San Francisco's taxi system is somewhat famously clunky. The city keeps tight control on the number of cabs through a strict medallion-issuing system. So cabs are scarce, and it can be impossible to hail a taxi outside a few touristy areas in the city's downtown.
Abuses to the system seem to be making the situation even worse, according to this SF Weekly story:
The arrangement, however, is riddled with scammery. A person with a medallion who doesn't want to drive can make thousands per month simply renting the medallion out to a cab company or another driver -- and apparently scores of them are doing just that.
Image via Flickr, from michael valdez