This study examines the frontal crush behaviour of the car population and shows that the specific energy absorption per unit mass properties of the car population are independent of car size. Examination of the car to car collision equations in this context shows that the mean deceleration experienced by a car is inversely proportional to car length, is related to the square root of collision closing speed and to the inverse of the fourth root of mass ratio and of crush depth. It is hypothesised for any specific car population and given degree of occupant protection within this population that Relative Injury Risk is proportional to the 2.5 power of mean deceleration. The model so derived is compared with published Relative Injury Risk data for collisions between similar cars, dissimilar cars. for overall risk to cars of specific size within the car population and for individual car ratings derived from analysis of accident data and very high correlations obtained.