Key Lab of Transportation Engineering, Beijing University of Technology, Beijing 100124, China
Copyright © 2012 Chang-qiao Shao and Xiao-ming Liu. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The saturation flow rate is a fundamental parameter to measure the intersection capacity and time the traffic signals. However, it is revealed that traditional methods which are mainly developed using the average value of observed queue discharge headways to estimate the saturation headway might lead to underestimate saturation flow rate. The goal of this paper is to study the stochastic nature of queue discharge headways and to develop a more accurate estimate method for saturation headway and saturation flow rate. Based on the surveyed data, the characteristics of queue discharge headways and the estimation method of saturated flow rate are studied. It is found that the average value of queue discharge headways is greater than the median value and that the skewness of the headways is positive. Normal distribution tests were conducted before and after a log transformation of the headways. The goodness-of-fit test showed that for some surveyed sites, the queue discharge headways can be fitted by the normal distribution and for other surveyed sites, the headways can be fitted by lognormal distribution. According to the queue discharge headway characteristics, the median value of queue discharge headways is suggested to estimate the saturation headway and a new method of estimation saturation flow rates is developed.