Every year approximately 10,000 Americans are killed in rollover accidents, accounting for about 30% of all light vehicle occupant fatalities.
The relationship between injury levels and intrusion or roof crush has been statistically established,[2] but the mechanism has been thought sometimes to be somewhat obscure.
[4] Observations from school bus and heavy truck rollovers also suggests that the fall and dive theories are incorrect [5] and that another theory of the mechanism of injury in rollover accidents is required, one that relates injury to the intrusion of the roof structure into the occupant compartment or more simply to "roof crush".
[6] Today it is generally realized that the primary injury mechanism in light vehicle rollover accidents is not crushing.
Since the severity of an impact depends to a large extent on the relative velocity between the impacting objects; the impact theory of injury causation in rollovers has sought to explain the increase in injuries associated with increased roof crush with an increase in the relative velocity between the occupant and the vehicle's interior which is generated by the roof crush.