Travel Time Minimization Versus Reserve Capacity Maximization in the Network Design Problem

In the classical continuous network design problem, the most common approach is to determine optimal capacity enhancement by minimizing the total system cost under a budget constraint while taking into account the route choice behavior of network users. An alternative objective, maximization of network reserve capacity, is examined with the expectation that it is equivalent to total system cost minimization to a certain extent. The two approaches are tested and the results are compared in order to assess the level of equivalence and their effectiveness. The level of equivalence of the two objectives is found to vary by level of congestion. When the level of congestion is low, maximization of network reserve capacity is found to have the simultaneous effect of total cost minimization. This objective becomes more desirable when the level of congestion is low. When the level of congestion is high, maximizing the reserve capacity can only minimize the total travel cost to a certain extent. In this case, a combined objective can also be considered by applying different weightings on the two objectives.