From a societal perspective, it is desirable for all transportation services to pay their full social
(private and external) costs. If the full social cost were re¯ected in the prices shippers pay, trans-
portation users could choose the amount of each form of service to consume on the basis of the
true cost of this service to society. By ``internalizing'' external costs, policy makers would eec-
tively create a market through which transportation users could weigh the bene®ts of consuming
a particular transportation service against the true costs.