[1FreeAmerican]

You may not enter the intersection when the light is red. Otherwise you may enter the intersection if the light is green or yellow. You have entered the intersection at the moment your front tires are on or over the limit line - the extra wide white line (that is in front of (before) the crosswalk, if there is one).
It is your responsibility to make sure the intersection is clear before you enter even on a green light. If the traffic has blocked your entry you may not enter until it is clear. This means that just because the light is green you do not necessarily have the right of way to enter.

[21twelve]

I was stuck at a notorius light in Seattle today. Trying to turn left (one-way to one-way). The busy main road is flowing with traffic, then comes crawling to a stop. Then our light turns green (another major road - two lanes trying to turn).

Two to four cars can make it onto the cross street before our light turns red again. It is ALWAYS like this. Although I didn’t see any cars blocking the intersection today. (Every so often some idiot will). So much for automated traffic cameras and lights. I have no idea how many light changes I waited on - probably 10 or 12.