Investing for reliability and security in transportation networks

Alternative transportation investment policies can lead to very different network forms in the future. The desirability of a transportation network should be assessed not only by its economic efficiency but also by its reliability and security, because the cost of an incidental capacity loss in a road network can be massive. This research concerns how investment rules shape the hierarchical structure of roads and affect network fragility to natural disasters, congestion, and accidents and vulnerability to targeted attacks. A microscopic network growth model predicts the equilibrium road networks under two alternative policy scenarios: investment based on benefit-cost analysis and investment based on bottle-neck removal. A set of Monte Carlo simulation runs, in which a certain percentage of links was removed according to the type of network degradation analyzed, was carried out to evaluate the equilibrium road networks. It was found that a hierarchy existed in road networks for reasons such as economic efficiency but that an overly hierarchical structure had serious reliability problems. Throughout the equilibrating or evolution process, the grid network studied under benefit-cost analysis had better efficiency performance, as well as error and attack tolerance. The paper demonstrates that reliability and security considerations can be integrated into the planning of transportation systems.