower density, flow rate, horizontal alignment characteristics (curve radius, tangent length), and average speed are investigated. The results show that the horizontal alignment characteristics have a significant effect on the follower density, especially curve radius value, by decreasing radius, the follower density increases (i.e., traffic performance decreases). It is also noticed that a value of horizontal curve radius falling between 400 and 450m seems to be a threshold of the significant impact of curve radius on traffic performance. It is worth to mention that the results and relationships presented in this paper is a step towards understanding the interaction between horizontal alignment and traffic performance at two-lane highway facilities and more investigation at other locations and under different traffic conditions are recommended.