"THANK YOU!" - with all the sarcasm I can layer on ...

Got right hooked on my motorcycle some years back. Rode up beside the dude at an intersection further up the road and his window was down.

Me: "You know you could have killed me back there, right" (overly politely)

Them: "Mumble mumble sorry mumble." (said while looking like kid caught flushing sister's stuffies down the toilet)

Me: "Please be more careful next time. Thanks. Have a nice day" (Green light and go)

I find extremely polite and articulate to really put people off balance when they expect/deserve profanity and a tongue lashing.