So I'm always driving or riding around San Francisco seeing these little street signs with a bicycle on them and a number indicating some kind of a route number like a highway or interstate, but for bikes. Well I found out what it is. The San Francisco Bicycle Coalition maintains a map of all of the SF bicycle routes. The map is amazing. I recommend downloading it and checking it out. It indicates routes to get from district to district utilizing low grade (not steep) and low traffic directions. Enjoy.