To signal to other drivers that there are police in the area.

Example:

2 way road, I pass a cop, cop is catching speeders with radar/laser, I see incoming vehicle, I high beam once or twice to notify other driver of police presence, and the other driver slows down and avoids a speeding ticket. (this is assuming that the other driver knows what I meant with the high beaming)

I always did this to try to save others, and I do not know why this does not catch on, like how when bikers pass each other they wave. So can't cars use some type of signaling (2-3 fast highbeams) to one another to warn of police trying to catch speeders over the ridge or bend?