I have been using a right hand extended for 35 years, because the left hand up is car specific and I really don't think it ever made any sense to use it on a bike. I have seen very few cyclists using the left hand, and when they do it looks wrong. They usually fail to make it look like a turn signal anyway, and nobody knows what they are doing.
I really don't know why hand signals in a car are legal anyway. I saw a guy in a car making a right turn with hand signals, and it took me forever to realize that was what he was doing. It's not different enough from someone with their hand out the window enjoying the breeze to really make sense.