During the past 30 years, there has been much debate about the association between roof crush in rollovers and serious head and neck injuries. Some studies have reported that roof strength and injury are not causally related but that occupants are injured as they "dive" into the roof before it crushes.
Conversely, other researchers maintain that injuries occur when the roof buckles into the occupant compartment and contacts the people inside.
The debate about how people are injured in rollovers has obscured the fact that a strong vehicle "safety cage" is an essential part of crashworthiness design in all types of crashes. Institute testing using front and side impact configurations shows that limiting intrusion in the occupant compartment is necessary to provide space for the occupant restraint systems to prevent injury. The same principle applies to rollovers. A 2008 Institute study found that strong roofs reduce the risk of fatal or incapacitating injury in rollover crashes. This was confirmed by a second IIHS study using a different set of vehicles.
These were the first studies to demonstrate the link between roof strength and injury risk. They showed that stronger roofs reduce the risk of ejection and the risk of injury for occupants remaining in the vehicle. While the crash databases used in the studies did not specify how occupants were ejected, it is possible that strong roofs allow windshields and side windows to remain intact and doors to remain closed more often in rollovers. Other research has found these openings are common causes of ejection.
Since the IIHS studies, two NHTSA studies have produced a similar finding when taken together. The first found that reduced roof intrusion is associated with lower injury risk, and the second showed a relationship between higher roof strength and lower roof intrusion.