Empirically, the wearing of motorcycle helmets has greater protective effect at slower accident speeds. The logic is that accidents (crashes) at high speeds and involving sudden decelerations such as hitting immovable objects, oncoming and path-crossing vehicles are more likely to inflict multiple serious injuries resulting in the death of the rider than low-kinetic energy crashes. Perversely, the life/death benefit of wearing a helmet on a freeway (motorway) is less than low speed urban riding. Other benefits though are eye and hearing protection with full-face helmets, not so with DOT minimal helmets. See if you can estimate the lifetime personal and societal costs of surviving a severe head injury compared to the comparable costs of death, then compare the head injury severities of helmet versus no-helmet.