People not paying -- and not getting caught -- is one reason collections fall short. Another possible explanation, suggests this San Francisco Chronicle story, is the large amount of handicap placards, which allow people to park for free. The city has 23,000 coin-fed meters, while city residents hold 90,000 handicap parking placards, issued by the state.
"While the [Municipal Transportation Agency] supports the legitimate use of disability placards, there's no doubt that they have an effect on our parking meter revenue," Judson True, spokesman for the city's Municipal Transportation Agency, told the paper.