When the ex and I visited San Francisco in 1990 we decided to catch a bus to Fisherman's Wharf. We were severely lambasted by the driver for not having the correct change and he didn't have change. He begrudgingly let as on for free but luckily a passenger came to our rescue and paid for us with his spare change
(Another pointer if you are visiting the US, if the taxi fare is going to be say $20 make sure you have more than the $20 on you!!)