"NHTSA dropped the ball," says Clarence Ditlow, the center's executive director. "They had no data or procedures for the test to back up the conclusion that it wasn't electronic controls."
The mounting skepticism spells more trouble for the world's largest automaker as it struggles to stamp out complaints about unintended acceleration linked to scores of accidents, injuries and deaths in Toyota and Lexus vehicles.
Since last fall, the carmaker has recalled 8.1 million vehicles to address unintended acceleration -- more than it sold globally last year. Meanwhile, it has embarked on an unprecedented cessation of sales and production of some of the affected vehicles in the United States.
And worse could still be around the corner.
"We're not finished with Toyota," Transportation Secretary Ray LaHood said Tuesday, Feb. 2.
That same day, the Transportation Department said it would revisit whether faulty electronic controls are linked to unwanted acceleration in Toyotas as well as other brands. NHTSA said the agency will meet with manufacturers, suppliers and independent experts to get a better understanding of vehicles' electronic throttle controls -- which are computerized gas-pedal systems.
Suspicion of Toyota's electronic control system stretches back to the first investigations of unintended acceleration in the early 2000s. Critics argue that Toyota's proposed fixes -- one addressing floor-mat interference and the other sticking pedals -- fail to cover all the cases.
"Neither floor mats nor sticking accelerator pedals explain many, many incidents," the auto safety consultancy Safety Research & Strategies Inc. says on its Safety Record Blog.
Toyota has embarked on two massive recalls to fix the acceleration problem.
The first recall, announced last fall and expanded in January, targets floor mats in the United States and Canada that could jam the gas pedal into an open-throttle position.
The second recall, announced last month, targets vehicles on five continents and aims to fix sticky accelerators.
Toyota is adamant it knows what's wrong.
"We've studied the events of unintended acceleration, and we're quite clear that it's come down to two different issues," Jim Lentz, president of Toyota Motor Sales U.S.A. Inc., told Matt Lauer last week on NBC's "Today Show." "Between those two things, this will be under control."
In the interview, Lentz ruled out any glitch in the electronic control systems.
Yet questions persist. By Toyota's own admission, it has not tracked the pedal problem to any accidents. And there are other acceleration crashes where the floor mats can be ruled out.
The day after Christmas, for instance, a 2008 Toyota Avalon sped off the road and landed upside down in a Texas pond, killing four people. The cause of that crash is still undetermined. But police have ruled out floor mats, which were found in the trunk.
During the 2007 investigation of unintended acceleration in the Lexus ES 350, NHTSA surveyed 1,986 owners of the car. Of the 600 responses received, 59 said they had experienced unwanted acceleration. But only 35 of those said they were using the all-weather floor mats that eventually were blamed for the surging cars and recalled.
Hiroyuki Yokoyama, Toyota's managing officer in charge of quality, dismisses suggestions of glitches in the electronic controls.
Toyota's system has dual sensors backstopping each other in monitoring the accelerator pedal's position, along with two more sensors double-checking the throttle position.
Meanwhile, a control computer actuates the throttle, and a monitoring computer surveys all the computer signals in the circuit. If any abnormal signals are detected, the engine is immediately returned to idle, Yokoyama said.
Furthermore, Toyota says the throttle control is reliable under extreme conditions of electromagnetic waves, temperature and vibration.