Here is a recap of efforts to replace the gasoline tax and the relatively less revenue collected because the federal gas tax hasn’t risen in years and the future decrease in gas consumption with more hybrids, electric cars, and fuel-efficient vehicles:

The favored answer of road engineers? Taxing by the mile driven. A handful of states — Oregon, Minnesota and Nevada — have already tested ways to use GPS and other electronics to adjust taxes. In the Nevada and Oregon tests, drivers had devices installed on their cars that sent data to special fuel pumps; those pumps automatically adjusted their fees based on how far the vehicles had driven, without revealing data that would amount to tracking drivers.

The GAO told Congress this week it should allow a similar test on electric vehicles and commercial trucks, and estimated that a pay-by-the-mile tax of 0.9 cents to 2.2 cents per mile designed to replace fuel taxes would raise a typical driver’s costs from $98 to between $108 to $248.

But it’s not the only answer to filling this financial sinkhole. Washington state lawmakers have put a flat fee of $100 a year on electric vehicles to make up for the gas taxes they don’t generate, and Oregon lawmakers may follow suit. In Virgina, Gov. Bob McDonald has proposed abolishing the gas tax entirely, replacing it with a sales tax and a new $100 fee on “alternative fuel” cars and trucks. That idea has already drawn fire from critics who point out that it would make Virginians who never drive pay for roads while letting people who travel through the state do so for free.

I’ve covered the proposals in some of these states earlier (see here) but I haven’t heard of the electric car flat fee. I imagine a flat fee will not be specific enough to target electric cars – why not just go by a reduced mile-driven rate as well to account for all of the roads being used?

I suspect the first state to institute this will encounter lots of protests. At what point can a tax like this be implemented: before taxes start to decline or only once it is really clear that gas tax revenues aren’t enough to cover road costs? A case could be made that we are already at the second scenario and need more revenue to cover federal roads.