An optimum link improvement model with a computer algorithm

This paper presents a multimodal network design model with a computer algorithm to solve it. The selection of link improvements in the network is assumed to affect practical capacities as well as mean free times. The equilibrium flows and the modal sharea are affected accordingly. The improvements are assumed to be continuous variables with concave marginal costs. This provided a good approximation for the behaviour of the discrete model. The algorithm is based on Hooke and Jeeves’ Optimization Methodology which requires a solution of a multimodal equilibrium assignment problem for each functional evaluation. The algorithm is implemented in such a way that only some functional evaluations need a solution of an equilibrium problem, implying efficiency for large-scale problems. Computational results show that although flow congestion decreased due to link improvements, modal shares may change towards private users. The latter contributes to congestion again and possibly makes the overall decrease in congestion less significant.