A 3-year prospective study of road traffic accidents was performed in Oxfordshire by a combined team of surgeons and engineers in which injury severity was accurately recorded and related both to seatbelt use and velocity change during the collision--an objective, computer-derived index of impact severity. Analysis of the injuries sustained in relation to the average crash severity from which they resulted has shown that for frontal impacts the use of seatbelts has had a major protective effect in reducing injury severity.