I most states its legal for cycles (motor and non) to pass cars and pull to the head of the line at a light. Not sure why; but that's the way it is. Stopped cars + agile cyclist = no problem.
When traffic is moving after the light, the car sure as heck better leave some room for the cycle, because any error there means an ambulance.
Try staying for a minute or two behind a vehicle that has its engine running and burning fuel. In a line at a light inhaling exhaust is enough to make you dizzy—at least that's what happened to me when I tried it once. Always pulling right to the light afterwards.
It's legal in exactly one state: California. I think it's a wink and a nod for bicycles (or maybe it is legal for pedal bikes, I just don't care enough to look), but it is most definitely illegal outside of CA for motorcycles. One lane == one vehicle. I can even dig out the relevant section of the Revised Code of Washington, 'cause it sure would be sweet to lane split in WA but the RCW says "no".
A lot of lights in London have a space marked out at the front reserved for cyclists and motorcyclists so that they don't get cut up by vehicles making a turn.