it's AFAIK 14mph, and motor bike helmets are tested at 17mph.
Note that in both cases you can still die from head injuries while wearing one. And in both cases accidents happen at a much higher speed than the tested speed (also they do resist much greater impacts)
Note that in both cases you can still die from head injuries while wearing one. And in both cases accidents happen at a much higher speed than the tested speed (also they do resist much greater impacts)