When making a left turn at a traffic light, you should NOT stay put at the white stop line when the light turns green. You should instead proceed fully into the intersection while you wait for a lull in oncoming traffic that will let you safely complete your turn.
This is a fundamental driving rule that most of us were taught when we first got our learner’s permits at age 16. Pulling into the intersection promotes safety and efficiency in at least three ways: First, drivers behind you who want to proceed straight ahead can more easily get past you if you are in the intersection. Second, drivers behind you who are also waiting to turn left have a better chance of completing their turns before the traffic light turns red. If, in contrast, you sit at the stop line while you wait for an opening in oncoming traffic, you might not be able to turn at all, forcing you and those behind you to wait for yet another signal cycle. Or, if you’re more aggressive, you may try to make a last-minute turn from the stop line, risking a collision with drivers on the cross street who get a quick jump on their green light. Third (and related to points one and two), by properly entering the intersection, you minimize the risk that you will be the victim of a road-rage assault by a driver frustrated by your ineptitude.
You need not be concerned about the light turning yellow or even red while you are in the intersection. You can lawfully complete your turn even after the light has turned red. Don’t believe me? Take a look at this article from the Tacoma News Tribune:
According to the Washington State Patrol, it’s completely legal to pull out into the intersection and wait for a clearing to turn left. You don’t have to wait at the stop line.
“If you have a green light, go ahead and get your position in the intersection,” said State Patrol spokesman trooper Guy Gill.
You can take your left when it’s safe, Gill said, even as the light turns from yellow to red.
If you’re eager to read more about this burning issue, take a look at this discussion on The Straight Dope.