Transportation Secretary Ray LaHood wants to tax the number of miles you drive, not the amount of gasoline you buy:
“We should look at the vehicular miles program where people are actually clocked on the number of miles that they traveled,” the former Illinois Republican lawmaker said.
….”What I see this administration doing is this — thinking outside the box on how we fund our infrastructure in America,” he said.
I’m with Atrios on this: it’s a dumb idea. There’s a place for London-style congestion charges in crowded urban cores, but outside of that a gas tax 
and a VMT tax are practically the same thing. The only real difference is that a VMT tax isn’t remotely feasible, and won’t be for years, while we could raise the gas tax right now if we wanted to. So jabbering about a VMT tax is basically just a good way to avoid taking any serious action to reduce gasoline consumption.
What’s more, in some ways a gasoline tax is better than a VMT tax anyway. A VMT tax motivates you to drive less, which is great, but it doesn’t motivate you to go out and buy a fuel-efficient car. A gas tax does both. It’s true that as cars become more fuel efficient — and as electric cars displace gasoline cars — the revenue from a gas tax will go down, but there’s an easy fix for that: raise the tax. That accomplishes the same revenue smoothing as a VMT tax, and does it with no muss and no fuss.
Eventually, electric cars will become widespread enough that we’ll need to figure out how to make them pay for using the roads even though they don’t pay any gas taxes. Maybe the answer will be VMT, maybe it will be something else. But that’s years away. So study away, but in the meantime if you’re serious about changing driving habits, raising the gas tax is the only serious alternative.