Drivers of vehicles that do poorly in the Institute's side-impact crash tests are 3 times as likely to die in a real-world left-side crash than drivers of vehicles that perform well, a new analysis finds. The study includes only passenger vehicles with side airbags, demonstrating that airbags, while crucial, are far from the whole story in side crash protection.
"This was our first look at how our ratings correlate with actual crash data since we started side tests in 2003, and the numbers confirm that these are meaningful ratings," says Institute chief research officer David Zuby. "Vehicles with good side ratings provide occupants with far more protection than vehicles that do poorly in our test."
Studies of frontal crashes have shown similar results: Drivers of vehicles with good ratings in the Institute's frontal offset crash tests are much less likely to die in frontal crashes (see "Test performance predicts outcomes in real-world crashes," Feb. 7, 2004, and Status Report special issue: front crash test verifications, March 29, 2006).
Side-impact crashes accounted for 27 percent of passenger vehicle occupant deaths in the United States in 2009. Such crashes can be particularly deadly because the sides of vehicles have relatively little space to absorb energy and shield occupants.
The ultimate goal of the Institute's testing program is to encourage automakers to produce safer vehicles. Knowing that consumers consult the ratings before buying, manufacturers design cars and trucks with the Institute's tests in mind. As a result, 78 percent of current vehicle designs that have been tested have good side ratings, compared with only about a third of vehicles tested in the program's first two years.
Such improvement is important to the extent it predicts performance in a real-world crash. To gauge how well the test does that, the Institute looked at federal data on side crashes from 2000 to 2009. Only crashes involving Institute-rated vehicles with standard side airbags to protect both the head and torso were included in the analysis.
By limiting the study to vehicles with side airbags, the researchers were able to bring other factors such as structure into sharper focus. Previous research has shown the importance of side airbags (see "Surviving side crashes: Side airbags are reducing driver deaths," Oct. 7, 2006), and no vehicle without head-protecting side airbags has ever earned a good rating from the Institute.
Researchers first compared outcomes of left-side crashes according to the Institute's side ratings. They found that drivers of vehicles rated good survived such crashes much more often than drivers of vehicles rated poor. Vehicles rated marginal — one step above poor — at first appeared to do a slightly better job of protecting people in real-world crashes than vehicles rated acceptable.
A deeper analysis helped explain this initially puzzling fact. Each vehicle's rating takes into account injury measures for a crash test dummy in the back seat, as well as one in the driver's seat. In contrast, the analysis of real-world crashes could consider only driver death risk because a federal database used for the study doesn't keep track of all uninjured passengers.
So the researchers recalculated the ratings without the passenger dummy measures. Using these driver-only ratings, and after controlling for age, gender, and vehicle type and weight, a driver of a vehicle rated marginal is 49 percent less likely to die in a left-side crash than a driver of a vehicle rated poor. A driver of a vehicle rated acceptable is 64 percent less likely to die. Drivers of vehicles with good driver-only ratings are 70 percent less likely to die in a driver-side crash compared with those rated poor.
In the Institute test, a vehicle is hit on the driver side by a deformable barrier weighing 3,300 pounds and traveling at 31 mph. The barrier's height and shape are designed like the front of a typical SUV or pickup.
Ratings are based on injury measures recorded on dummies, head protection, and vehicle intrusion during crash tests. In addition to looking at overall driver protection, researchers also looked at these components individually. They found that a vehicle's structure rating was by far the best predictor of fatality risk.
"We knew that our ratings would encourage manufacturers to add head-protecting side airbags, which would save lives," Zuby says. "It's great to see that other aspects of our evaluation, such as encouraging strong side structures, resulted in so much additional protection."
A key difference between the Institute's side crash test and one the government runs is the Institute's SUV-like barrier. The National Highway Traffic Safety Administration uses a lower barrier designed when the majority of vehicles on the road were cars.
Another important distinction is the type of dummy used. Until recently, both dummies used in the government's side barrier test represented average-size men, while the Institute's side test has always used dummies representing small women or 12-year-old children.
The choice of a small female dummy was a first for any consumer information test. The decision was based on the fact that women are more likely than men to suffer serious head injuries in real-world side impacts. Shorter drivers have a greater chance of having their heads come into contact with the front end of the striking vehicle in a left-side crash (see Status Report special issue: side impact crashworthiness, June 28, 2003).
The government recently started using the small female dummy in the back seat for its side barrier test, though a midsize male dummy still is used in front. The government also is using the female dummy for a new side test that involves crashing a vehicle into a pole.