People that change lanes for no reason. I'll be going driving down the Interstate in the middle lane at 75 mph and a car in the right lane going 65 mph will decide they want to be in the middle lane and just move over. There are no cars in their right lane in front of them. No hazards. Nothing on the shoulder. They just prefer the middle lane. It creates the situation where the middle lane is the slow lane and left and right lanes are the passing lanes. That's not how it is supposed to be.