Transportation network improvement and tolling strategies: The issue of intergeneration equity

Existing transportation network design studies focus on optimizing the network for a certain future time but without explicitly defining the time dimension within the formulation. This study extends the consideration by formulating the time-dependent network design problem. With this extension, one can plan for the optimal infrastructure improvement timetable, the associated financial arrangement, and tolling scheme over the planning horizon. In addition, this extension enables the pursuit of important considerations that are otherwise difficult, if at all possible, with the traditional timeless approach. Through the time-dependent framework, this study examines the issue of intergeneration equity according to the user and social perspectives. Basically, should the present generation build the full-blown network, or should users at the time pay for future incremental upgrades? Using a gap function to measure the degree of intergeneration equity achieved, this study illustrates that there are tradeoffs between societal and individual perspectives. Nevertheless, this study suggests ways whereby the planner can trade the level of equity to be attained with the overall network performance. In this way, some gradual measures can be introduced to the network design to compromise between these two perspectives.