Accident studies show that frontal collisions, both as regards the number of people killed and those seriously-injured, are by far the type of crash with the most serious consequences. In order to improve this situation, it is necessary to ensure that the means used to restrain occupants work as efficiently as possible, whilst preserving the occupant compartment and thus by eliminating intrusion on the occupant restrained by seat-belts and pretensioners.
In frontal collisions where vehicle intrusion is minor, the main lesions caused to occupantss 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 nothing is done in order to limit the restraining forces, occupant lesions will be mainly thoracic, since airbags will have reduced the risk for the head, and lesions of the lower members, linked to intrusion, will be limited.
In order to reduce loads on occupants restrained by seat-belts, 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 first-generation Programmed Restraint System (PRS), with a seat-belt force threshold of 6 kN; thirty seven frontal accident cases involving this type of restraint were investigated. The corresponding data, crash severities and occupant injuries, are reported in this paper. 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 situation with respect to a European standard belt + air bag system is discussed. The paper provides data on the validation of the PRS II in various frontal collisions and static out-of-position (OOP) tests.