Estimating the Capacity of the Basic Freeway Section Using Headway Method in Developing Countries (Case Study: Iran

Document Type: Research Paper


1 Civil and Environmental Engineering Faculty, Amirkabir University of Technology, Tehran, Iran

2 Civil Engineering Faculty, Imam Khomeini International University, Qazvin, Iran


Although there are remarkable differences in drivers’ behavior as well as vehicles characteristics, road capacity estimation
methods in developing countries are same as developed ones. So, the discrepancy between the theoretical and
practical values of capacity is inevitable. Although, capacity estimation methods can be classified based on numerous
parameters, 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 section
regarding to the lack of available data in developing countries such as Iran. According to headway method theory, the
distribution function of headways needs to be estimated in order to find the capacity of a road facility. However, in
previous 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 the
input data that are related to following vehicles and just before queue. 2) Statistical modelling using the Minitab 16
software. Evaluating models by three parameters (applicability, validity and reasonability), it was proven that the lognormal
distribution functions, is the best fit to observed data. According to results, the capacity of the basic freeway
section is 12% less than the values proposed by HCM, which are the current bases for traffic studies in developing
countries such as Iran.


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