As we have mentioned before, the gas tax will eventually come to an end. The following graphic illustrates the issue.
Imagine all gasoline vehicle users pay for all transportation costs. Imagine total expenses are $100,000,000 and the total number of users are 1,000,000, and all gasoline powered cars get 30 MPG. In that case, if all vehicles are gasoline powered, the gas tax will be $0.30/gallon, in line with current costs. Now imagine, only half of all cars pay the gas tax, the tax jumps to $0.60 to cover costs, still quite tolerable, but as the gas tax rises, the number of gasoline powered cars should be expected to fall. The following image shows the expected gas tax based on the above assumptions with a varying number of gasoline powered cars on the road. Note especially this is a log-log scale. At 50,000 cars with gasoline engines (95% non-gasoline powered), the tax jumps to $6.00 per gallon (above European levels), but the last car has to pay $300,000 per gallon. The move away from the gas tax is a positive feedback system that will accelerate. A replacement is required.

The possibility most vetted is some form of mileage tax using GPS technology. The following article from the Albany Democrat describes
Oregon’s proposal, which as the comments on the article indicates, will not come without some political struggle.
The longer version of the report is
Oregon’s Mileage Fee Concept and Road User Fee Pilot Program
along with a report by Starr McMullen and Lei Zhang on
Techniques for assessing the socio-economic effects of vehicle mileage fees