“That would be a user fee per mile adjusted by vehicle weight.”
I've heard it argued that gas taxes pretty much do this already.
Although not perfectly linear, fuel mileage tracks roughly with vehicle weight - the heavier a vehicle, the more fuel it uses per mile, and thus, a per gallon tax on fuel means that heavier vehicles pay more per mile than lighter vehicles.
It's true that the relationship isn't perfect, but it produces roughly the same effect with a lot less government than I've seen proposed to charge per-mile taxes, which usually include tracking the actual driving of individual drivers by the government. Not a conservative solution at all.
sitetest
You are correct. However, when mileage improvements are made in vehicles the amount collected per mile goes down while the wear inflicted on the road remains roughly the same.
Not exactly the same, since the higher mpg vehicle is also usually somewhat lighter.
According to this site, 1976 F150s average 10 mpg, 2014 models average 16 mpg.
http://www.fuelly.com/car/ford/f-150
That’s a huge difference in gas taxes collected.
Luckily, average national gas taxes have roughly remained the same per gallon over this period, adjusted for inflation. But with more miles traveled per gallon, that’s less taxes paid per mile.
The 1976 F150 owner pays about $5.00 in gas taxes per 100 miles driven, the 2014 model owner pays about $3.12.
There is also the issue of whether the inflation-adjusted cost per mile of maintaining the system is the same as in 1976. I have no idea whether it is or not.