In HEAD INJURY: WOULD A HELMET HAVE HELPED?, bicycle accident reconstruction expert witnesses at Walters Forensic Engineering write:
Helmets significantly reduce the possibility of a skull fracture. The impact force of a striking object against the helmet is applied over a small area at the outer edge of the helmet. The purpose of the helmet is to distribute this force over a larger area at the interface of the helmet with the head. This results in a decreased maximum force at the skull relative to an equivalent non-helmeted impact and decreases the probability of a skull fracture.