Elementary calculations show that the mass of the passenger's vehicle should have an important influence on risk of injury, greater mass yielding greater protection independent of the other mass(es) in the collision. This holds for collisions treated either as purely elastic, or as quasi-inelastic. Passengers in more massive vehicles thus would seem better off no matter what the size or weight of other vehicle(s) in a collision. The approximations used suggest that highway vehicles should be at least 20 times the mass of the average passenger, or injuries in a collision will be disproportionately grave.