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Status Report, Vol. 48, No. 7 | September 27, 2013 Subscribe

ESC, strong roofs reduce but don't eliminate all rollover injuries

The odds of dying in a rollover crash have fallen in recent years, thanks to safety improvements such as electronic stability control (ESC), side curtain airbags and stronger roofs. As more and more vehicles on the road are equipped with these features, rollover crashes and deaths can be expected to fall even further.

As important as these three features are, they won't make rollover injuries and deaths disappear entirely. To help determine the next steps in improving rollover crashworthiness, IIHS researchers have been analyzing rollovers of vehicles that had some or all of these features.

"As the number of rollover crashes that result in injuries decreases, the characteristics of the remaining ones are changing," says IIHS senior research engineer Matthew Brumbelow. "If we want to cut rollover deaths and injuries even further, we need to look at how these remaining crashes are different."

Vehicles roll in just 2 percent of crashes, but these crashes account for more than a third of passenger vehicle occupant deaths. In recent years, fatal rollovers have been rapidly decreasing (see "Dying in a crash," June 9, 2011). The rate of rollover driver deaths per million registered vehicles 1-3 years old has fallen from 27 in 2000 to 8 in 2011, much faster than the rate of other crash deaths. Some of the recent decline reflects the economic downturn, but key reasons for the rollover drop in particular are more stable SUV designs and the increasing prevalence of ESC, which prevents the sideways skidding that can lead to rollovers. ESC has been required on all new passenger vehicles since the 2012 model year.

Meanwhile, airbags and strong roofs have increased the odds of surviving a rollover. In many vehicles, head-protecting side airbags are linked to rollover sensors, causing them to deploy when the vehicle flips, even if there isn' t an initial side impact. These help prevent occupants from being ejected and protect them from contact with the ground or vehicle interior. Stronger roofs also prevent ejection and injury in a rollover, as occupant space is better maintained (see "New ejection rule may spur changes in side airbags," April 26, 2011, and Status Report special issue: roof strength, March 24, 2009).

To find out what kind of rollovers remain, the Institute turned to a federal database containing a sample of crashes and looked for rollovers that occurred from 2003 through 2011. Researchers looked for rollovers of at least two quarter turns involving vehicles with good IIHS roof-strength ratings and found 61 cases to examine. For a good rating, a vehicle's roof must resist a force of 4 times the vehicle's weight before reaching 5 inches of crush.

Since the database doesn't indicate whether a specific vehicle has ESC if it was optional on that model, researchers compared vehicles known to have standard ESC with ones that had no ESC or had it only as an option. In nearly all of the 19 crashes of models with standard ESC, the rollovers were preceded by an impact. In other words, they were pushed over by another vehicle or they flipped after hitting an object. In contrast, 37 percent of the rollovers of vehicles without standard ESC weren't preceded by an impact, meaning in all likelihood they lost stability during a maneuver.

"Rollovers of vehicles with ESC tend to be more complex crashes," Brumbelow says. "If the rollover is preceded by an impact, then the side airbags may have already deployed by the time the vehicle flips. Safety belts may have more slack, and the occupants may be out of position. That makes it a little more complicated to ensure there is good protection during the roll."

There always have been rollovers caused by an initial impact, but since it is more difficult for ESC to address them, they will account for a larger proportion of the remaining rollovers as ESC becomes more common. Although they are more complex, rollovers of vehicles with ESC tend to be less severe. Only 9 percent of the crashes of models with standard ESC involved six or more quarter turns. In contrast, 41 percent of the rollovers of vehicles without standard ESC did.

The rate of serious injury in rollovers of vehicles with good IIHS roof-strength ratings was 5 percent. Vehicles without good ratings, which weren' t part of the main analysis, were found in a separate calculation to have a 14 percent injury rate.

Among the strong-roof group, there were no serious injuries in any of the 14 vehicles that had ESC and in which both side curtain airbags deployed.

"In this small sample of crashes, the combination of ESC, side curtain airbags and good roof strength seemed to work well to prevent rollover injuries," Brumbelow says. "We' ll continue to study the issue as data on more crashes become available, but it' s already apparent the effects of these technologies need to be taken into account when developing countermeasures for the remaining rollover problem. For example, rollover crash tests should be developed to represent the complexity of these crashes."

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