Tax on Miles Driven?

Here's a new concept I just recently heard about...how about paying a tax on the amount of miles you drive rather than on gas at the pump? Apparently a few European countries are implementing just that...it's called a VMT or 'vehicle miles traveled' tax. The argument is that now that cars have become more fuel efficient, the government is not getting enough money out of the current gas tax to fund maintenance and construction on our highway and road system. Even Oregon tried out this concept in 2006. They installed GPS trackers on vehicles that calculated miles driven and adjusted the tax amount at the gas pump...sounds simple enough. My only question is how would you track this for electric vehicles? Although I was first shocked to hear about this concept, I can understand the need to finance our immense infrastructure. Any thoughts?