And of course, there's always the possibility of some grandma rear-ending you at 25 mph at a stop light.For the original question, if you end up in a roll keep the arms close, tucked into the chest, hands under the chin. Be a pencil. Enjoy the earth-sky-earth-sky experience. Otherwise you're flailing and whipping your limbs into the floor or worse and breaking bits.
I've never considered an air bag vest for street riding, it seems to me like overkill unless I was riding crazy, in which case it's time to slow down. That's what track days are for
That said, if I lived somewhere cold I'd probably consider one for spirited rides, I mean, if there's no downside I guess why not. It's just too hot here to wrap up in even more crap.