Did you know that flashing your lights at an oncoming car is a method of warning of any road hazard behind you (and thus ahead of them)?
Reasons I have done this:
Deer in the road.
A mattress in the road.
A car accident partially blocking the road.
A half-naked man chasing a German Shepherd puppy down the centre-line of the road.
Kids playing kickball beside the road.
I've never done it to warn of a speed trap, and I certainly wouldn't have tried to warn a frickin' cop car of a speed trap. Yes, ossifer, it means any road hazard, and there are deer in the road about a quarter-mile or less behind me. You got nice U-turn skillz, though.