This is a field study of cars that sustained very little passenger compartment intrusion in frontal crashes. It has been possible to define the key limits on seat belt effectiveness in these, the most common type of crash.Data are taken from the in-depth investigations by the Co-operative Crash Injury Study and Rover Group in the U.K. Vehicle crashworthiness is assessed in detail and then matched with medical information from coroners reports, hospital records or questionnaires sent to survivors.The study describes and compares important injury mechanisms such as driver head to steering wheel contacts, front passenger torso injuries caused by seat belt loads and front occupant leg injuries. 14% of occupants were rated two or above on the Maximum Abbreviated Injury Scale.It is realistic to design seat belt systems for use in passenger compartments that can be expected not to deform in the more common frontal crashes. The discussion is concerned with the importance of these results when developing safety systems in that context.