German In-Depth Accident Study (GIDAS) data from 1999 to 2007 was compared to Hanover Medical School (MHH) data from 1973 to 1987 and it was found that the relative percentage of passenger cars sustaining more than one impact in a crash, so called multiple impact crashes, has increased by about one third within the last two decades. An analysis of 9316 GIDAS accidents from 1999 to 2007 showed a three-fold greater risk of severe injury and a four-fold greater risk of fatal injury for a multiple as opposed to a single impact crash.

This study analyses multiple impact crashes in general and in particular occupant protection by out-of-position mitigation between impacts.

It was found that in two thirds of all multiple impacts with severe injury outcome, the irreversible restraint systems, front airbags and pretensioners, were not activated in an initial front impact. The corresponding proportion for nonactivation of side and curtain airbags in initial side impacts was approximately fifty percent.

To evaluate the risk of occupant out-of-position and the effect of one type of reversible system, a retractor pre-pretensioner, a finite element sled model including the human body model THUMS, was used.

In the simulation of initial front impacts with different changes of velocity, followed by a braking sequence, the pre-pretensioner leads to an obvious reduction in the forward chest displacement of the human model. Furthermore, depending on the pre-pretensioner force, the human model may be pulled back into its initial seating position.

The calculated time distribution between initial and subsequent impacts with a median of 0.6 to 0.8 seconds, was used for the evaluation of “pre-crash” measures.

The effectiveness of pre-pretensioning to position the occupant between impacts, ranges from 24% with 200N, to 93% with 400N pre-pretensioning force.