A 3-year prospective study of road traffic accidents was performed in Oxfordshire by a combined team of surgeons and engineers. This liaison permitted the identification of injury sources and allowed the accurate comparison of injuries with collision severity. This was measured using the velocity change of the occupant's vehicle during the impact and was calculated using the CRASH 2 computer programme. It was shown that the pattern of chest injury varied with the direction of impact and the severity was related to the velocity change. Seatbelt use resulted in a reduction in injury severity and also modified the pattern of injuries sustained.