For many years there has been an acceptance that the level of protection provided to car occupants was influenced not only by the characteristics of the car they occupied but also by the characteristics of the other car involved. Heavy and stiff cars have been said to be aggressive and with other aspects, such as the height of the front structure, these characteristics have been seen as the major factors influencing compatibility. Despite these concerns, comparatively little research has been carried out to identify the most important characteristics influencing compatibility or to quantify their effects.

For the UK Department of Transport, the Transport Research Laboratory has embarked on a programme of research which will attempt to answer these questions. This paper discusses what is currently known and details some of the early findings from the research. Already, some of the simplistic views held in the past are being questioned.