Tax mileage, not gas?

Oregon may tax drivers based on the miles they rack up on their cars, instead of using gas taxes to fund new roads.  This is a novel approach; one that I had not thought of.  It does make sense, since cars will eventually get to 50mpg or more, and at that point the gas tax will have to be extremely high, or other taxes (like this) will have to bridge the gap.  It is definitely a fair tax, since drivers would pay for the amount that they use the roads.

The one concern that I have (which is also mentioned in the article), is privacy.  However, if detailed tracking information is not stored, I do not see much of a problem.