Wait a few seconds before doing anything. There might be a reason you don't know about, such as a disabled pedestrian being slow about getting across the street. If it's more than a few seconds, then tap your horn, once, lightly. A long, obnoxious BEEEP, the second the light changes, is liable to get a person shot here in Dallas. Road rage is very common here. One of my pet peeves is when I've turned on my blinker to make a right turn (almost a block before the actual turn) to let the person behind me know I'm going to turn, and then when I slow down to turn, the driver behind gets right on my bumper and lays on the horn until after I've completed the turn.