The role of accessibility indexes and pricing policies in urban planning

The activity system of an urban area can be decomposed into three subsystem consisting of the households living in each zone, the economic activities located in each zone, and the floor-space (and the relative market prices) available in each zone for various uses. For instance, the number of households in a zone depend on employment opportunities (i.e. the subsystem of economic activities), the location of economic activities depends on the distribution of households, and both households and economic activities in each zone depend on the availability of specific types of floor-space and on the relative prices. In this context, the transportation system study and design represents a relevant element in the urban area planning because the relative 'accessibility' generated among different activities could support or limit the development of these activities. Indeed, the need to use different urban functions on different places yields the potential transportation demand; but this potential demand has to (or should) be satisfied by suitable transportation system. Hence, since household members make their travel choice in function of some characteristics of transportation services offered by the different travel modes (individual car, bus, metro, walking), it is necessary to have some synthetic indexes that allow decision-makers to evaluate and forecast effects of transportation systems on urban activity development. Moreover, the relative accessibility of a zone in urban contexts influences also the real-estate market: zones with high level of accessibility generally are characterised by high real-estate prices. Hence, users locate their residence also in function of their value of time, which can be considered as a measure of their income level (i.e. people with high incomes, and therefore with an high value of time, locate their residences in place with high accessibility characterised by high real-estate prices). These remarks are confirmed by the fact that the construction of a new metro station generally yields an increase in the real-estate prices. Many authors (see for instance Bath et al., 2000; Cascetta, 2001) agree that synthetic indexes for evaluating transportation system effects on the external environment are accessibility indexes which can be defined as "a measure of the ease of an individual to pursue an activity of a desired type, at a desired location, by a desired mode, and at a desired time" (Bath et al., 2000). In urban contexts, the variation of level of transportation services (and therefore of accessibility among different zones) is hard to be implemented by means of the construction of new transportation infrastructures. Therefore, a simpler and more feasible approach for yielding these variations is the optimisation of existing transportation systems as for instance by means of pricing policies (road pricing, cordon pricing, parking pricing, integrated mass-transit system fare, etc.). Indeed, these policies allow to modify substantially the modal split between road and mass-transit systems and therefore performances of each single transportation mode. Obviously these fares have to be designed taking into account the accessibility indexes in order to avoid that an odd pricing strategy could reduce accessibility and impoverish a zone. Hence, pricing policies supported by accessibility indexes can be considered an useful tool for planners for taking into account influences that transportation systems yield on the urban environment.