This paper provides a description of "Unintended Acceleration" (UA) in passenger cars, presents data pertaining to the scope of the problem as defined by the number of accidents and injuries reported annually to NHTSA that are alleged to be associated with UA, discusses the causes of UA and countermeasures to reduce the incidence of UA, and provides analysis to assess the reductions in reported UA-related accident rates which have resulted from the use of automatic shift locks.

UA reports are defined as incidents of high-powered unwanted vehicle acceleration from a stationary position or very slow speed, accompanied by reportedly ineffective brakes. Previous studies and investigations conducted by the National Highway Traffic Safety Administration (NHTSA), and by Canadian and Japanese government agencies have concluded that the major cause of such incidents has been drivers unknowingly depressing the accelerator instead of the brake pedal on automatic transmission-equipped cars.

Based on data obtained from NHTSA’s computerized consumer complaint file system, and information obtained in the course of agency defect investigations, the paper concludes that the best known countermeasure to UA has been factory installation of automatic shift lock systems which prevent the driver from shifting the transmission out of Park unless the brake pedal is simultaneously applied . A comparison of reported UA accident rates (accidents per vehicles produced) for automatic transmission-equipped cars indicates that vehicles equipped with shift locks have experienced approximately a 60 percent reduction in UA-related accidents compared to similar cars without shiftlocks. Also, the effect on UA of retrofitting shift locks on one particular make/model is assessed.