Road pricing and network equilibrium

The paper presents a deterministic static equilibrium model for urban transport networks with elastic demand and capacity constraints. It is shown that this model may have no solution which satisfies the capacity constraints, but additional costs can be imposed on some road links so that an equilibrium solution that satisfies the capacity constraints always exists. These additonal costs could be used as road pricing, in order to prevent traffic flow becoming unstable and/or producing unacceptable environmental damage. A method to calculate the additional costs is proposed and is used in a simple numerical example. The results so obtained are compared with the congestion tolls derived from the traditional economic theory.