want the I81 user to pay the same gas tax as everybody else when he fills up, PLUS a toll that most other users wouldn't. How is that close to the "pay for the roads you use" that you state you want in your first sentence?

The closest thing we can come to paying for the road that you use without tolling every road is a gas tax. Sure, some cars get better mileage than others so they pay less, but they also tend to be lighter and do less damage to the roads. With a gas tax you are almost paying per mile travel. The more "useful" roads are by definition more heavily traveled so users would pay more in taxes. The most "useless" and costly per vehicle roads are the lightly traveled back country roads. If you want to make people pay their fair share those are the roads that should be tolled!

Also, the I81 toll as proposed is NOT about taxing people that use the roads. Daily commuters would be mostly exempt! It is about gutless politicians that are trying to raise somebody else's taxes. It's has nothing to do with fairness.