Our analysis has focused on truckload general freight trucking between US cities. Intercity
travel accounts for a very large share of the total ton-miles of transportation, and it enables us to
make two important simpli®cations in the analysis. We are able to ignore congestion costs
because very little congestion exists in rural areas. Additionally, we are able to use single values
for the costs of various air pollutants. In metropolitan areas where ambient pollution levels vary,
the social cost of further vehicle-generated pollution will dier as well.