Roll over in your SUV, and you want the roof to hold up so you're protected from injury, including harm from the roof caving in on you. Every passenger vehicle meets federal requirements for roof strength, measured in a test, and some exceed the requirements by substantial amounts. The question has been whether stronger roofs actually reduce injury risk in real-world rollover crashes. Some studies have concluded that the strength of a vehicle's roof has little or no effect on the likelihood of injury, but a new Institute study indicates that roof strength definitely influences injury risk.
Researchers tested SUVs in a procedure similar to what the government requires automakers to conduct to assess roof strength and then related the findings to the real-world death and injury experience of the same SUVs in single-vehicle rollover crashes. The main finding is that injury risk went down as roof strength increased.
Injury rates vary considerably among vehicles in rollovers, and there's still a lot researchers don't know about these crashes. For example, is injury risk primarily from the sudden crushing of the roof? Is it because people crash into the roof when the vehicle is upside down? Or does the main risk come from full or partial ejection of occupants when vehicle doors and windows break open during rollover crashes?
"We don't know just what happens to people in these crashes or what the injury mechanisms are. What we do know from the new study is that strengthening a vehicle's roof reduces injury risk, and reduces it a lot," says Institute president Adrian Lund.
Extent of the rollover problem
About 35 percent of all occupant deaths occur in crashes in which vehicles roll over. This problem is worse in some kinds of vehicles than others. About 25 percent of occupant deaths in crashes of cars and minivans involve rolling over. The proportion jumps to 59 percent in SUVs.
Of course, the best way to prevent these deaths is to keep vehicles from rolling over in the first place, and electronic stability control is helping. It's reducing rollover crashes, especially fatal single-vehicle ones, by significant percentages.
"But until these crashes are reduced to zero, roof strength will remain an important aspect of occupant protection," Lund points out.
What the U.S. government requires
Federal Motor Vehicle Safety Standard 216 establishes minimum roof strength for passenger vehicles. Compliance testing involves the application of a metal plate to one side of a roof at a constant speed. The roof must withstand a force of 1.5 times the weight of the vehicle before reaching 5 inches of crush. Thus, a vehicle weighing 4,000 pounds has to withstand 6,000 pounds of force while sustaining 5 or fewer inches of crush.
This requirement, in effect since 1973 for cars and 1994 for other passenger vehicles, is in the process of an upgrade. One of the government's main proposals, issued in 2005, is to boost the specified force to 2.5 times vehicle weight (see "Plan to boost strength of vehicle roofs is worthwhile but not enough," Jan. 28, 2006). Last month the government indicated it may consider further altering the standard by testing both sides of vehicle roofs instead of applying the force to one side only. When the changes were proposed in 2005, the Institute voiced general support but noted the "surprising lack of evidence" connecting the requirements of the standard to real-world rollover crash outcomes.
The new Institute study provides some missing evidence. Across 11 SUVs at 3 different degrees of roof crush — 2, 5, and 10 inches — the strongest roofs are associated with injury risks 39 to 57 percent lower than the weakest roofs. Peak roof strength at 2 and 10 inches of crush is more highly related to injury risk than at 5 inches. Based on these findings, the researchers estimate that if the roofs on every SUV the Institute tested were as strong as the strongest one, about 212 of the 668 deaths that occurred in these SUVs in 2006 would have been prevented.
"These are big risk reductions, bigger than what the government or anybody else has established," Lund says.
The researchers estimate that a 1-unit increase in peak strength-to-weight ratio — for example, from 1.5 times vehicle weight to 2.5, as the government proposed in 2005 — reduces the risk of serious and fatal injury in a rollover crash by 28 percent. Increasing roof strength requirements beyond 2.5 times vehicle weight would reduce injury risk even further.
Strong vs. weak
The difference in roof strength was obvious when the Nissan Xterra (left) and Ford Explorer, both 2000 models, were subjected to a crushing force of up to 10,000 pounds. The Xterra's roof crushed about 2 inches, and damage is hardly visible except for a cracked windshield. Meanwhile the Explorer's roof crushed 10 inches, caving far into the occupant compartment even before reaching 10,000 pounds of force.
New findings vs. previous studies
Before the Institute's study, there was no conclusive evidence about the specific contribution of a vehicle's roof strength to occupant protection. The government estimated that proposed changes in federal roof strength requirements would save 13 to 44 lives per year.
"This was based on assumptions that were conservative in the extreme," Lund explains. "For example, the government assumed zero benefit for unbelted occupants. We don't know exactly what the benefit of an upgraded roof strength standard would be for these occupants, but it would be likely to exceed zero."
Meanwhile two studies sponsored by automakers, one in 1995 and the other a decade later, found no relationship at all between roof strength and injury risk in rollovers. Findings of the first study prompted General Motors to tell The Detroit News in 2002, "Good science, long established and well reviewed in the technical literature, has conclusively demonstrated that there is no relationship between roof strength and the likelihood of occupant injury given a rollover." Four years later, Ford told the government that "substantial and compelling real-world crash data and laboratory testing have confirmed that simply increasing roof strength will not measurably reduce the risk of injury or death to vehicle occupants in rollovers."
A main problem with these studies is that they included all kinds of passenger vehicles with their substantial differences in driver demographics, rollover propensity, and other factors that confound the results. In contrast, the Institute's new study focuses on one kind of vehicle, midsize 4-door SUVs, and tightly controls for other factors that could confound the results. While the findings are about a limited number of SUVs, the researchers conclude that the overall finding of reduced injury risk as roof strength increases would hold for other kinds of vehicles, although the magnitude of the injury rate reduction may differ among vehicle groups.
Lund adds that the findings "prompt us to expand our research on roof strength with an eye to supplying consumers with comparisons of how well vehicles protect people in rollover crashes. A dynamic test with dummies instrumented to measure injury risk in rollovers would be desirable, but there's a sticking point. First we have to understand how the movement of dummies in controlled tests could reflect how real people move in real-world rollovers. Meanwhile, simpler roof strength measurements could provide useful consumer information."
Details of the study
The Institute study is a two-part analysis involving vehicle testing and examination of the outcomes of real-world rollover crashes. Eleven midsize 4-door SUVs were subjected to a test similar to the one run by automakers to comply with federal roof strength requirements (the manufacturers' own test data aren't public information). The 11 SUVs exclude features that might affect injury rates in rollovers such as side curtain airbags and electronic stability control. To assess the range of roof strength among the SUVs, researchers applied force to the roofs until crush reached 10 inches, measuring the peak force required for 2 inches of crush, 5 inches, and 10 inches. Because crush in a rollover can depend on vehicle weight as well as roof strength, the researchers calculated strength-to-weight ratios for each degree of crush. They also measured the amount of energy absorbed by each roof at each degree of crush and, again taking vehicle weight into account, the height from which the vehicle would have to be dropped to produce equivalent energy absorption.
By almost any of these measures, the strongest roof was on the 2000-04 Nissan Xterra while one of the weakest was on the 1999-2004 Jeep Grand Cherokee. Within 5 inches of crush, the Jeep withstood a force as high as 6,560 pounds, which amounts to 1.64 times the weight of the 4-wheel-drive version and 1.72 times the weight of the 2-wheel-drive. The corresponding figure for the Xterra was 11,996 pounds, or 2.93 times the weight of the 4-wheel-drive and 3.16 times the 2-wheel-drive.
Having established the range of roof strength among the SUVs, the researchers studied almost 23,000 real-world rollovers of the same 11 SUVs during 1997-2005. This information was collected from 12 states with sufficient data on police-reported crashes to comply with study criteria.
Logistic regression was used to assess the effect of roof strength on the likelihood of driver injury in the rollover crashes of the 11 SUVs. The regression controlled for state-to-state differences in methods of reporting crashes, terrain, urbanization, etc.; vehicle stability; and driver age. Results indicate the various injury risks given the various SUV roof strengths.
The drivers of these SUVs died when their vehicles overturned. It's a big problem — more than half of all occupant deaths in SUVs occur in rollover crashes. New research indicates that strengthening vehicle roofs would reduce this problem. If the roof on every SUV were as strong as the best one the Institute tested, injury risk in rollover crashes could be reduced 39 to 57 percent. These are very big risk reductions, bigger than the federal government or anybody else has established.
"No matter what measurement of roof strength we used or whether we measured at 2 or 5 or 10 inches of crush, we found a consistent relationship between roof strength and injury risk," Lund points out.
The relationship between roof strength-to-weight ratio and injury risk was stronger at 2 inches than at 5 inches, the crush specified for testing under the federal standard (the government doesn't require automakers to assess roof strength at 2 or 10 inches). At 5 inches, the predicted injury risk for people in SUVs with roof strength-to-weight ratios as strong as the Xterra's would be 39 percent lower than for people in vehicles with roof strength like the Grand Cherokee's. At 2 inches of crush, the difference in predicted injury risk is 51 percent.
The 11 SUV designs in the study include the 1996-2004 Chevrolet Blazer, 2002-05 Chevrolet TrailBlazer, 1998-2003 Dodge Durango, 1996-2001 Ford Explorer, 2002-04 Ford Explorer, 1996-98 Jeep Grand Cherokee, 1999-2004 Jeep Grand Cherokee, 2002-05 Jeep Liberty, 1997-2004 Mitsubishi Montero Sport, 2000-04 Nissan Xterra, and 1996-2000 Toyota 4Runner.
Importance of ESC and side airbags
Vehicle roof strength is crucial to occupant protection in rollover crashes. Other features are effective, too, in both preventing such crashes in the first place and protecting people when their vehicles do roll. Researchers estimate that electronic stability control, or ESC, reduces the risk of a fatal single-vehicle rollover by about 69 percent for all passenger vehicles and 72 percent for SUVs in particular. Side curtain airbags are expected to reduce the risk of death in the rollovers that still occur.
"These technologies are essential," Institute president Adrian Lund points out, "but electronic stability control doesn't completely eliminate rollover crashes, and side airbags aren't the only protection occupants need if they do roll over. This is why we have to pay attention to the roof. If a vehicle's roof is strong enough to absorb the energy of a rollover without caving in on its occupants, injury risk goes down."
Electronic stability control monitors vehicle response to driver steering and applies the brakes on individual wheels to maintain the path that's indicated by the steering wheel position (see "ESC reduces multiple-vehicle crashes as well as single-vehicle ones," June 13, 2006). This technology is standard or optional on about two-thirds of all current passenger vehicle models. Side airbags are standard or optional in about 80 percent.