Vehicle rollover as an indicator of mechanism of injury was investigated in a study examining accident databases and the medical literature. Only 2.4% of crashes involved rollovers but they accounted for one third of occupant deaths.
Some facts on vehicle rollover from the article:

• Rollover is defined as a vehicle overturned by at least one quarter turn (at least onto its side).
• Some rollovers involve many quarter turns and the final resting position may be on the vehicle’s side, roof, or back on its wheels.
• Factors that cause a vehicle to roll over include trajectory (i.e., turning vs. straight), vehicle type, and speed (precrash velocity may be the most predictive factor)

The importance of vehicle rollover as a field triage criterion
J Trauma. 2009 Aug;67(2):350-7