Bike NOPA has images and facts on the bike counters placed under city streets. They're used to tally the details of cyclists' routes and numbers. This one here is Baker Street and Golden Gate Avenue. Cute, yes? According to BN, here's how they work:

The sensors are placed from one to three inches under the road surface and can distinguish between a bicycle, a pedestrian, or a vehicle passing over them. The system records a count once it detects an "electromagnetic signature" from the bicycle. The counters require a minimum of maintenance and are cost-effective, using batteries that last a year. All that is visible from the street is a diamond shape in the bike lane and a line connecting it to a similar sensor in the opposite bike lane across the street. The SFMTA first experimented with the automatic counters in March 2009 on Fell between Scott and Divisadero. The agency found the system accurate when compared to manual counts undertaken during the same time period.

Read more about counters at Bike NOPA.

The 2010 bike report, which can be found here, provides interesting facts: a scant 28% of cyclists are female, 71% of cyclists use a helmet, and, of course, cycling in San Francisco increased in 2010.