Road space is a scarce resource, and traffic engineers and planners need to ensure that roads are able to accommodate as much traffic as possible, subject to safety and environmental constraints. In other words, we need to maximise the capacity of the road. But how is capacity measured, and what influences it? These questions lie at the core of traffic flow theory, a science which has attracted both theoretical and empirical analysis. In this chapter there is space only to introduce the concepts, the principal parameters, and some relationships which enable us to estimate capacity. We do this initially for an individual length of road, and subsequently for road networks. Further developments of these concepts are presented in a useful text: DL Gerlough and MJ Huber - Traffic flow theory: a monograph, Transportation Research Board Special Report 165, published 1975. Much of what follows is taken from an earlier textbook, which expands on the engineering implications (O'Flaherty, 1996). The publisher's permission to do this is acknowledged.
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