Seriously. That's all it takes to make me not want to kill you, skin you, and feed you to my pet fire ants. Usually.
If I slow down and hold up the 20 cars behind me in traffic to let you turn left onto the road in front of us all, how difficult is it to raise your fucking hand in a gesture of "hey, thanks for holding up those 20 cars back there and letting me get on the road".
If you and I are in two right-turn lanes (you in the far right, me in the next lane over), and upon turning, you decide to drift over 3/4 of the way into my lane, making me pull left and go 3/4 of the way into the oncoming traffic lane. AND you don't notice that I'm blaring my truck horn at you for 5-7 seconds *right outside your driver's side window* while you do this...just wave! It's really that easy. Sure, I wanted to kill you for those 5-7 seconds and maybe the next 2 seconds until you *did* wave, but then the tension was allievated when you did the sheepish "gosh, i'm sorry, that was a dumbass move and maybe next time I'll be a little more careful" wave. Then at least you realized you fucked up and nearly caused my death by head-on collision. And I appreciate you noticing.
Yeah, I realize this is basically asking people to be polite. And I realize I'd have better luck standing on the streetcorner asking people to shoot donuts out of their asses through a ring of fire. But at least I can ask, right?