The Looming Era of VMT
"What's needed now is not a higher gas tax, but a whole new way of looking at how we pay our fair share for using public roads. The report's authors home in on what has become the consensus favorite solution of transportation wonks. We shouldn't fill road coffers according to how much gas we buy, but how many miles we drive.
The gas tax has ceased to function in the way it was originally intended - as a user fee (by sparing us their tail-pipe emissions, this means those 50-mile-per-gallon Prius drivers are also not really paying for their full use of common roads). Today, the most accurate measure of use is what's known as Vehicle Miles Traveled."
Some argue that the success of variably-priced vehicle tolling is a sign that charging by the mile would not be a political bomb.