Sudden Unintended Acceleration: Causation and Drive-By-Wire Technology

Recent cases of sudden unintended acceleration (“SUA”) in cars and trucks, particularly as they have involved vehicles featuring drive-by-wire technology, may offer a preview of the types of potential catastrophes and subsequent questions of causation that may accompany emergent driverless vehicle technology.

In 2009, a late-model Lexus ES350 hurled toward a San Diego dead-end intersection at more than 100 miles per hour. With no avenue of escape, the Lexus crashed through a fence, became airborne, and finally landed in a riverbed before bursting into flames. All four passengers died.

The driver of the vehicle was an experienced California Highway Patrol officer. Just moments before the crash, his brother-in-law called 911 from inside the car. He told the 911 operator that the car’s accelerator pedal was stuck and, despite the driver’s frantic efforts, the car would not slow down whatsoever.

Like many modern cars, the Lexus ES350 in question had an on-off ignition button. According to Lexus, the button should have forced the engine to quit had the driver pushed it down for at least three consecutive seconds. In addition, it remains unknown whether the driver attempted to put the car in neutral. As is the case with many new cars, the ES350’s console-mounted gear lever was actually an electric switch designed to look and feel like a traditional, mechanical gear selector. Finally, eyewitnesses reported seeing smoke billowing from behind the wheels of the runaway Lexus, which suggests the driver had overheated the brakes to the point of failure while attempting to stop the car from high speeds.

An investigation revealed that someone had fitted the ES350, a loaner car from a local Lexus dealership, with an improper and ill-fitting driver’s side floor mat. Apparently, the non-standard floor mat had caused “pedal entrapment.” Simply, the floor mat had wedged the accelerator pedal at full throttle.

Although the parties settled a subsequent lawsuit, and Toyota, because of this case and other similar cases, recalled thousands of vehicles to install modified floor mat anchors and adjust the angles of accelerator pedals, some pundits have nevertheless questioned whether “pedal entrapment” had actually caused this and other related accidents involving Toyota vehicles featuring drive-by-wire technology.

Many modern cars have electronic accelerators. Generally, accelerator pedals no longer actuate cables that mechanically open or close engine throttle bodies according to the position of a driver’s right foot. Today, an accelerator pedal may simply be a spring-loaded data input switch that digitally communicates with an Electronic Throttle Control (“ETC”).

Ultimately, the ETC orchestrates engine combustion. Thus, in modern cars, pressing the accelerator pedal can be, depending on the circumstances, akin to a suggestion by the driver. The ETC may, or may not, comply with the driver’s input. This is particularly true in vehicles that have advanced stability control systems. The driver may “floor it,” but the ETC, in concert with the brakes, will likely keep wheel spin to a minimum. Even further, many luxury vehicles now feature steer-by-wire and brake-by-wire systems. The latest S-Class Mercedes Benz may ignore all but the most basic driver inputs when faced with certain environmental or emergency conditions.

Some have argued that an ETC could malfunction and possibly hurl a car toward a dead-end intersection at over 100 mph, regardless of driver input or lack of input. To date, however, even after extensive testing of Toyota’s ETC by scientists at NASA, the NHTSA has found no objective scientific evidence linking Toyota’s ETC to cases of SUA, including that of the San Diego tragedy.

However, in a world with driverless vehicles, causation of SUA may not be as readily ascribed to misplaced floor mats, or comparatively to drivers who may have neglected to push down on-off ignition buttons for at least three consecutive seconds or shift electric gear selector switches to neutral.

L.A. Times: Data point to Toyota’s throttles, not floor mats. http://articles.latimes.com/2009/nov/29/business/la-fi-toyota-throttle29-2009nov29

NHTSA: NHTSA-NASA Study of Unintended Acceleration in Toyota Vehicles. http://www.nhtsa.gov/UA

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