I grew up in San Francisco, and believe it or not, in those days we could and did easily get away with riding on Mt San Bruno. My best friend and racing buddy lived in Brisbane near S. San Fran and we could ride one block on the streets and be in the dirt. We had many practice tracks there. We even made an illegal (but never busted or even hassled) track near the San Francisco zoo. You could sometimes hear the animals if it was feeding time, and of course they could probably hear us. LOL.

edit for typo