In HEAD INJURY : WOULD A HELMET HAVE HELPED?, bicycle expert witnesses at Walters Forensic Engineering write:
Those of us who work in the legal and insurance industries are aware of the benefits gained through use of protective equipment such as seatbelts and helmets. However, when an accident occurs and liability is in issue, the plaintiff who failed to make use of protective equipment will typically argue that the equipment, if used, would not have reduced the injury suffered. These liability determinations are made by the judge or jury, and establishing the necessary causal connection between the injury suffered and equipment used (or not used) is a matter of expert scientific evidence.