User pay.
I favor charging by the pound, actually. It's weight that tears up roads.
And on that note:
Aren't we GLAD we abandoned/gave away all those miles of railroad, thereby insuring that most of the nation's freight would move by the very Interstates now in such awful shape! Ever wonder how much damage semis do, as compares to cars?
Here quoting one discussion of the subject, noting that it was written in 2007. How many miles of interstate have been utterly destroyed before their time since then?
Governing.com wrote:
Engineers estimate that a fully loaded truck--a five-axle rig weighing 80,000 pounds, the interstate maximum--causes more damage to a highway than 5,000 cars. Some road planners say that the toll is even higher, that it would take close to 10,000 cars to equal the damage caused by one heavy truck. When the trucks are overloaded, as quite a few of them are, the damage is exponentially worse. Increasing a truck's weight to 90,000 pounds results in a 42 percent increase in road wear. Pavement designed to last 20 years wears out in seven.
Link to source