Amir Reza Mamdoohi - Assistant Professor, Department of Civil and Environmental Engineering, Tarbiat Modares University, Tehran, Iran
Mahmoud Saffarzadeh - Professor, Department of Civil and Environmental Engineering, Tarbiat Modares University, Tehran, Iran
Siavash Shojaat - Ph.D. Candidate, Department of Civil and Environmental Engineering, Tarbiat Modares University, Tehran, Iran

Existence of capacity drop phenomenon, as the difference between pre-queue and queue discharge flow rates, has been one of the controversial concepts of traffic engineering. Several researches have focused on capacity drop existence and also its estimation issues. This paper aims to estimate capacity drop based not only on a comparison betweenbreakdown and queue discharge flow rates, but also on the estimation of the capacity distribution function beforeand after breakdown. In the empirical case, speed and flow rate data are collected in a section of Iran’s most crowdedfreeway for four months, based on which the threshold speed as the boundary between congested and non-congested flow is determined, and breakdown flow rates and their subsequent queue discharge flows are detected. Paired t-testbetween pre-queue and queue discharge flow rates is conducted to find the mean difference. Also, the distributionfunction of capacity under non-congested and congested flow is estimated using maximum likelihood and productlimit methods. Based on the 11,600-record data set, it was observed that end results of both methods are consistent, revealing roughly five percent drop in capacity for the section under investigation

Capacity drop, stochastic approach, probability distribution function