When I fly through SFO, I usually do an overnight at the Marriott or Hyatt near the airport. I hate waiting for the shuttle. Even though they say it runs every 15 minutes, I've waited for over 1/2 hour many times. Thinking next time I'll just take a cab. Question is, how much should I tip on such a short cab ride? I'm guessing the fare would only be around $10 but I feel guilty since I realize the cab driver will probably have to go back and wait his turn which might be 1/2 hour or longer to pick up his next fare. Is a $20 tip enough?