  We study a network design problem (NDP) where the planner aims at selecting
the optimal single-link intervention on a transportation network to minimize
the travel time under Wardrop equilibrium flows. Our first result is that, if
the delay functions are affine and the support of the equilibrium is not
modified with interventions, the NDP may be formulated in terms of electrical
quantities computed on a related resistor network. In particular, we show that
the travel time variation corresponding to an intervention on a given link
depends on the effective resistance between the endpoints of the link. We
suggest an approach to approximate such an effective resistance by performing
only local computation, and exploit it to design an efficient algorithm to
solve the NDP. We discuss the optimality of this procedure in the limit of
infinitely large networks, and provide a sufficient condition for its
optimality. We then provide numerical simulations, showing that our algorithm
achieves good performance even if the equilibrium support varies and the delay
functions are non-linear.
