A vehicle is classified as rolling over if it tips onto its side or roof at any time during a crash. Many rollovers lead to partial or full ejection of occupants, which increases the likelihood of injury or death.

How big is this problem?
Rollovers account for only 3 percent of vehicles in police-reported crashes, but about 20 percent of vehicles involved in fatal crashes roll over. About 10,000 people die in rollover crashes each year.

Why do vehicles roll over?
Usually because a driver loses control, and the vehicle begins to slide sideways. Then something like a curb or uneven ground 'trips' the vehicle. Rollovers also occur when drivers try to turn too aggressively. About 3 of every 4 rollover crashes involve 1 vehicle only.

Are SUVs more prone to rolling over?
Yes, rollover is much more common for SUVs and pickups than for cars — and more common for SUVs than for pickups. In 2004, 62 percent of SUV occupants killed in crashes were in vehicles that rolled over. This compares with 45 percent of deaths in pickups and 23 percent of deaths in cars. In large part this is because of physical differences among the vehicles. SUVs and pickups are taller than cars and have greater ground clearance, causing their mass to be distributed higher off the road relative to the width of the vehicle. Adding passengers and cargo increases the center of gravity even more, and vehicles with high centers of gravity are more prone to rolling over.

What to do?
Adding electronic stability control reduces rollovers by as much as 80 percent (see Status Report, June 13, 2006; on the web at iihs.org). More and more vehicles have this feature, but it won't prevent all rollovers. When a vehicle does roll, a strong structure, including the roof, is essential. The best protection is to buckle up. More than 70 percent of the people killed in rollovers aren't using safety belts. For answers to 17 questions about rollover, go to iihs.org/research.