That system would be ridiculous. It would add on another thing that we'd have to deal with. How would we report it? How would it be tracked? How would they prevent cheating? To actually make a system that would work it would cost a HUGE amount of money and it would be a headache for all of us. I'd much rather have the tax rolled into the cost of gas because then everything is easily dealt with and we don't need this huge organization to monitor our driving.
The last thing we need is another government organization that costs money to support. That would just end up taking more money out of all of our pockets.