The Key Role of FFS Computation Methods in the HCM 2016 Performance, Case Study of a Weaving Segment

Free-flow speed (FFS) is defined as the theoretical speed when density and flow rate are zero. This concept forms a fundamental part of the Highway Capacity Manual (HCM). On the other hand, weaving segments are considered as one of the most important elements in the freeway capacity and the level of service analysis in the HCM. Hence, this study used three different methods to compute FFS of a ramp-weaving segment based on field data, and then employed the HCM 2016 model to analyze the weaving segment. Ultimately, model results were compared to field data and the relative superiority of the HCM 2016 method for field measurement of FFS was proven. By contrast, the unsatisfactory performance of the HCM 2016 in predicting the speed of non-weaving vehicles negatively affected the final performance of the model and caused under-predicted results. Thus, this study proposed a method which produces significantly more desirable results. The merit of researches like this study becomes even more apparent by considering the circumstances in which references analogous to the HCM are developed in. The HCM models depend on several contextual factors. Therefore, it can be logically deduced that the most recent editions of this manual may not always be the most accurate ones for the traffic condition throughout the Iran highway networks.

Keywords:

Free-flow speed , Highway capacity manual , weaving segment , operating speed , Isfahan