It's protocol here in Australia to give people a little wave in your rear-view mirror, if they have courteously allowed you to merge into their lane, in front of them. Alas, however. this is a practice that has also been taken-up by ultra-rude, barge-arse drivers, who dangerously barge in front of you in traffic, for no apparent reason.
One is driving along at speed and for no reason, whatsoever, some drongo will barge in front of you and give you a jaunty little "Thank you kindly!" wave in their rear-view mirror. It's hard not to take it as a sarcastic, "Up yours!" gesture, even though it does take the form of a limp little wave. If they're going to barge dangerously in front of me, I would personally prefer that they don't give me a familiar, cheery wave, thanks all the same. We're not friends, so please don't wave at me. And besides, you twit, you've just been driving like a moron. Keep away from me!
Sheesh, I think I'll go back to public transport!