Never lived in SF but it definitely happens in NYC all the time. Not some deep conversation perhaps but people exchange small talk with strangers on the subway, in parks, cafes etc.
Traffic lived up to my expectations though. Driving to and from the hotel was a little stressful.
But being considerate in New York usually looks like staying out of others' way. It might seem like unfriendliness to out-of-towners, but it's a (perhaps counter-intuitive) form of courtesy.