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A comprehensive understanding of speed-flow-density relations is an essential requirement in planning, design and operation of transportation systems. Many researchers have investigated the nature of speed-flow-density relationships, commonly known as stream models, under homogeneous traffic conditions. However, those models may not be the best under Indian traffic conditions. Indian traffic differs significantly from homogeneous traffic and may need a different treatment for analysis. Not much research has been reported on this important aspect so far from India. The present study is an attempt to develop an optimal traffic stream model suitable for the heterogeneous traffic flow condition existing in India, taking Chennai as a case study. A two-regime speed-density relationship was developed using a conventional optimisation algorithm. Using this, the speed-flow and flow-density relationships were theoretically derived. The results indicate that the proposed model is able to represent the steady-state macroscopic behaviour of the heterogeneous and less lane-disciplined traffic stream under study with reasonable accuracy. [ABSTRACT FROM AUTHOR]