Soheil Sohrabi - Civil and Environmental Engineering Faculty, Amirkabir University of Technology, Tehran, Iran
Khosrow Ovaici - Civil and Environmental Engineering Faculty, Amirkabir University of Technology, Tehran, Iran
Mina Ghanbarikarekani - Civil Engineering Faculty, Imam Khomeini International University, Qazvin, Iran

Although there are remarkable differences in drivers’ behavior as well as vehicles characteristics, road capacity estimationmethods in developing countries are same as developed ones. So, the discrepancy between the theoretical andpractical values of capacity is inevitable. Although, capacity estimation methods can be classified based on numerousparameters, in developing countries, the availability of data is a remarkable factor to choose the appropriate method.Headway method is one of the most popular methods which is used to estimate capacity of basic freeways sectionregarding to the lack of available data in developing countries such as Iran. According to headway method theory, thedistribution function of headways needs to be estimated in order to find the capacity of a road facility. However, inprevious studies the mean of observed headway is used for capacity estimation.The data applied in this study have been collected from Iran. Modelling had accomplished in 2 steps: 1) Filtering theinput data that are related to following vehicles and just before queue. 2) Statistical modelling using the Minitab 16software. Evaluating models by three parameters (applicability, validity and reasonability), it was proven that the lognormaldistribution functions, is the best fit to observed data. According to results, the capacity of the basic freewaysection is 12% less than the values proposed by HCM, which are the current bases for traffic studies in developingcountries such as Iran.

Capacity, headway, following vehicle, follower vehicle