There are few things that get my goat more than the lack of courtesy wavers on the road. A quick flutter of the fingers to acknowledge your fellow driver that you snuck in front is not too big of an ask is it?
When someone doesn't give a polite wave, I am sorely tempted to re-inact this Flight of the Conchords clip:
However my Mum would probably box my ears if I attempted this (plus I'm a chicken with slow reflexes). So I think the next time someone doesn't follow the courtesy wave conventions then I'll have to trial some of these more inventive hand gestures: