a less common signal these days is when you meet a motorcyclist approaching in the other lane and he points at his helmet or head, it doesn't mean he thinks you're crazy.... ..usually. Older police cars had a single red rotating light on top, resembling a bubble gum machine so he is telling you that in your very near future there will be a friendly law enforcement officer, probably with a radar gun. Since a helmet resembles a bubble, etc. Don't see it used much any more.
Have fun out there, stephen