Evaluation of Methods for Computing Free-Flow Speed and Its Significance in the HCM 2010; Case Study: A Ramp-Weaving Segment

Abstract

The core methodology of the Highway Capacity Manual (HCM) 2010 for freeway analyses is based on Free-Flow Speed (FFS). Moreover, weaving segments are major elements of freeway facilities that form where two one-way traffic streams intersect by merging and diverging maneuvers. Hence, this study used three different methods to compute FFS of a ramp-weaving segment, and then employed the proposed HCM 2010 model to analyze the weaving. Comparison of the model results and the field data indicated that the HCM 2010 method for field measurement of FFS outperformed the other two methods. Nonetheless, the considerably poor performance of the HCM 2010 model in predicting the speed of non-weaving vehicles adversely affected the ultimate outcome of the model and caused under-predicted results. Thus, by app.lying slight modifications to the field measurement method of FFS for non-weaving flow, this study proposed a novel method which produces significantly more favorable results compared to other methods. Considering these results alongside the context in which manuals like the HCM are calibrated in, elegantly demonstrates the merit of this study because the HCM traffic models depend on several contextual factors including vehicle technical characteristics and traffic behavior. This implies that most recent editions of manuals may not always be the most accurate ones for the traffic conditions throughout the highways of Iranian cities.