Combined horizontal and vertical alignment designs on highways have been shown to have impacts on operational and safety problems. Previous studies have evaluated vehicle operations on combined alignments but have not been focused on horizontal curve components. A horizontal curve may be conveniently separated into three successive components: approach transition, circular, and departure transition curves. Vehicle operations on these three components have not examined before. Such a study is needed because speed consistency, defined as changes in speed that occur as a driver drives through the three components, can affect safety. Previous studies focused on characteristics such as radius, and length and grade of horizontal curves, while changes in design characteristics such as length differences, and grade changes between successive components were neglected. The objective of this study is to identify the influence of geometric design on horizontal curve of combined alignment. In this study, the relationship between geometric design characteristics of horizontal curves combined with downgrade alignments, and speed consistency was investigated. To analyze drivers’speed choices, speed consistency across these components was categorized into positive and negative speed differences, and was separately examined using Tobit models. Results showed grade changes, component length differences, and component turning directions of influenced speed consistency. Performance differences among test drivers were accounted using Random Effects Tobit models. The results showed that there is no heterogeneity among different drivers.
Xuesong Wang, Xiaomeng Wang*, Tarko. A., Rongjie Yu. The Effect of Combined Horizontal and Vertical Curves on Speed Change Behavior. Transportation Research Board 96th Annual Meeting, Washington D.C., USA, 2017. 01. 08-12.
The Effects of Combined Horizontal and Downgrade Alignments on Speed Consistency.pdf