Sudden unintended acceleration (SUA) is the unintended, unexpected, uncontrolled acceleration of a vehicle, often accompanied by an apparent loss of braking effectiveness. It may be caused by some combination of driver error (such as pedal misapplication), or mechanical or electrical problems. The US National Highway Traffic Safety Administration estimates 16,000 accidents per year in the United States occur when drivers intend to apply the brake but mistakenly apply the accelerator.
Definition and background In the 1980s, the U.S. National Highway Traffic Safety Administration (NHTSA) reported a narrow definition of sudden acceleration only from near standstill in their 1989 Sudden Acceleration Report:
The report is taken from a study begun in 1986, in which the NHTSA examined ten vehicles suffering from an "above average" number of incident reports and concluded that those incidents must have resulted from driver error. In the lab tests, throttles were positioned to wide open prior to brake application in an attempt to replicate the circumstances of the incidents under study. However, the newest vehicle involved in the study was a 1986 model and no test vehicles were equipped with the electronic control (drive by wire) systems common in 2010. All vehicles were equipped with automatic transmissions, that is, no vehicles had manual transmissions with left foot clutch pedal disengagement of engine power.
These tests were meant to simulate reports of the time suggesting that the vehicles were at a standstill and accelerated uncontrollably when shifted from park. With modern drive by wire fuel controls, problems are believed to occur exclusively while the vehicle is under way.
General Motors cars of the 1950s with automatic transmissions have the R for reverse at the furthest clockwise position in the rotation of the column-mounted shift lever. L for low position is just adjacent as the lever moves one notch counterclockwise. Because it is very easy to select L, a forward position when desiring R, to reverse, there are many unintended lurches forward while the driver watches toward the rear, expecting to reverse the automobile. By the 1960s, gear selection arrangements became standardized in the familiar PRNDL, with reverse well away from the forward positions and between the Park and Neutral selections. The elimination of push-button drive control on all Chrysler products began after 1965 to eliminate the ease of selecting an unintended direction.
The most prominent incidents of sudden unintended acceleration took place from 2000–2010 in Toyota and Lexus vehicles, resulting in up to 89 deaths and 52 injuries in the USA. The NHTSA first opened an auto defect investigation into Toyota vehicles in 2004, but the Office of Defects Investigation (ODI) within the NHTSA closed the investigation citing inconclusive evidence. Toyota also claimed that no defects existed and that the electronic control systems within the vehicles were unable to fail in a way that would result in an acceleration surge. More investigations were made but were unsuccessful in finding any defect until April 2008, when it was discovered that the driver-side trim on a 2004 Toyota Sienna could come loose and prevent the accelerator pedal from returning to its fully closed position. It was later discovered that the floor mats of the affected Toyota vehicles could cause them to accelerate suddenly and that Toyota had known about these problems but had misled consumers and continued to manufacture defective cars. In March 2014, the Department of Justice issued $1.2 billion of financial penalties against Toyota in a deferred prosecution agreement.
In January 2021, engineer Colin O'Flynn was able to induce unintended acceleration with a similar Toyota vehicle using electromagnetic fault injection (EMFI) on a test bench. He used an ECU and components from a wrecked 2005 Toyota Corolla. O'Flynn's experiments were conducted without access to the ECU source code, and without access to the confidential report by the Barr Group.
In late May 2022, Ford issued a recall for the 2021 Ford Mustang Mach-E due to an unintended acceleration issue resulting from the powertrain control module functional safety software potentially failing to detect a software error, resulting in unintended acceleration, unintended deceleration, or a loss of drive power.
Causes Sudden unintended acceleration incidents are often posited to involve the simultaneous failure of a vehicle's acceleration and brake systems. Acceleration system factors may include: Pedal misapplicationHighway Accident Report – Rear-End Collision and Subsequent Vehicle Intrusion Into Pedestrian Space at Certified Farmers' Market Unresponsive (entrapped) pedals Electronic throttle control, throttle position sensor fault or cruise control failure (see drive by wire) Stuck throttle (unrelated to pedal position) Shorting of tin whiskers Diesel engine runaway: Diesel engine power is governed by the amount of fuel supplied. Excessive pressure in the crankcase can force mist of engine oil into the intake manifold, which can be burned in the same fashion as Diesel fuel, thus causing runaway. One-pedal driving mode and lack of brake application in deceleration and reverse actions.
Unintended acceleration resulting from pedal misapplication is a driver error wherein the driver presses the accelerator when braking is intended and is unable to quickly realize the mistake. Some shorter drivers' feet may not be long enough to touch the floor and pedals, making them more likely to press the wrong pedal due to a lack of proper spatial or tactile reference. Pedal misapplication may be related to pedal design and placement, as in cases where the brake and accelerator are too close to one another, or the accelerator pedal is too large. The NHTSA estimates that 16,000 accidents per year in the United States occur when drivers intend to apply the brake but mistakenly apply the accelerator.
An unresponsive accelerator pedal may result from incursion. For example, blockage by a foreign object or any other mechanical interference with the pedal's operation, and may involve the accelerator or brake pedal. A design flaw in some Toyota models enabled accelerator pedals to become trapped by floor mats. Throttle butterfly valves may become sluggish in operation or may stick in the closed position. When the driver pushes harder on the right foot, the valve may "pop" open to a point greater than that wanted by the driver, thus creating too much power and a lurch forward. Special solvent sprays are offered by all manufacturers and aftermarket jobbers to solve this very common problem.
Other problems may be implicated in the case of older vehicles equipped with carburetors. Weak, disconnected, or mis-connected throttle return springs, worn shot-pump barrels, chafed bowden cable housings, and cables which jump their tracks in the throttle-body crank can all cause similar acceleration problems.
For drive-by-wire automobiles, a brake-accelerator interlock switch, or "smart throttle", would eliminate or at least curtail any instance of unintended acceleration not a result of pedal misapplication by causing the brake to override the throttle. An unintended acceleration event would require the failure of such a mechanism if it were present. Such a solution would not be applicable to older vehicles lacking a drive-by-wire throttle.
Analyses conducted in the mid to late 1990s on Jeep Cherokee and Grand Cherokee vehicles concluded that hundreds of reported sudden accelerations in these vehicles were likely caused by an undesired current leakage pathway that resulted in actuation of the cruise control servo. When this occurred, typically at shift engage (moving the shift lever from park to reverse), the engine throttle would move to the wide open position. While the brakes were operational, operator response was often not quick enough to prevent an accident. Most of these events occurred in close confines in which rapid operator response would be necessary to prevent striking a person, fixed object, or another vehicle. Many of these events occurred at car washes, and the Jeep Grand Cherokee continues to experience sudden acceleration at car washes across the United States. A statistical analysis of SAIs in 1991 through 1995 Jeeps revealed that the root cause of these incidents could not be human error, as had been historically posited by NHTSA and auto manufacturers.
Physical analysis conducted on Toyota's electronic engine control system including accelerator pedal position sensors (APPSs) in 2011 showed the presence of a significant number of tin whiskers. Tin whiskers are elongated or needle-like structures of pure tin that grow from pure tin and tin alloy surfaces. Toyota's APPS were found to use tin finishes. These tin finishes can produce conductive tin whiskers capable of creating unintended electrical failures such as short circuits. The use of tin finish in Toyota's APPS is therefore a cause for concern. Similarly in 2013, materials used in an automotive engine control unit (ECU) from a 2008 Toyota Tundra truck were analyzed. It was found that pure tin with a nickel underlayer was used as the connector finish in the unit, and analysis revealed tin whiskers on the connector surface. Further testing under a standard temperature-humidity cycling showed tin whisker growth, raising additional reliability and safety concerns. These studies show that poor design choices, such as the use of tin finishes, result in unintended failures.
Laboratory replication Sudden uncommanded opening of engine throttle in certain models of modern, ECU-controlled automobiles have been replicated multiple times in laboratory settings. A negative surge of voltage (down to 7V) can cause rebooting of the ECU, resulting in the output APS (accelerator position) and TPS (throttle position) reaching 100% while the physical accelerator is still consistently held at 37%. In the study cited, a laparoscopic camera positioned inside the engine confirmed that 100% TPS corresponded with the physical full-opening of the throttle. A review of the logs did not show any abnormalities. A road test with the same conditions resulted in a sudden uncommanded acceleration 4 hours into the study.
Reported incidents Reported incidents of sudden unintended acceleration include: 1987: The 1982–1987 Audi 5000's sales in the United States fell after recalls linked to sudden unintended acceleration. There were 700 accidents and 6 deaths. 1988: 1986 Honda Accords were documented to have had sudden acceleration incidents due to the Vehicle Speed Control component, as reported to the NHTSA. 1997: Sudden acceleration in Cherokees and Grand Cherokees was reported by Diane Sawyer in a March 1997 ABC News Primetime segment. 2000: Several Ford Explorers were reported in the UK where the vehicle was already moving and experienced sudden acceleration. 2004: The NHTSA sent Toyota a chart showing that Toyota Camry models with electronic throttle controls had over 400% more "vehicle speed" complaints than those with manual controls. 2005: Incident observed in a Toyota Camry. The cause was initially suggested to be a tin whisker, however this was later proven not to be the case. 2005: Ida Starr St John had an SUA accident with her Toyota Camry which immediately accelerated without the pedal, and the brake pedal did not stop it. 2006: The 2004 model year Ford Mustang Cobra was recalled by Ford for accelerator pedals that failed to return to idle after being fully pressed. 2008: Incidents involving the 2005 Kia Amanti and Kia Sephia had been reported that were preceded by a racing or highly revving engine. 2009: Toyota Avalon displays unintended acceleration without the floor mat, observed by the dealer.