Observations from school bus and heavy truck rollovers also suggests that the fall and dive theories are incorrect 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". . Today it is generally realized that the primary injury mechanism in light vehicle rollover accidents is not crushing. Rather, it is widely acknowledged that the principle injury process for contained,i.e., non-ejected, occupants involves the impact between the occupant and the vehicle interior. 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.. In the simplest terms, when the occupants hits a collapsing roof, they hit harder because the roof is moving in on them. If the roof was not collapsing, i.e., not moving in toward them, their velocity relative to the roof would be lower and the impact less severe.
Roof crush has also been identified as a cause of both full and partial ejection in rollover accidents because of ejection portals created by the collapsing roof structure. These chiefly involve broken windows but occasionally also involve the body structure. The current Federal regulation involving roof strength -49 CFR 571.216 (FMVSS 216) -has been found to offer little benefit and is currently being reviewed. Many European manufacturers provide stronger roofs than do U.S. or Asian manufacturers despite the fact that their is no European (EEC) roof strength regulation for light vehicles. The Volvo XC 90 may be a good example of this.