Measurement of free-flow conditions on multilane intercity highways under heterogeneous traffic conditions

Vehicles travelling at their desired speed under free-flow conditions (FFC) on a highway without getting influenced by other vehicle types travelling in the same lane exhibit a good level of service (LOS) on that particular highway facility. Few studies could be identified that provide guidelines on the measurement of FFC on multilane highways, including the latest version of the United States' Highway Capacity Manual (TRB 2010). As these guidelines are mostly based on homogeneous traffic conditions, they may or may not be applicable to heterogeneous traffic conditions. Previously used methodologies (for two-lane and multilane highways) were examined in the current study, but were found to be unsuitable for multilane highways. Therefore a new measure, named speed difference (SD), was introduced in the present study. SD is the difference in speed of two consecutive vehicles travelling in the same lane. A relationship was established between SD and gap (the difference between two consecutive vehicles from back to front bumper while travelling in the same lane), instead of headway (the difference between two consecutive vehicles from back to back or front to front bumper while travelling in the same lane). A gap value of 10 seconds was identified, beyond which vehicles would be travelling in FFC. The guidelines proposed in the present study will be helpful for traffic engineers and planners in developing countries.