Interesting. At 1.10 cars per household, and 2.26 people per household, about 47% of SF residents own cars. (Though subtracting out the under-18 population means 55% of driving-age residents own a car.) Also, given there must be some people who own more than one, the actual percentage is probably lower. Still, that's a lower number than I imagined.
http://www.bayareacensus.ca.gov/counties/SanFranciscoCounty....
40% drive alone, while a bare majority 51% use a car, bus or van for transport (including carpools).
It's so popular, nobody does it anymore.