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 from the vehicle, increasing the likelihood of injury or death. Although rollovers account for less than 3 percent of passenger vehicles in all police-reported crashes, they make up about 23 percent of passenger vehicles in fatal crashes.¹ A total of 10,182 passenger vehicle occupants died in crashes where their vehicle rolled over in 2007.

Most rollovers occur when a driver loses control of a vehicle, and it begins to slide sideways. When this happens, something can "trip" the vehicle and cause it to roll over. This tripping object could be a curb, guardrail, tree stump, or soft or uneven ground on the side of the roadway. Rollovers also can occur when a driver attempts to turn a vehicle too aggressively — at a high velocity or with a tight turning radius. In such conditions, the frictional force between the tires and road surface can cause the vehicle to tip up and then roll over. These crashes generally are referred to as "untripped" or "frictional" rollovers. Though less frequent, rollovers can be caused by other factors, such as when one side of a vehicle is flipped up suddenly by a guardrail or other ramp-like object or when a vehicle falls sideways or front-first down an embankment.

A multiple-vehicle crash can initiate a rollover if it causes a driver to lose control, or a vehicle may roll directly after being struck in the side by another vehicle. However, about three-fourths of vehicles that roll over in fatal crashes are in single-vehicle crashes.² More than half of all occupants killed in single-vehicle crashes are involved in rollovers.

Rollovers are much more common for SUVs and pickups than for cars, and more common for SUVs than for pickups. This has been true in the past and continues to be so. In 2007, 59 percent of SUV occupants killed in crashes were in vehicles that rolled over. In comparison, 46 percent of deaths in pickups and 25 percent of deaths in cars were in rollovers.

Pickups and SUVs tend to be involved in rollovers more frequently than cars largely due to the physical differences of these vehicles. Light trucks 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. Additional passengers and cargo can increase the center of gravity even more. Other things being equal, a vehicle with a higher center of gravity is more prone to roll over than a lower riding vehicle.³

Driver behavior may contribute to the increased rollover involvement rate of SUVs and pickups. Pickups and SUVs are more likely than cars to be driven on rural roads, where rollovers occur more frequently.³ Lower belt use among pickup occupants⁴ means they are more likely to be seriously or fatally injured when rollovers occur.

The significance of rollover crashes can be perceived as increasing, decreasing, or staying the same, depending on what is being compared. The annual number of fatalities in rollover crashes on US roads has increased as SUVs have become more popular. However, the size of the US vehicle fleet has grown more rapidly than the number of rollover fatalities, so the fatality rate based on the number of registered passenger vehicles in the fleet has declined consistently during the past 20 years, from 32 driver deaths per million registered vehicles in 1986 to 19 deaths per million in 2007. The percentage of fatalities in rollover crashes for each vehicle type has remained relatively unchanged.

Manufacturers are creating more stable vehicle designs. The static stability factor (SSF), a measurement of a vehicle's geometrical ability to resist rollover based on its width and center of gravity height, increased an average of 6 percent for new SUVs between 1998 and 2003, after remaining constant for 20 years.⁵ Electronic stability control (ESC) also has become more common. This technology helps prevent the sideways skidding and loss of control that can lead to rollovers.

Since 2001 the National Highway Traffic Safety Administration (NHTSA) has assigned rollover resistance ratings to vehicles (1 to 5 stars). These ratings can provide some indication about which specific vehicles are more likely to be involved in rollover crashes. Between 2001 and 2003, the ratings were calculated using SSF only. SSF is calculated by dividing half of a vehicle's track width (the distance between the right and left tires) by its center of gravity height. Wider vehicles with centers of gravity closer to the ground tend to be more stable, but this measurement does not account for dynamic effects such as those due to a vehicle's suspension. Beginning in 2004, the rollover resistance rating system was revised to combine the SSF with results from a dynamic handling test, but this test changes the star ratings of only a few vehicles.⁶

ESC is a vehicle control system comprised of sensors, brakes, engine control modules, and a microcomputer that continuously monitors how well a vehicle responds to a driver's steering input. The computer compares a driver's commands to the actual travel of the vehicle. In general, when the sensors indicate the vehicle is leaving the intended line of travel, ESC applies the brake pressure needed at each wheel to bring the vehicle back on track. In some cases, ESC also reduces engine speed. ESC has been found to reduce single-vehicle fatal crash involvement risk by 51 percent and could reduce the risk of rolling over by as much as 72 percent.⁷

Germany's Robert Bosch GmbH was the first supplier to bring ESC to market on the 1995 Mercedes-Benz S-Class in Europe.⁸ The technology made its way to the American market a few years later as optional equipment on luxury cars. By the 2001 model year it was standard on a number of high-selling vehicles and available as an option on many more. Since then automakers have been putting ESC on their vehicles, particularly SUVs, at a steady rate. The systems are marketed under various names, including dynamic stability control, vehicle stability control, dynamic stability and traction control, among others. The percentage of vehicles with this technology has increased tenfold since the 1998 model year. For the 2008 model year, ESC was standard on 63 percent of new passenger vehicle models and optional on 15 percent. ESC was standard on 64 percent of car models, 95 percent of SUV models, and 12 percent of pickup models. NHTSA has issued a standard requiring all passenger vehicles to be equipped with ESC by 2012 model year.

See vehicles equipped with electronic stability control (ESC)

Safety belt use is the most effective way to reduce the risk of injury or death in a rollover. Sixty-six percent of people killed in passenger vehicle rollover crashes in 2007 were unbelted. Without safety belts, occupants in vehicles that roll can be thrown from the vehicle, greatly increasing the risk of serious injury or death.

When occupants are contained in the vehicle during a rollover, the performance of restraint systems and structural components is crucial to preventing injury. Head-protecting side curtain airbags triggered by rollover sensors can prevent the upper body from contacting the ground and also prevent occupants from being ejected from the vehicle. Good safety belt designs with tensioners that remove slack are important to hold occupants in their seats and away from the roof as much as possible. Finally, the roof and other vehicle structures must be strong enough to resist occupant compartment intrusion that can increase the risk of head and neck injury.

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.^9,10,11 Conversely, other researchers maintain that injuries occur when the roof buckles into the occupant compartment and contacts the people inside.^12,13

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.¹⁴ This was the first study to demonstrate the link between roof strength and injury risk.

Even when all vehicles eventually are equipped with ESC, rollover crashes will not be eliminated. NHTSA estimates that 5,000 to 6,000 rollover fatalities per year would still occur in a fleet fully equipped with ESC.¹⁵ ESC can help a driver maintain control in some situations but not all. For example, ESC may not prevent a rollover-initiating impact with another vehicle or with a roadside obstacle, tire failure, or complete loss of traction with the road surface due to weather conditions. Vehicles with ESC still need strong roofs and effective restraint systems to protect occupants in rollover crashes.

Federal Motor Vehicle Safety Standard (FMVSS) No. 216, Roof Crush Resistance, establishes a minimum requirement for roof strength to "reduce deaths and injuries due to the crushing of the roof into the occupant compartment in rollover crashes." This is a quasi-static test in which a rigid plate is pushed into the roof at a slow rate. The roof must be strong enough to prevent the plate from moving 5 inches when pushed at a force equal to 1½ times the weight of the vehicle. The test went into effect in 1973 and has remained essentially unchanged, although modifications were proposed in 2005. No federal standards provide performance requirements for occupant restraint systems in rollover crashes.

NHTSA in 2005 proposed an upgrade to FMVSS 216, which for the first time would regulate the roof strength of many SUVs and pickup trucks by extending coverage to vehicles with gross weight ratings up to 10,000 pounds. The current standard applies only to vehicles with ratings up to 6,000 pounds, which means about 44 percent of the SUV and pickup fleets currently are exempt.¹⁶ The proposal would require that a roof withstand an applied force equal to 2½ times the vehicle's weight while maintaining sufficient headroom for an average size adult male. The current requirement is that the roof withstand an applied force equal to 1½ times the vehicle's weight, with a limit of 5,000 pounds for cars.

The Institute generally supported NHTSA's proposal but noted a "surprising lack of evidence" that the quasi-static test procedure used in the standard had any relationship to protection in real-world rollover crashes. Without knowledge of such a relationship, it is not possible to determine an adequate level of roof strength for the vehicle fleet. Based on the need for real-world crash data, the Institute studied the relationship between roof strength measurements using the FMVSS 216 procedure and the risk of injury or death in rollover crashes.¹⁴ The 2008 study found that strong roofs significantly reduce the risk of injury or death in rollover crashes, and they continue to do so beyond the roof strength level proposed by NHTSA.

The NHTSA proposal will improve the rollover crashworthiness of the fleet, but stronger requirements would save even more lives. NHTSA estimated that 13 to 44 lives per year would be saved by the 2005 proposal that roofs withstand 2½ times the vehicle's weight. However, the Institute's 2008 study suggests that the actual number would be far higher.¹⁴

The Institute's study found that the quasi-static test used in the standard can evaluate roofs in a way that relates to real-world protection. However, it also found that occupants in vehicles that meet the current strength requirements by a narrow margin have elevated injury risks compared to occupants in vehicles with stronger roofs. It is impossible to say exactly how many lives would be saved by strengthening roofs, partly because many new vehicles already have very strong roofs due to other structural improvements. Nevertheless, a standard requiring roof strengths to increase to a level of 3 or 3½ times the vehicle's weight would be expected to save hundreds of lives per year.

The quasi-static regulatory test addresses only the structural aspect of rollover crashworthiness. Dynamic testing offers the possibility of assessing not only the effects of vehicle structure on the risk of injury in a rollover, but also the effects of occupant restraint systems and occupant movements. Dynamic testing also may more accurately reproduce the forces acting on roofs in real-world crashes.

However, because there is a wide range of rollover crashes, it is difficult to identify a single dynamic evaluation that is representative of most rollover crashes. Repeatability has been a problem with dynamic tests, as slight differences from one test to the next can significantly change the outcome. In addition, more research is needed to determine how to use dummies in rollover tests in a way that represents the movements and injury risks of people in real crashes. While certain dynamic tests hold promise, more work is necessary to address these issues.