From Transit Systems to Models: Purpose of Modelling

From Part I of the book, it will be obvious that public transport plays an essential role in providing mobility to people, especially in dense urban areas. The social welfare generated by good public transport comes at a price, however. Almost all forms require large investments into infrastructure, vehicles and operation. With limited finance, ideal public transport remains a distant goal, and a lot of effort goes into finding an optimal allocation of budget to investment options. The key question for these decisions is: How big is the total benefit of a proposed investment? To answer it, one needs to predict how the potential users will make use of the hypothetical improved public transport. For responsible decision-making, this prediction should be rational, transparent and accountable. It is no surprise therefore that models are typically used to produce the predictions. These models span the whole range of mobility decisions made by individuals, from long term to short term. Passenger route choice, the focus of Part III, accounts for only a part of the complex decision hierarchy. Before zooming into route choice models, this chapter looks at the planning process as a whole, explains the role of models in decision-making and gives an overview of the whole decision hierarchy. The last two sections introduce the general mathematical framework, in which decision models are formulated and set the stage for the description of specific models.