I always wave to the person behind me when I merge in traffic. That's friendly thanks-for-letting-me-in shoulder high kind of wave.
I wave if someone lets me drive up a crowded street first. That's a smiley 'thankyou' mouthed silently as you pass them and wave.
I wave if they let me out of the driveway (we live on a semi-busy road). That's a quick nod and wave as you drive off.
I nod and flick my lights to allow someone else to merge, then I nod and wave...that one's a one finger wave.
What about you?