okay so the great mystery is tapping your head mean slow down because my brother and i have got into discussion and never agree on this.everything else we do.i know theres a post here somewere.not alot of computer time.
In Europe in car signs:
flash - police ahead (illegal)
brake lights + hazard lights - emergency braking(sight from behind) usually on express way due to high speed
hazard lights - thank you (sight from behind)
driving with left blinker on behind somebody - let me pass you/make me some space (pull to the left)
Have been using them there and I think they're very useful and could be applied in US.