I've had SO MANY people ask me this, I thought I better spell it out.
SO....if you're on a Petrol Forecourt, or in a car park, and the guy in front of you flashes his reverse lights at you ....and then pulls away....
Its cos he's got an AUTOMATIC GEARBOX and to move the Gear Selector from Park to Drive, you have to go through REVERSE on the way....
So for a split second the lights flash....its not cos he's trying to stress you out, or make you think he's gonna reverse into you!
Here endeth the lesson