The main objective of this paper, based upon the extensive empirical research of free flow in local conditions, is to quantify the unfavourable impact of the flow structure on the road capacity using PCE (Passenger Car Equivalent) values as a function of longitudinal grade. Based on literature reviews and empirical research, it has been proved that the PCE value for all vehicle classes is directly correlated with the road gradient. The PCE values in free flow conditions have been determined for the approved vehicle classes. Based on the measured values, models for determining the average PCE value depending on the upward grade on two-lane roads have been developed. Comparison of the developed models in conditions of free traffic flow with the Highway Capacity Manual (HCM) models has shown lower PCE values in this research. Models for the percentage of PCE values PCE15%, PCE50% and PCE85% have also been established.
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