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The time taken to travel from one point to another in a road network is an important feature of many urban models. Proper estimates of such a quantity involve taking account of the effects of congestion. The basis for doing this is to model the relationship, on a link of a road, between speed and the flow of vehicles on the link - the so-called speed-flow rel ati onship. A third variable can be used to achieve the best insight into the form of the relationship: the density (or concentration) of the traffic on the link.