DON'T DO IT. If any idiot thinks he/she is doing you a favor by stopping traffic behind him/her to let you out - honk your horn, flip them off and tell them what an 'effin idiot they are and to keep moving.
Today I had at least three idiots stop traffic behind them to put me into a risky situation by letting me out. NEWSFLASH - look in the rearview mirror asshole and see the destruction you bring to the traffic flow by stopping to let someone who does not have the right of way into a stream of traffic when in a few seconds the way will be clear anyway.
Call me jaded but I have driven enough miles to go to the moon and back twice.