I've always thought taht calling "car up!" and "car back!" stuff were as much for *me* to keep me in the "I'm riding my bike... not in my car... or on my couch... or on the hammock... or with a mouse..."
It's like doing the cell phone while driving. Somehow it really is different than the radio in the car; it feels different.
I confess if I come up behind someone and the reason they can't hear me the first time is 'cause of crap in their ears, I *will* be loud when I'm closer and yes, I hope I startle you!