In some ways, taxing people for the miles they drive makes sense. After all, we need money to keep roads in good shape and it already happens today, indirectly, through gasoline taxes. But when anyone talks about taxing the miles directly – i.e., through a mileage or "vehicle miles traveled" tax – hackles get raised.

Nonetheless, a new VMT tax is being proposed in the San Francisco Bay area, where officials from the Metropolitan Transportation Commission and the Association of Bay Area Governments just approved studying how a per-mile tax would affect revenue, pollution and traffic. The study will look at rates between a penny and a dime a mile. The Mercury News figures that, at the higher end, the tax could raise up to $15 million a day. The study will last through January and the commission could vote on it next April.

What's tricky in the Bay Area proposal is that vehicles would need to have "GPS-like odometers or other devices" installed in order to track how far they've gone. Sure, many people already have such trackers in their cell phones they carry with them all the time, but to require this from drivers would be a new step. A mileage tax project that was tested in Oregon had a clever workaround. Yes, the cars had GPS units, but they only broadcasted information when the car was filled up, allaying some privacy concerns. In 2009, Transportation Secretary Ray LaHood also talked about a mileage tax, but the idea was quickly shot down.