It's ridiculous. All shouting accomplishes is more aggravation.
It can be. Especially when people are damned and determined to get their point across with on the spot hand gestures. I've also encountered people who know the alphabet and nothing more and try to painstakingly sign everything letter by letter. I can read lips, though, so as long as I can see the mouth I can usually follow along.
Kindergarten or earlier. but adults tend to forget those early life lessons when it suits them.