We model a road network as a directed graph G(V,E) with a source s and a sink t, where each edge e has a positive length l(e) and each vertex v has a distribution function αv with respect to the traffic entering and leaving v. This paper proposes a polynomial time algorithm for evaluating the importance of each edge e E whicn is defined to be the traffic f(e) passing through e in order to assign the required traffic Fst(0) from s to t along only shortest s-t paths in accordance with the distribution function αv at each vertex v.