A driver climbs in, starts the engine and moves a foot toward the brake. The car lunges. The harder the driver “brakes,” the faster it goes, until the episode ends in a crash. This is what drivers describe as sudden unintended acceleration, and for decades investigators have argued over what causes it.
The term itself dates to the 1980s. A surge of reports then covered almost every brand of car, according to a peer-reviewed analysis of North Carolina police accident reports. Three federal agencies, automakers and private research firms went looking for an electrical or mechanical fault. None was ever found.
What police reports showed
The researchers behind that paper went through crash reports from 1979 to 1995 and isolated 3740 accidents caused by pedal misapplication, meaning the driver’s foot landed on the accelerator when it was meant for the brake. Of those, 2640 came from a foot slipping off the brake and 821 from simply hitting the wrong pedal. More than 92% happened while the car was already being driven, not at start-up, and 2938 of the crashes came in unhurried conditions, against 268 hurried and 255 distracted.
Shift interlocks, which keep the lever in Park until the brake is pressed, cut the start-up type of episode by roughly 60%. The authors read that as evidence for driver error over a defect. They estimate that only about 1% of episodes fit the narrow start-up definition NHTSA used.
Floor mats and sticky pedals
Pedal error is not the only cause, and the Toyota recalls showed it. In testimony to a House subcommittee on February 23, 2010, Transportation Secretary Ray LaHood described two separate problems: pedals trapped by floor mats, and pedals that stuck or returned slowly. He called the floor mat problem “far more serious” and said NHTSA knew of five deaths from it, among them four people killed near San Diego in August 2009.
NHTSA first learned of the issue in 2007, with a Lexus ES350, and opened an investigation that March. Toyota recalled certain all-weather mats that September. After the San Diego crash, on August 28, 2009, the agency pushed for more. Toyota announced a recall to replace or reshape pedals in 3.8 million vehicles and notified NHTSA on October 5, 2009. On January 27, 2010 it added 1.1 million more.
The department pressed Toyota to add brake override on models with keyless ignition. With it, the control system gives the brake signal priority and returns the engine to idle if both pedals are pressed. LaHood’s advice to owners was plain: if the accelerator sticks, press the brake steadily, shift to neutral, stop somewhere safe and call the dealer.
The Tesla petition
The same question came up again in 2021. TechCrunch reported on January 8 of that year that NHTSA had denied a petition from Brian Sparks, who wanted a review of 662,109 Tesla Model S, Model X and Model 3 vehicles. The agency examined 232 complaints he supplied, plus 14 others and all available crash data, and found the driver caused every crash cited. “There is no evidence of any fault in the accelerator pedal assemblies, motor control systems, or brake systems that has contributed to any of the cited incidents,” the report said.
Sources
- Response by Toyota and NHTSA to Incidents of Sudden Unintended Acceleration | US Department of Transportation
- NHTSA determines sudden acceleration complaints in Tesla vehicles were due to driver error | TechCrunch
- Cars Gone Wild: The Major Contributor to Unintended Acceleration in Automobiles is Pedal Error (peer-reviewed paper, PubMed Central)
