Simulation models are mathematical/logical representations of real-world systems. Microscopic traffic simulation models have been playing an important role in traffic engineering and particularly in cases, in which field studies, involving extensive data collection, over a wide range, is difficult or expensive to conduct. As the available simulation models can only replicate homogeneous traffic flow, a model of heterogeneous traffic flow, named, HETEROSIM was developed to simulate heterogeneous traffic flow. In this model, a dynamic stochastic type discrete event simulation is adopted in which the aspects of interest are analysed numerically with the aid of a computer program. The model was validated using field observed data of traffic flow. Then, the model was applied to measure one of the fundamental charactertistics of traffic flow, namely concentration. It is a broader term encompassing both density and occupancy. Occupancy takes into account the traffic composition and speed, and hence, occupancy is more meaningful than density. The concept of occupancy can not be directly applied under heterogeneous traffic conditions, as the traffic has no lane discipline. In this paper, a new concep named,'area-occupancy'is proposed to measure traffic concentration. © ECMS.