Capacity Estimation of Multi-lane Rural Highway: A Comparative Study

Abstract

The highway capacity manual (HCM) of the United States of America provides techniques for evaluating the capacity and the quality of service of any facility. The present study is an attempt to determine the adequacy of the techniques in US HCM to predict the traffic performance of highways in Al-Anbar, Iraq. Capacity of the multi-lane rural highway that connects two cities in Al-Anbar Governorate, Iraq was estimated. Twenty sections of the multi-lane rural highway were selected and observed for this purpose. Data were collected using the video recording technique. Data were abstracted, processed and then analyzed using computer programs developed for this purpose. Three methods for estimating capacity were applied, which are, the enveloping curve technique, the best-fit technique, and the US HCM methodology. Results showed that the capacity estimation methodology using US HCM has limited application for the heterogeneous traffic situation prevailing in the studied sites. This finding is consistent with that obtained by previous studies in developing countries, such as India, Thailand, and Indonesia. The capacity estimate showed that the US HCM methodology tends to underestimate capacity unlike other techniques because the adjustment factor for the traffic composition is not applicable for all vehicle types in the traffic stream that has different equivalency units. Further, the underestimation is attributed partly by differences in driver behavior, vehicle characteristics, and roadside activities.