Originally Posted by
JanMM
A more reliable indicator of an action that is about to happen is an arm signal - I make sure to signal before a lane change or turn. A headcheck communicates nothing in particular.
I didn't say it was the most effective form of communication, just that it is a form of communication (in addition to telling you what's going on back there.)
It tells people that you might be about to do something.
It tells people that you're aware of the traffic behind and to the side of you.
dahut covered it pretty well.