I was quoting information from "Bicycling Science" published by MIT and based on test information from British and American tests of rear wheel brakes versus both wheels being braked. Tests done on bikes with brakes in excellent condition.

They listed rear brakes as providing a maximum *********** of about .25G maximum and dual brakes able to give a bit over .5G before risking a header. BTW modern automobile anti lock brakes, properly used, can get close to 1G stopping power so the average car can outbrake any bike by a considerable margin in an emergency stop.