In order to be able to calculate the social welfare for any given combination of infrastructure projects, a transport ministry should be able to foresee the changes in travel behavior of citizens, which are the result of new investments. Travel behavior of citizens is summarized in the form of a network equilibrium model that describes behavior of both car users and public transport users. A transport system is in equilibrium, when each network user (citizen) chooses the route with the smallest transport costs, given route choices of all other users. None of the network users finds itworthwhile to deviate from his equilibrium route, if the others do not deviate. Network equilibrium depends upon a network structure and hence is influenced by infrastructural improvements. By solving network equilibrium model for each combination of infrastructure projects, a transport ministry is able to get the forecast of travel behavior of citizens and hence calculate the social welfare measure. In order for a transport ministry to beable to consider significant number of infrastructure projects and hence increase the probability that chosen combination is close to the ultimate optimum, solution time of the network equilibrium model should be as smallas possible. The optimality of chosen combination of investment projects also depends upon the functional form of social welfare measure as well asupon the effective investment budget restriction. The present paper has the following related aims: the first one is to formulate the simultaneous network equilibrium for both car and public transport in the form of MixedComplementarity Problem and incorporate it into a bi-level programming framework. The second aim is to investigate the role, which functional form of social welfare measure plays in optimal infrastructure investment choice using Oslo case-studio. The model presented in the paper has a structureof bi-level programming or leader-follower game, where a transport ministry is the leader and citizens are the followers. The lower level of the bi-level programming represents changes in travelling behavior of citizens for a given allocation of investments. The upper level represents the choice of investments allocation by a transport ministry in order to maximize social welfare. This formulation makes it possible to implement network equilibrium without use of special transport packages as well as to incorporate it into the bi-level programming framework, with the help of which one may evaluate welfare benefits of a large number of infrastructure project combinations during reasonable amounts of time and allocate network investments in the most efficient way. For the covering abstract see ITRD E145999
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