[throwaway0a5e]

Its contextual.

Car coming the other way flashes and it means hazard ahead.

If it's at night and your DRLs are the only thing on it means "turn on your lights moron"

At an intersection it usually means "go" to whichever party should be yielding to the flashing party.

Of course if you're in the left land and flash your brights at someone who's clogging up the left lane there's a good chance they won't understand it because if they could understand the context they wouldn't be clogging up the left lane in the first place.