Knowing less after you read a sign than before

I've discussed how some things are fixed in documentation - people choosing to put up signs that warn you of some thing rather than fixing the thing in the first place. This one's different. I was contentedly waiting for the red light to change until I saw this sign. After reading the sign, I knew less what to do than before.

I think that this means that traffic going in either this direction or the opposite gets a green light sooner than the other. That probably makes a difference if I'm going to take a left turn, which I am not.