I've commuted by bike in both San Francisco and Amsterdam. In Amsterdam it's a lovely (and safe) experience, whereas in San Fransisco it was positively hazardous. The key difference being the thoughtful lane designs in The Netherlands where you have dedicated bike lanes physically separated from the car traffic. In San Francisco you are often mixed with or adjacent to car traffic or parked cars - leaving you susceptible to parked car doors suddenly opening, or being caught in a moving car's blind spot.