Chapter 21: Congestion Modelling

Traffic congestion is one of the major liabilities of modern life. It is a price that people pay for the various benefits derived from agglomeration of population and economic activity. Because land is scarce and road capacity is expensive to construct, it would be uneconomical to invest in so much capacity that travel were congestion-free. Indeed, because demand for travel depends on the cost, improvements in travel conditions induce people to take more trips, and it would probably be impossible to eliminate congestion.

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. Early researchers hoped to develop models based on fluid dynamics that would not only be accurate, but also universally applicable. However, unlike fluid flow, congestion is not a purely physical phenomenon, but rather the result of peoples' trip-making decisions and minute-by-minute driving behaviour. One should therefore expect the quantitative — if not also the qualitative — characteristics of congestion to vary with automobile and road design, rules of the road, pace of life, and other factors. Models calibrated in a developed country during the 1960s, for example, may not fit well a developing country in the early twenty-first century.