Transportation and economic systems have a reciprocal relationship as transport supply and demand are mutually interdependent. For instance, the construction of a highway interchange favors the concentration of commercial and service activities, which will generate additional transport demand, which in turn will favor the location of new activities and a reorganization of the regional spatial structure. This interdependence can be conceptualized with three major elements:
- Transport system. It is mainly composed of infrastructures conferring a level of transport supply, from which accessibility levels can be derived. A variety of models have been developed to measure transport supply and its enabling (or constraining) effects on mobility. For instance, traffic assignment models take an existing spatial interaction structure and infer flows within a transportation network. Conceptual flows between origin-destination pairs consequently become a physical reality.
- Spatial interactions. The assumption is that mobility between locations is mainly related to a function of spatial impedance, which reflects friction of distance. Many spatial interaction models rely on distance decay parameters to estimate flows. Another dimension of spatial interactions concern the modes involved in urban trips, particularly which mode will be used for which trip.
- Land use. Represents the level of spatial accumulation from which transport demand is derived. There is a wide base of spatial economic models estimating transport demand, mainly through the generation and attraction of traffic by different types of land use.