NEW YORK – A Purdue University research team that studied five years of motor vehicle accidents in Washington State concludes antilock brakes and airbags don't minimize accidents or injuries because those systems may encourage more aggressive driving.
Fred Mannering, a Purdue professor of civil engineering, led the study. The results, which are bound to be controversial with auto makers and safety experts, say the innovations designed to improve safety also make drivers less vigilant.
Mannering calls this behavior “offset hypotheses.”
He notes that when ABS debuted, insurance companies said accident rates actually increased for vehicles equipped with the systems that prevent wheels from locking up while braking.
"We decided to see whether the offset hypothesis could explain this phenomenon," Mannering says.
His team analyzed accident data from Washington over a 5-year period beginning in 1992.
"We used that time period because that's when airbags started getting introduced very rapidly, and we wanted to track the same drivers over that timeframe to see whether the new safety features reduced their accident and injury rate," he says.
"Our findings suggest the offset hypothesis is occurring and that it is sufficient to counter the modest technological benefits of airbags and antilock brakes," Mannering concludes.
Using accident data and actual driving records, the researchers calculated the probability of being involved in an accident for drivers of different ages and demographics. Mannering says the research models indicate ABS and airbags do not affect the probability of an accident or suffering an injury.
“Our findings actually reflect offsetting behavior," Mannering says. The latest 2005 National Highway Traffic Safety Admin. data reporting fatalities per mile driven have increased, he notes.
"This adds some aggregate validation of our findings," he says.
The study examined 1,307 drivers involved in 614 accidents, 16 of which resulted in injuries, from 1992-1996. During the period, 271 of the drivers studied switched from a vehicle without an airbag to one that had some type of airbag or supplemental restraint system.
All but one of the drivers also switched to a vehicle that had ABS.
Meanwhile, drivers who continued to use vehicles without ABS and airbags served as a control group. This created a complete matrix of drivers and vehicles with and without safety features.
"There are no indications Washington State drivers are unrepresentative of U.S. drivers in general," Mannering adds.
He also cites his own experience to back up the study's conclusions. When he drives his vintage MG roadster, he doesn't tailgate or accelerate quickly on wet roads because the car doesn't have ABS or airbags.
Mannering says the offset hypothesis will continue as an issue as other active and passive safety systems are introduced, such as electronic stability control systems that purportedly help to prevent rollover accidents.
Auto makers and safety experts asked to comment on the Purdue study’s conclusions are wholly unconvinced.
"The sample size is too small to make meaningful conclusions," aCorp. spokesman says.
A NHTSA spokesman says, "That (conclusion) runs counter to our own research."
Russ Rader, a spokesman for the Insurance Institute for Highway Safety, terms the findings "ridiculous."
"Airbags are very effective in reducing injuries," he says. Rader also questions whether systems that don't generate direct feedback to the driver cause more aggressive driving. On the other hand, he says an improvement such as better headlights that provide direct feedback may cause some drivers to go faster.
A Volvo Car Corp. spokesman says his company's studies find a "significant injury reduction with airbags.” However, he says Volvo does not have data that can correlate whether drivers become more aggressive in vehicles equipped with airbags.
"I don't think so, but that is my personal opinion," he says.
Volvo is famous for in-depth safety research, and the Volvo spokesman counters with a company study that analyzed accidents involving 3,672 Volvos, in which 2,734 had no airbags, while 838 did.
That study found a 62% reduction of 3+ Abbreviated Injury Scale head injuries for airbag-equipped Volvos, which he terms "significant."
The AIS is a scoring system first introduced in 1969 to measure anatomical injuries and has been updated several times, the last being in 1990. The AIS is monitored by a committee of the Association for the Advancement of Automotive Medicine.