Development of link-capacity functions for Istanbul freeways

Abstract

The relationships between traffic flow and travel time are called link-capacity functions and are fundamental inputs for capacity restrained trip assignment methods. In order to estimate realistic results for travel demand, trip assignment is to be handled carefully and for this purpose link-capacity functions are necessary. Unfortunately, there is not an ultimate link-capacity function being valid for any road; instead there are many different types of functions with different parameters. These parameters show dependency to the links, vehicle and driver characteristics of that specific region. Istanbul, being the most crowded city of Turkey has its own specific parameters, too. This study aimed to calibrate suitable link-capacity functions and the estimates of their parameters for Istanbul. This was achieved by first investigating the basic relationships between traffic stream parameters and after finding out the characteristic inputs for the functions, such as free flow speed, and capacity. The two dimensional traditional approach to the relationships between these traffic parameters and the estimation of them is followed by a three dimensional newer approach to a small part of the existing data in order to see its relevance, experimentally. The results of this study may lead the transportation planners of Istanbul to convenient forecast analysis.