Shot dead before it was even born …
Via Techdirt, from Fox: Obama Administration Shoots Down LaHood Mileage Tax Idea
The Mileage tax, proposed in Oregon and some other states, and due to be proposed nationally, had the following debut for the US:
From the article:

“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.
Asked about the claim, transportation department spokeswoman Lori Irving immediately shot it down.
“The policy of taxing motorists based on how many miles they have traveled is not and will not be Obama administration policy,” she said.

While I might think it a good idea from the point of view of managing traffic and replacing the gas tax as a source of funding, it will likely be more expensive to administer and politically unpopular, suggesting now is not the right time to pursue such a strategy.
The gas tax should be milked for all it is worth, until enough vehicles have switched away that it ceases to be viable. Scientific experiments on the technology underlying road pricing, and on the behavioral response are all worthy endeavors, but deployment is clearly premature.