On the change in surpluses equivalence: measuring benefits from transport infrastructure investments

Reductions in transport costs resulting from infrastructure improvements generate benefits that can be measured as surplus changes either at an economy-wide scale (social welfare changes) or, as is common practice in cost-benefit analysis (CBA), at a transport market level as transport users’ surplus changes. In this paper we look at an economy with spatially separated markets embedded in a transport network (a spatial price equilibrium model) to study the equivalence between these two benefit measures. Three different product market competition arrangements are considered. A similar question and strategy is presented in Jara-Diaz (1986) employing a two-node network and extreme competition assumptions on the production side: perfect competition and monopolistic production with arbitrage. We extend his work by additionally considering perfect collusion (monopoly without resale) and Cournot-Nash oligopoly under flow-dependent transport costs (i.e. congestion in transport). Numerical simulations in a three-node network with and without transshipment nodes, illustrate our main results.