Well, I have seen that several states have talked about doing something like this, including Oregon and California. And in the cases where I have read about this they were looking at this as an addition the existing gasoline tax, not a replacement. And if you don't collect a lot of data and make assumptions about what the vehicles do to the roads, you end up treating all vehicles the same. Suddenly a Hummer is paying the same to use the road as a Toyota Tercel.
I attempted to sign up for a test program that the University of Iowa was launching last year, but did not get picked. I was not a fan of this sort of thing but I was curious and wanted to know what data they would be collecting and how it might be used. I know that the stories I had seen talked about how they wanted to have billing based not only on the number of miles driven but the time of day and where they were driven. So, you might pay less on a rural highway than a congested city street. You would pay more for travel in the city at rush hours than after hours. (I think of this when I see the IBM commercial where they talk about a system of fees or something that helped a city reduce congestion.)
Nonetheless, it is intrusive, but then so might be the smart grid and other technologies. I guess we should just get used to not having our privacy.