This happens a lot where I live. Your behind another car going 30-40, they decide to let a car out so bring themselves and you to a standstill to do so. The other car doesn't react quickly to what is happening so waits a bit longer until they join the traffic. I can understand if its busy or stuck in a q, but all the time it happens where the road is free behind or one more car behind. So if they just carried on they could go after we pass. The only thing it does is delay everyone and waste fuel.