If you've finally accepted that automobile ownership is linked to pretty much every malaise suffered by this town, yet you still love to drive, we may have found the service for you. City CarShare is a sort of by-the-hour car rental agency that may just help end the relationship between automobile use and urban congestion. To use the service, you first have to be approved by City CarShare's insurance company and put down a $300 deposit; but once you're signed up, going for a midnight drive up Mount Tam is merely a matter of making a reservation on the CarShare Web site, which programs an electronic box on one of its cars (stored at various locations around the city) to accept your CarShare-issued magnetic key fob. Then turn the key and drive, drive, drive, for as little as an hour at a time. At the end of the month, you get a bill charging you a $10 monthly fee, $2.50 per hour driven, and 45 cents per mile. You don't pay for gas, insurance, repairs, or anything else. Because people using the program drive only when they need to, one can hope that ever more of them will decide that is quite good enough, and will choose not to buy cars or will give up the costly, space-eating ones they already own.