  We present an efficient algorithm to identify which edge should be improved
in a traffic network to minimize the total travel time. Our main result is to
show that it is possible to approximate the variation of total travel time
obtained by changing the congestion coefficient of any given edge, by
performing only local computations, without the need of recomputing the entire
equilibrium flow. To obtain such a result, we reformulate our problem in terms
of the effective resistance between two adjacent nodes and suggest a new
approach to approximate such effective resistance. We then study the optimality
of the proposed procedure for recurrent networks, and provide simulations over
synthetic and real transportation networks.
