Transportation researchers have long struggled to find satisfactory ways of describing and analysing congestion, as evident from the large number of often competing approaches and models that have been developed. Congestion in transportation is not limited to roads: it is also a problem at airports and in the airways, at harbours, on railways, and for travellers on bus and subway networks. For modelling purposes useful parallels can often be drawn between traffic congestion and congestion at other facilities. Attention is limited in this chapter, however, to road-traffic congestion. Broadly speaking, traffic congestion occurs when the cost of travel is increased by the presence of other vehicles, either because speeds fall or because greater attention is required to drive safely. Traffic congestion can be studied either at a microscopic level, where the motion of individual vehicles is tracked, or at a macroscopic level, where vehicles are treated as a fluid-like continuum. This chapter is organized as follows. Section 2 concerns the modelling of homogeneous traffic flow and congestion on an isolated road under stationary conditions. It also sets up the supply-demand framework used to characterize equilibrium and optimal travel volumes, Section 3 provides an overview of macroscopic and microscopic models of non-stationary traffic flow. It then describes how trip timing can be modelled, and discusses the essence of dynamic equilibrium. Section 4 reviews the principles of static and dynamic equilibrium on a road network in a deterministic environment, and then identifies equilibrium concepts that account for stochasticity in demand and capacity. Section 5 addresses conceptual and practical issues regarding congestion pricing and investment on a network. Finally, Section 6 concludes.
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