Shortly after Secretary of Transportation Roy LaHood raised the idea of a transportation tax based on mileage, rather than gas, the idea was shot down in both a written statement from the department and by White House spokesman Robert Gibbs. LaHood's idea, which would have placed GPS devices in cars, would have taked people by how much they drive, rather than how much gas they purchase. LaHood said that the plan, which could have raised additional funds for infrastructure, was something "we should look at."
According to a Department of Transportation statement: "The policy of taxing motorists based on how many miles they have traveled is not and will not be Obama administration policy." The idea of a mileage tax is still being floated around by a few states - namely, Idaho, Rhode Island and North Carolina. Oregon has even run a pilot program, and found the program to be viable.
The tax would have environmental implications both good and bad. When people buy more efficient vehicles, they use less gas, which is good. They're still driving just as much, though, and their cars have the same effect on the road, so the government doesn't get as much revenue from these drivers to fix our highways and bridges. As more and more people begin driving hybrids and using less fuel, this will become a bigger problem.
At the same time, green bloggers have pointed out that presently, a mileage tax would punish them for consuming less gas, and take away the incentive for others to purchase hybrids. Morgan Clendaniel of Good points out that an optimal solution would be a weight-per-mile tax, which would reward drivers of lighter vehicles, like the Prius, which presumably do less damage to the road. The problem with all of these ideas, though, is that they still require a GPS to be put in each car - a privacy concern. Surely, someone is hard at work on a better solution.