[m463]

flashing high/low/high/low means "your high beams are on" but you can take away their night vision.

flashing on/off/on/off means "turn on your headlights" or "speed trap ahead"

[jayphen]

Flashing high/low/high/low from behind here in Austria at least means “you are annoying me but not in a way that immediately threatens my safety” (beeps are reserved for slow starters at lights or near-collisions). If going in the other direction it means cops ahead.