Traffic engineers aim to maximise road capacities. Traffic flow theory is concerned with theoretical and empirical methods of measuring road capacity, and investigating the factors that influence it. The number of vehicles on a road can be counted in terms of 'traffic concentration', the number per unit road length in one road lane, or 'traffic flow', the number passing a given point per unit time. Similarly, an interval between successive vehicles on a road can be defined as 'space headway' ('spacing') or 'time headway' ('headway'), and an average speed may be defined as 'space mean speed' or 'time mean speed'. For stable traffic flow, the following fundamental relationship holds: traffic flow is the product of traffic concentration and space mean speed. It is also possible to derive simple flow-concentration, speed-flow, and speed-concentration curves. Several attempts have been made to fit empirical relationships between these variables to observed data. Under suitable conditions, any of these relationships can be used to provide a theoretical estimate of the capacity of a road from a pair of observed data points. A numerical example is presented to illustrate these concepts and formulae. Care is needed in using these empirical relationships, because no scientific laws explain traffic behaviour. For the covering abstract, see IRRD 892228.
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