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A free moving vehicle is a vehicle that travels at their desired speed without any influenced from other vehicles within the same traffic stream or in the opposite traffic stream and the speed is considered as free-flow speed (FFS). This FFS or in another word desired speed can achieve only when the vehicle is travelling in the FFC. FFC is one of the key parameters which is directly related to the LOS assessment for two-lane highways. In the past decades, different researchers used different methodologies to calculate the FFC but all of them were based on homogeneous traffic conditions while in India heterogeneous traffic condition occurred. US HCM 2010 recommend calculating FFS at low traffic volume as vehicle will travel under FFC. On the other hand, other researchers suggested different critical headway value in order to identify the FFC. So, there was a need to examine the previously proposed guidelines or methodology in order to examine their suitability for heterogeneous traffic condition. Different five study sites were selected from different regions of India and different relationships were established between previously used measures to identify the FFC. It was concluded that headway can't be used under heterogeneous traffic condition as well travel mean speed and 85th percentile speed also. So, a new measure namely; speed difference (SD), which is the difference in speed of two consecutive vehicles in the traffic stream introduced in this study. A relationship was established between SD and headway threshold but it found inappropriate for the existing heterogeneous traffic condition at all the study sites. Finally, the gap is used instead of headway and found suitable for heterogeneous traffic condition. A critical gap threshold value of 10 seconds was observed at all the study sites which showed that the vehicles would be in FFC after that critical gap value and ultimately FFS can be calculated by considering those vehicles which were travelling greater than 10 sec gap value.
Boora et al. (Sun,) studied this question.