As cars get more fuel efficient , they become a less profitable source of tax dollars. So what's a city to do? Raising gas taxes is certain political death. For San Francisco Bay officials, creativity is the key.The Metropolitan Transportation Commission and Association of Bay Area Governments are currently examining a plan to tax drivers by the number of miles driven instead of number of gallons burned. To do that, cars registered in the area would be fitted with a GPS device to track the number of miles traveled.Drivers could be charged less than a penny or as much as a dime per mile. One estimate puts potential revenue from the system at $15 million a day.If the thought of your city government knowing your every move sounds frightening, the Commission's spokesman, Randy Rentschler, says not to worry.

"The last thing we're interested in is where you go and what you do," Rentschler tells the Associated Press. "What we're trying to do is get people to figure out a way to raise revenue that they could support."

Still not convinced?Don't get worked up yet – it could take Bay Area governments as long as a decade to implement such an idea. By then we'll surely have flying cars, electric scooters or even molecular transporters.