The physics of road wear means trying to more accurately assign costs would result in higher costs for consumer goods (mostly a regressive tax) due to increased cost for big rigs while cheaper transportation costs for passenger vehicles.

Passenger vehicles currently pay more than their fare share in road maintenance costs.

Overall, roads and infrastructure are a profit center for governments, returning $1.5 to $3 for every dollar spent.

http://www.frbsf.org/economic-research/publications/economic...