I try not to pass cars waiting at lights (we have no bike lanes -- that would be different) because I know it's annoying for them to wait behind me, then pass me, only to have to pass me again. Also, drivers around here are very lax with using turn signals. Granted, I'm not usually behind a long line of cars either, nor am I a particulary fast biker. I have done it in the past, but I'm trying to mend my ways.