I've often wished there was a signal one could give. I have a friend who is a classic car buff, they put their arms up high with a thumbs up when they see a car they like - this is understood by the driver in question. I've been riding bikes with him and had him do that to drivers, they almost always honk and wave - it's understood! Motorcyclists show a downward fist with the palm toward the rider. I've also seen this done car to motorcycle, and I have several cycling friends who'll do that bike to motocycle - and the response is always the same symbol back from the motorcyclist.
Why don't we have such a thing? Especially when I'm driving my truck, I'd love to signal a bicyclist some way that I am friendly.
Last week I cut a cyclist off - totally my fault and I felt terrible, I would have loved to wave a thumb or fist in such a way that he would have understood that it was a brain f*&% on my part and not that I was being a jerky driver...