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

Document Type: Research Paper


1 Isfahan University of Technology

2 Transportation Research Institute


A weaving segment occurs where two one-way traffic streams intersect by merging and ‎diverging maneuvers. These segments are considered as one of the most important elements in ‎the freeway capacity and the Level of Service analysis in the Highway Capacity Manual (HCM). ‎Free-flow speed (FFS) plays a major role in this analysis. Hence, the current study used three ‎different methods to compute FFS of a ramp-weaving segment based on field data, and then ‎employed the proposed HCM 2010 model to analyze the weaving segment based on the three ‎FFS values obtained. Ultimately, model results were compared to field data and it was found ‎that the proposed method by the HCM 2010 outperformed the other two methods in field ‎measurement of FFS. However, the considerably poor performance of the presented model by ‎the HCM 2010 in predicting the speed of non-weaving vehicles negatively affected the ultimate ‎performance of the model and caused under-predicted results. Thereby, by applying 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. The merit of the study becomes even more apparent by considering the context ‎in which manuals like HCM are calibrated in. Traffic models including those mentioned in ‎HCM depend on several contextual factors including vehicle technical characteristics and traffic ‎behaviour. This implies that most recent editions of manuals may not always be the most ‎accurate ones for the traffic condition throughout the highways of Iranian cities.‎