> Due to the fourth power law, they should be taxed about 5 times higher than a sedan simply for road maintainence.

In this model wouldn’t 18 wheelers dominate and it doesn’t matter what personal vehicles do?

Indeed a max-load trailer truck is equivalent to 10,000 cars in road wear. However the benefit of them (since they transport goods for many) is somewhat outweighed. Regardless, that doesn’t obviate taxing personal vehicles at all, nor heavier vehicles more than lighter ones. Some quick searching shows there are about 60x more personal vehicles than trailer trucks in the U.S.