How to thank motorists? Often motorist do something that makes me wish they had stayed at home, but they also frequently do nice things too. Like slowing down to let me in to the lane, or making sure that they overtake with plenty of room etc...  

Is there an unambiguous gesture that means "thank you"? I don't want to confuse people about my intentions (by using a gesture that could be interpreted as something else) but it would be nice to show appreciation.
A polite nod, wave or smile usually suffices.

It doesn't need to be much, where I live in the UK raising one or two fingers from the steering wheel is plenty when driving. Not sure about down south, I've heard it's a fairly local thing, but I don't imagine it being too different.