As tepples mentioned, the road wear goes up exponentially, which is why the discussion of fuel taxes and hybrid drivers is so frustrating. Automobiles have a very limited impact on roads compared to heavy, loaded, tractor trailers.
According to this GAO report (http://archive.gao.gov/f0302/109884.pdf) a fully loaded tractor trailer does ~9,600 times the damage to roads that a car does. An average driver does ~15k miles per year and gets 25 mpg. Using the Michigan gas tax, that car would pay about $110/year in gas tax which is supposed to support the roads. If we're interested in assigning fees based on road wear and impact, that truck needs to pay over $1M. It's a regressive tax that has general drivers subsidizing business use of the roads. I know we need goods, but if we forced businesses to assume the cost to move their goods, they may get a lot better at distributing goods more efficiently (trains, boats, etc.)