It would be a pointless change that wouldn't do much but add all sorts of new bureaucracy and paperwork (How many miles did you drive this year? Prove it or get audited!).
People already pay for how much they drive by how much gas they use; the current system even has the advantage that heavier vehicles (which cause more damage to asphalt road surfaces) generally use more gas so those people pay more per mile to keep the roads up.
Linking it to fuel consumption as it is now is about the best way it can be done, since it's completely hands-off. No paperwork, no bureaucrats, no checking and calculating what you'll owe down the road. You just pay as you go, as you use the roads, and there's no way to over or under-pay.
I like the idea of getting people to drive less, but on the other hand if you tax on miles driven rather than fuel used, there's really no incentive to upgrade to more fuel efficient vehicles.
That too, though I'm sure they'll come looking for a new source of taxes once gas consumption drops in any significant way. But this idea is about the worst way to do that. More likely they'd have to go to something like taxing tires, but that's still not as directly linkable to road wear as gas use.
