Q&As: Rollover and roof strength

April 2010

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. Vehicles roll over in less than 3 percent of all crashes1, but these crashes account for more than a third of passenger vehicle occupant deaths. In 2008, 25,428 passenger vehicle occupants died in crashes of all kinds. Of those, 9,023 died in crashes where their vehicle rolled over.

2 What causes a rollover?

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.

The forces in a multiple-vehicle crash may also cause a vehicle to roll. For example, a vehicle struck in the side may be pushed over by the striking 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.

3 Are rollovers more common for SUVs than for other vehicles?

Rollovers are much more common for SUVs and pickups than for cars, and more common for SUVs than for pickups. In 2008, 58 percent of SUV occupants killed in crashes were in vehicles that rolled over. In comparison, 47 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.2 Lower belt use among pickup occupants4 means they are more likely to be seriously or fatally injured when rollovers occur.

4 How has the number of rollover fatalities changed over time?

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 16 deaths per million in 2008. The percentage of fatalities in rollover crashes for each vehicle type has remained relatively unchanged.

5 What is being done to reduce the occurrence of rollovers?

Manufacturers are creating more stable vehicle designs. The static stability factor (SSF), a measurement of a vehicle's geometrical ability to resist rollover, increased an average of 6 percent for new SUVs between 1998 and 2003, after remaining constant for 20 years.5 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.

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.

6 What are the rollover resistance ratings assigned by NHTSA?

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. 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

7 How does electronic stability control work?

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 75 percent for SUVs and by 72 percent for cars.7

Q&A: Electronic stability control

8 Are electronic stability control systems widely available?

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 new passenger vehicle models with standard ESC has increased from 9 percent in the 2000 model year to 85 percent in the 2010 model year. 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)

9 What can be done to reduce the likelihood and severity of injuries when rollovers occur?

Safety belt use is one of the most effective ways to reduce the risk of injury or death in a rollover.9 Sixty-seven percent of people killed in passenger vehicle rollover crashes in 2008 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 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.

10 Does roof strength really matter in rollovers?

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 crushes10,11,12 Conversely, other researchers maintain that injuries occur when the roof buckles into the occupant compartment and contacts the people inside.13,14

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 and this was confirmed by a second IIHS study using a different set of vehicles.15,16 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 study 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 are common causes of ejection.17

11 Will roof strength matter in the future if all vehicles have ESC?

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.18 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.

12 What is the current federal standard for rollover crashworthiness? What changes have been announced?

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." In this test, a rigid plate is pushed into one side of the roof at a constant speed. 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 remained essentially unchanged until an updated rule was announced in 2009.

The new rule will require that a roof withstand an applied force equal to 3 times the vehicle's weight while maintaining sufficient headroom for an average size adult male. While both sides of a vehicle's roof were required to meet the former standard, only one side was tested on any given vehicle. The new rule requires a second test of the same vehicle's roof on the opposite side. Beginning with 2013 model vehicles, a certain percentage of each manufacturer's fleet must meet the new standard. Phase-in will be complete for the 2017 model year.

The updated FMVSS 216 will regulate the roof strength of many SUVs and pickup trucks by extending coverage to vehicles with gross weight ratings (GVWRs) up to 10,000 pounds. (GVWR is the weight of the vehicle plus the maximum load of passengers and cargo specified by the manufacturer.) In the past, the standard applied only to vehicles with GVWRs up to 6,000 pounds, which meant about 44 percent of the SUV and pickup fleets were exempt.19 While roof strength regulation now applies to these vehicles, they are not subject to the same force requirements. Instead of a force equal to 3 times the vehicle's weight, vehicles with GVWRs over 6,000 pounds are subject to a force equal to 1½ times their weight.

13 Why does IIHS rate roof strength?

The Institute generally supported NHTSA's initial proposal in 2005 but noted a "surprising lack of evidence" that the federal test procedure was in any way related to protection in real-world rollovers. 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 rollovers.15 The 2008 study found that strong roofs significantly reduce the risk of injury or death in rollover crashes, and this was confirmed by a second IIHS study using a different set of vehicles with strengths up to 4 times their weight.16 Both studies found that strong roofs reduce the risk of ejection and the risk of injury for non-ejected occupants.

Based on these findings, the lack of consumer information on roof strength, and continued delays in the rulemaking process, IIHS began rating new vehicles for roof strength in March 2009. To qualify for a "good" rating, a vehicle must be able to withstand a force equal to 4 times its weight prior to 5 inches of crush. NHTSA's final rule, announced in May 2009, eventually will force vehicles with GVWRs 6,000 pounds or less to have roofs nearly as strong as that required for a good rating. However, the IIHS rating program allows consumers to identify which vehicles already have stronger roofs. It also may encourage manufacturers to make roofs even stronger than the new regulation will require, especially for heavier vehicles subject to the less strict requirements. Finally, the rating program will allow IIHS researchers to determine whether additional benefits exist for roofs stronger than those studied in the past.

14 Should dynamic tests of roof strength in rollovers be conducted?

The current federal test addresses only the structural aspect of rollover crashworthiness. No federal standards currently provide performance requirements for occupant restraint systems in rollover crashes. However, a 2009 proposal would require restraint systems such as side curtain airbags to reduce ejection risk in rollovers. The intent of the standard would be to ensure such systems were sufficiently large and strong enough to prevent ejection through the side windows. While this could be a positive step for further reducing rollover injuries, the standard would not test whether the restraint systems deploy properly in actual rollover crash conditions.

Dynamic testing that simulates an actual rollover offers the possibility of evaluating the effects of vehicle structure, occupant restraint systems and occupant movements in an overall crashworthiness assessment. Dynamic testing also may more accurately reproduce the forces acting on roofs in real-world crashes than the test method used in FMVSS 216 and in the IIHS roof strength rating program.

However, because there is a wide range of rollover crashes, it is difficult to identify a single dynamic evaluation that is representative of most rollovers. 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.

References

¹National Highway Traffic Safety Administration. 2009. Traffic safety facts, 2008. Report no. DOT HS-811-170. Washington, DC: US Department of Transportation.

²Deutermann, W. 2002. Characteristics of fatal rollover crashes. Report no. DOT HS-809-438. Washington, DC: National Highway Traffic Safety Administration.

³Robertson, L.S. and Kelley, A.B. 1988. Static stability as a predictor of overturn in fatal motor vehicle crashes. Journal of Trauma 29:313-19.

⁴Pickrell, T.M.; and Ye, T.J. 2009. Seatbelt use in 2009 – overall results. Report no. DOT HS-811-100. Washington, DC: National Highway Traffic Safety Administration.

⁵Walz, M.C. 2005. Trends in the static stability factor of passenger cars, light trucks, and vans. Report no. DOT HS-809-868. Washington, DC: National Highway Traffic Safety Administration.

⁶Insurance Institute for Highway Safety. 2004. New rollover ratings reflect dynamic testing, but tests don't affect ratings very much. Status Report 39(3):6-7. Arlington, VA.

⁷Insurance Institute for Highway Safety. 2009. Unpublished data analysis.

⁸Robert Bosch GmbH. 2005. Bosch reaches 10 year production milestone for electronic stability control. Press release, February 10. Gerlingen, Germany.

⁹Viano, D.C. and Parenteau, C.S. 2004. Rollover crash sensing and safety overview. SAE Technical Paper Series 2004-01-0342. Warrendale, PA: Society of Automotive Engineers.

¹⁰Bahling, G.S.; Bundorf, R.T.; Kaspzyk, G.S.; Moffatt, E.A.; Orlowski, K.F.; and Stocke, J.E. 1990. Rollover and drop tests – the influence of roof strength on injury mechanics using belted dummies. SAE Technical Paper Series 902314. Warrendale, PA: Society of Automotive Engineers.

¹¹James, M.B.; Nordhagen, R.P.; Schneider, D.C.; and Koh, S.W. 2007. Occupant injury in rollover crashes: a reexamination of Malibu II. SAE Technical Paper Series 2007-01-0369. Warrendale, PA: Society of Automotive Engineers.

¹²Orlowski, K.F.; Bundorf, R.T.; and Moffatt, E.A. 1985. Rollover crash tests-the influence of roof strength on injury mechanics. SAE Technical Paper Series 851734. Warrendale, PA: Society of Automotive Engineers.

¹³Friedman, D. and Nash, C.E. 2001. Advanced roof design for rollover protection. Proceedings of the 17th International Technical Conference on the Enhanced Safety of Vehicles. Paper no. 01-S12-W-94. Washington, DC: National Highway Traffic Safety Administration.

¹⁴Rechnitzer, G.; Lane, J.; McIntosh, A.S.; and Scott, G. 1998. Serious neck injuries in rollovers – is roof crush a factor? International Journal of Crashworthiness 3:286-94.

¹⁵Brumbelow, M.L.; Teoh, E.R.; Zuby, D.S.; and McCartt, A.T. 2009. Roof strength and injury risk in rollover crashes. Traffic Injury Prevention 10:252-65.

¹⁶Brumbelow, M.L. and Teoh, E.R. 2009. Roof strength and injury risk in rollover crashes of passenger cars. Traffic Injury Prevention 10:584-92.

¹⁷Digges, K.H.; Malliaris, A.C.; and DeBlois, H.J. 1994. Opportunities for casualty reduction in rollover crashes. Proc. 14th International Technical Conference on the Enhanced Safety of Vehicles. Paper no. 94-S5-O-11. Washington, DC: National Highway Traffic Safety Administration.

¹⁸Office of the Federal Register. 2006. National Highway Traffic Safety Administration – Notice of proposed rulemaking. Docket no. NHTSA-2006-25801; 49 CFR Parts 571 and 585 – Federal Motor Vehicle Safety Standards, Electronic stability control systems. Federal Register, vol. 71, no. 180, pp. 54712-53. Washington, DC: National Archives and Records Administration.

¹⁹Office of the Federal Register. 2005. National Highway Traffic Safety Administration – Notice of proposed rulemaking. Docket no. NHTSA-2005-22143. 49 CFR Part 571 – Federal Motor Vehicle Safety Standards, Roof crush resistance. Federal Register, vol. 70, no. 162, pp. 49223-48. Washington, DC: National Archives and Records Administration.