Over the past few decades, more than 10,000 automobile crashes involving sudden acceleration incidents (SAI) have occurred in which the driver claimed that their vehicle accelerated suddenly and unexpectedly at full throttle and the brakes were ineffective. Typically, the driver insists that their right foot was pressing hard on the brake pedal, that the pedal went to the floor, and that the brakes would not stop the car. After the episode, analyses of the vehicle's various mechanical and electrical systems revealed nothing that would explain the event.[1] Thus, investigators concluded that most SAI are caused by human error, not a mechanical defect in the vehicle.[2] In fact, almost all SAIs are the result of a pedal misapplication error in which the driver unintentionally pushes the accelerator instead of the brake.
The National Highway Traffic Safety Administration (NHTSA) developed a model to explain SAI. See Figure 1, adapted from Pollard and Sussman (1989).[3] The SAI scenario can be broken down into three main stages—initiation, persistence, and termination.
Initiation
Human factors research on SAI indicates that many pedal misapplications are initiated by a "startle effect."[4] A driver can be startled by the sudden unintended acceleration of the vehicle itself or by some other startling event (e.g., hitting another object, being hit by another object, distraction).
The “startle effect” can initiate an SAI by causing either a control or an execution error. An example of a control error is when a driver who has their foot on the accelerator pedal when they are startled and reflexively extends their foot/ankle in response and pushes the gas pedal to the floor without intending to do so. An example of an execution error is when a startled driver attempts to put their foot on the brake pedal but misses. The result of either type of error is a sudden acceleration of the vehicle due to a pedal misapplication (i.e., pushing the gas pedal to the floor instead of the brake).
A research study on SAI found that 56% of pedal misapplication errors while driving were caused by a slip whereas 44% were caused by the driver striking the “wrong pedal.” “Wrong pedal” accidents mainly occur in driving situations involving turning or being hit by another vehicle and are not simple slips from one pedal to another. A car being hit by another object was a factor in approximately 6% of SAI that occurred while driving and distraction was a factor in approximately 8% of SAI that occurred while driving.[5]
Persistence
Once the SAI has been initiated the event often persists until a collision occurs. In other words, the driver often does not correct the pedal error in time to avoid a collision. SAIs persist for several reasons. The driver may not have sufficient time to detect and resolve the pedal misapplication error before a collision occurs. In other instances, a driver who is under time pressure and high levels of stress may not detect a pedal error at all and may not consider or attempt a solution before a collision occurs. In a stressful situation such as an SAI, a driver's brain may only process the driver's intentions (e.g., to push the brake pedal) and actions (e.g., a pedal was pushed) but may not process enough information to figure out what actually occurred (e.g., the gas pedal was pushed instead of the brake pedal). Additionally, a driver's memory of the event may be subconsciously altered to protect their ego from admitting they made an error. In these cases, a driver’s account is frequently consistent with the human factors literature indicating that many drivers insist they were pushing the brake pedal during an SAI even though they were, in actuality, pressing the accelerator.
Termination
Once the SAI is initiated it will persist as long as the driver fails to recognize the pedal error and continues to press the accelerator. SAI episodes might last from less than 1 second to as long as 45 seconds, with the vehicle attaining speeds above 40 mph before a violent collision occurs.[6][7] The SAI will terminate when the driver recognizes their error and resolves the error or when they collide with an object.
Human Factors Investigations of SAI
Human factors investigations of SAI include evaluating potential causes of SAI (e.g., distraction, being hit by an object, hitting an object) as well as evaluating driver actions during all 3 stages of an SAI incident—initiation, persistence, and termination.
Does your auto accident case involve sudden acceleration? Contact Human Factors Expert Dr. Nancy Grugle to discuss your case.
