The National Highway Traffic Safety Agency says "pedal misapplication" is the most probable cause for the vast majority of sudden acceleration incidents in which no vehicle malfunction is evident.
The conclusions of the yearlong study by the U.S. safety agency closely parallels findings by Transport Canada, which last month closed its probe after determining sudden acceleration was due to driver error.However, NHTSA declined to characterize the cause of sudden acceleration as driver error, saying that term could imply "carelessness or willfulness in failing to operate a car properly."
NHTSA defined sudden acceleration as occurring when a car allegedly accelerates to full power from standstill after being placed in gear, and that stepping on the brakes does not stop the vehicle.
Pedal misapplication occurs when a driver unwittingly steps on the accelerator instead of the brake pedal, it said.
NHTSA also said that poor pedal design could play a role in sudden acceleration but was unwilling to recommend changes because additional studies are needed to determine if those changes would create other undesired effects, like increased braking response time.
NHTSA, which has conducted more than 100 investigations of unwanted power surges dating back to the late 1970s covering 20 automakers, said its findings "may be difficult to accept" by drivers who believe they were applying the brake while experiencing sudden acceleration.
However it said extensive testing failed to turn up any defects in cruise control systems, electronic idle control systems, brakes, or transmissions which would produce sufficient power to create what it calls sudden acceleration.
It also found that in all cases, a vehicle's braking power was sufficient to stop a car, even under full acceleration.
The agency also noted that not all reports of unwanted acceleration could be called sudden acceleration. It said other problems, like high idle speed, have been investigated routinely, and that recalls have been made to remedy those problems.
West German carmaker Audi, which bore the brunt of negative publicity over unintended acceleration in late 1986 and saw its sales plunge dramatically, applauded the report.
"We did not believe, certainly after the findings of the studies in Japan and Canada, that the agency could have come to any other conclusion," said Joseph Bennett, a spokesman for Audi of America in Troy, Mich.
"We've waited a long time for this," he said. "It takes the wind out of the sails of those who preached demons in cars."
Bennett said Audi would probably not sue CBS, which in November 1986 aired a report on its "60 Minutes" television program unfavorably portraying Audi's cars as prone to sudden acceleration.
"The odds are 90 percent that we will not sue CBS," he said. "It's a no-win situation which would only prolong the issue and keep it open in mind of the public for another two or three years. We want to get on with the business of selling cars."
In January 1987, Audi recalled 250,000 Audi 5000 cars with automatic transmissions from the 1978-86 model years to install an automatic shift lock, which prevents the car from being placed in gear unless the brake pedal is applied.
Several other domestic and import carmakers have since adopted the device, which both NHTSA and Transport Canada said have proven their worth as a deterrent to sudden acceleration.