Road damage goes up roughly as the fourth power of the axle weight; the damage to the road from cars, even a lot of cars, is trivial compared to the damage from heavily loaded trucks. Michigan claims that although they allow heavier trucks, they require more axles per unit of weight, which should offset that somewhat. Nonetheless, if you compare a truck with 20000 pounds/axle, vs. a car with maybe 2000 pounds/axle, you find that the truck would do about 10000 times as much damage to the road [[10^4) per mile.