If any horse broke rotary protocol, word would get around the coach house. It would only be a matter of time before the other horses pooped in his oats and peed on his salt lick. If you're a horse, it does not pay to be a schmuck. Nowadays we use horseless carriages, and we alone decide whether to be polite or to be Boston Drivers. And so, the rotary has come to have a distilled concentration of everything that makes driving in Boston such a contest for survival.
People drive so fast in the rotaries that their cars list visibly.
Normal levels of Boston Driving are amplified considerably, because we all know that those who enter must yield, but nobody really obeys that. The truly unnerving part is that as you line up to enter, each driver coming by may still be in the rotary or may be planning to exit in the street right after yours. He won't signal. Turn signals are for the weak. Nothing beats the thrill of riding a bicycle through a rotary.