There may be something we haven't taken into consideration. At a light, whether I am in front or waiting behind other riders, I try to get up to speed as quick as I can (remember I am riding long, heavy cruisers / choppers) and I find that I am often closing in on top speed while other much faster bikes are still slowly gearing up. I have no choice but to pass them in spite of cars being in close proximity and even though they will likely (but not always) pass me a minute or two later. People have different styles of riding and I am always surprised how long it takes these nice, light road-bikes to get up to a half-decent cruising speed (my chopper is often way faster). I have tried continuing behind these lazy-peddaling cyclists but it's much harder for me to go that slow, requiring alot more effort. That doesn't excuse the inconsiderate and selfish idiots that by-pass fellow cyclists waiting at the light, but it may explain, at least in part, why they do it: they can establish their cruising speed easier and stake their lane.