When we are driving 30 mph and trying not to kill you, we don't have the time or energy to decode your Navajo hand motions. Maybe we should, but we don't.
I understand that you received education to what these signals mean, but we didn't. All we know is you are making odd gestures while riding a small, engineless device trying to cross four lanes during rush hour.
So basically, when you make a motion with your arm, any motion, here is what we get out of it:
"Shit, that guy on the bike is gonna do something."