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Status Report, Vol. 35, No. 6 | SPECIAL ISSUE: NEW FEDERAL AIRBAG RULE | June 17, 2000 Subscribe

Science prevails in decision to test unbelted dummies at 25 mph

Industry, government, and safety groups agreed on virtually all of the requirements proposed in the complex airbag rulemaking. The lone issue that sparked intense debate was whether the maximum speed of rigid-barrier crash tests with unbelted dummies should be 25 or 30 mph.

After months of consideration, the National Highway Traffic Safety Administration (NHTSA) went with the 25 mph test speed. The agency says its "initial inclination" was toward the higher speed but later concluded it "would not be in the best interest of safety."

Pros and cons

The main claims before the agency from those supporting the 30 mph test speed were that half of all frontal crash deaths occur in impacts with velocity changes greater than this, and 30 mph testing wouldn't prompt a return to more aggressive airbags because almost all vehicles now meet such requirements with adult male dummies. Another claim was that the lower speed of 25 mph would fail to encourage advanced airbags.

On the other hand, advocates of 25 mph testing told NHTSA the higher speed would prompt a return to overly aggressive airbags, which can cause deaths in low- and high-speed crashes. Depowered airbags are providing effective protection in crashes at higher speeds while reducing harm to people in low-speed crashes. No convincing evidence was presented to indicate the lower test speed would decrease protection for people without belts.

Problems with 30 mph tests

"We firmly believe NHTSA made the right decision, going with the 25 mph speed," Institute president Brian O'Neill says. "While it may seem counterintuitive that higher test speeds don't necessarily lead to better protection in real-world crashes at higher speeds, the available evidence indicates that rigid-barrier tests conducted at 25 mph with unbelted dummies will result in better protection for unbelted people than if the same tests were run at 30 mph."

At issue is the airbag energy necessary to protect people, O'Neill further explains. "Higher test speeds require more airbag energy, and when occupants, usually unbelted, are out of position in their vehicles, airbag energy can be harmful rather than protective. So the key is whether there's any convincing evidence that today's depowered airbags have insufficient energy to protect people in crashes at higher speeds. And the answer, based on our review of cases from federal data files involving driver deaths in frontal crashes, is that none of the 'failures' — that is, cases when airbags didn't prevent deaths — occurred because of insufficient inflation energy. Instead, the deaths occurred because of major intrusion into the occupant compartments, or the occupants were ejected, or because of the inflation energy of the airbags themselves."

Neither intrusion nor ejection are addressed in the current airbag rule. However, airbag inflation energy is driven by the test speed choice. "Higher speeds mean greater inflation energy, which in turn increases the risks to unbelted people who often are out of position when their airbags begin to inflate. These increased risks aren't offset by any increase in the protection airbags afford. This is why the Institute opposed the 30 mph test speed. Adopting it would degrade rather than enhance overall protection for unbelted people," O'Neill says.

He further notes that "it's misleading to say more than half of all frontal crash deaths occur in impacts with velocity changes greater than 30 mph and to imply this is relevant to the choice of a barrier test speed. The velocity changes computed for real-world crashes cannot be directly related to barrier test speeds because, for any velocity change, virtually all real crashes occur over a longer time than rigid-barrier tests. This means the occupant compartment decelerations in real-world crashes are much lower than in rigid-barrier tests, which have very short durations — no real-world crash could occur over a shorter time. This fact has important implications for airbag performance. Short-duration high-speed barrier tests require airbag inflators to produce more gas faster, whereas many real-world crashes require less gas over a longer time."

Interim decision

NHTSA ultimately didn't agree with all of the arguments against 30 mph testing with unbelted dummies. But the many uncertainties associated with the higher speed were enough to decide the issue at least for the short term. The agency says it drew "no final conclusion about the appropriateness of that test speed in the longer run." Thus the 25 mph requirement has been issued as an interim final rule. Investigations of the real-world performance of depowered and advanced airbags are planned for possible reconsideration of the test speed.

Confirmed deaths from inflating airbags

NHTSA says technological changes to airbags "have already occurred that are reducing the number of persons killed by airbags."

Graph images
New airbag rule protects unbelted people

Airbags that meet a new government standard will be safer for out-of-position occupants even as they continue to provide protection in serious crashes.

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