This study deals with the development of a restraint system in order to improve occupant protection in frontal impact. In frontal collisions where vehicle intrusion is minor, the main lesions caused to occupants are thoracic, mainly rib fractures resulting from the seat-belt. In collisions where intrusion is substantial, the lower members are particularly vulnerable. In the coming years, we will see developments which include more solidly-built cars, as offset crash test procedures are widely used to evaluate the passive safety of production vehicles. If this trend will continue, restraint forces from the belt will increase and as a consequence more thoracic injuries will occur in frontal collisions.

In order to address this risk, it has become necessary to work on an optimized limitation of the restraining forces, while taking account of the broadest possible population, especially elderly people. A first step in this reduction was taken in 1995 with the introduction of the firstgeneration Programmed Restraint System (PRS), with a seat-belt force threshold of 6 kN combined with a belt pretensioner. Thirty seven frontal accident cases involving this type of restraint were investigated.

Analysis of these data combined with findings from the University of Heidelberg / NHTSA study, shows that it is necessary to go a step further by reducing the shoulder belt force to 4 kN. As this objective cannot be achieved with a standard restraint system, it was necessary to redesign the airbag and its operating mode that is, a new seat-belt + airbag combination called PRS II.

This paper summarizes the data obtained with the 6 kN load limiter restraint in real-world collisions. A description of the new system is given and its performance in offset crash configurations with respect to a European standard belt + air bag system is discussed.