Un sistema di supporto alle decisioni per l'analisi dei conflitti derivanti dall'implementazione delle politiche di pricing

In recent years several European cities (such as London, Stockholm and Milan) have introduced pricing policies as a tool for managing transport demand, especially to yield a temporal, spatial and modal redistribution of travel, and particularly rebalance the modal split between private vehicles and mass-transit systems. Indeed, the interaction between user behaviours (whose choices are affected by transportation network performances) and transportation networks (whose performances are depending on the number of travelling users/vehicles) brings the system about a condition, defined in the literature as User Equilibrium, which does not correspond to overall utility maximisation and fails to take account of external costs. The discrepancy between the User Equilibrium condition and the efficient use of transportation systems (condition indicated in the literature as System Equilibrium) comes from user behaviour in making mobility choices: an additional user, entering a traffic flow, considers a travel cost that does not include the cost increase imposed on the other travellers in the network. In other words, travellers try to maximise their own utility or private benefits instead of considering social welfare. It is shown that efficient transportation system use can be achieved by charging 'efficient tolls' on network links. The optimal situation can be reached by the imposition of a tax (or toll) that will reconcile the private cost and the social cost. These tolls, called Marginal Social Cost Pricing (MSCP) tolls, are equal to negative externalities (such as congestion cost, travel delays, air pollution, accidents) imposed on other travellers by an additional user and are one of the most popular tools for pricing applications. For several reasons (theoretical, political, social acceptability) it is impossible to charge 'efficient tolls' (first-best solutions) proposed in the literature; therefore in real networks sub-optimal tolls (second-best solutions) are applied. Moreover, one of the main problems related to pricing policy application is their acceptability among community and/or social categories (such as shopkeepers, residents, commuters, etc.) leading to conflicts and oppositions. In this paper we analyse the effects on optimal fare design when pricing revenues are wholly or partly used for improving public transport. In particular, we formulate a toll computation model through a multidimensional constrained optimisation problem according to economic theory in a multimodal and multiuser context, where multimodal features are calculated explicitly on the network for each fare configuration. The model is then applied on a trial network (built with heterogeneous values of relative accessibility among different traffic zones) and several second-best strategies are analysed with particular attention to possible effects of road pricing revenue use on social welfare and fare levels