H. Chen, L. Lu, J.J. Lu, S. Zhu, D. Wei

Pages: 19-42

Abstract
Due to their high transportation effectiveness, urban expressways have become a major means of alleviating traffic congestion in many metropolitan areas. To study the traffic performance of urban expressways, a definition of ramp combinations was developed by calculating the combination boundaries at the design speeds of 60, 80, and 100 km/h. Based on the layout of the on and off ramps, the ramp combinations were classified into four types: on-off, on-on, off-on and off-off. Additionally, field observations and video recordings of the traffic flow at typical urban expressways in Shanghai were conducted. As the most important factor observed, average speed significantly affects the functioning of urban expressways. The related factors for average speeds in different types of ramp combinations were analyzed, the value range of those factors was then determined and an appropriate number of samples were extracted with Latin Hypercube Sampling methodology. Quantitative models of average mainline speeds in different types of ramp combinations were developed by appropriately changing the form of the variables with a Statistical Package for the Social Sciences (SPSS) and by applying Multiple Linear Regressions. On the basis of these quantitative models, methods for enhancing the average speeds in combination areas were explored, and recommended ramp distances for the four types of ramp combinations were proposed. It was found that traffic volume had the greatest influence on the average speed in combination areas, especially the mainline upstream volume, and quantitative models of average speed for four ramp combinations invariably contain a ramp distance factor.

Keywords: urban expressway; ramp combinations; average speed; quantitative models; ramp distance