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Is human error a myth?

June 02, 2011
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Editor's Note: David Bizzak, a Ph. D. of mechanical engineering and member of the Society of Automotive Engineers, has specialized in the analysis of machine and consumer product design and manufacturing defects, as well as handled investigations of Jeep vehicles as a consulting engineer for Romualdi, Davidson & Associates. We have invited him to recall his earlier investigations, as well as suggest his own theory as to why human error is not the most likely cause of SUA.

In the process of investigating sudden acceleration incidents (SAIs), automotive manufacturers have long examined vehicles to identify any potential causes of accelerator pedal interference that might explain a “stuck” accelerator.

Most recently, Toyota issued a recall for certain models of their vehicles in which floor mats were considered to be the potential cause of SAI. In the absence of any evidence of such interference, or some other mechanical cause (e.g., sticking throttle cable/linkage), manufacturers have historically concluded that the cause of a SAI is human error.

This logic paralleled that of the National Highway Traffic Safety Administration (NHTSA).

A case of two studies

During the late 1980s, a NHTSA report on SAI concluded that human error — the driver mistaking the accelerator pedal for the brake pedal — was the most probable cause of SAI. It was posited that differences in reports of SAI in different makes and models of vehicles was likely due to the placement of the brake and accelerator pedals.

However, a NHTSA study performed by the Texas Transportation Institute in the early 1990s concluded that there is no preferred placement of pedals that might explain why some vehicles had higher rates of SAI than others.

Based upon the results of this study, there was no scientific, objective evidence to allow one to conclude that human error was the sole and proximate cause of SAI.

Nonetheless, the default conclusion that, in the absence of evidence to the contrary, human error is the cause remained the position of NHTSA and automotive manufacturers.

Factoring in the customer complaints

In the course of our investigation of sudden acceleration in the population of 1991-1995 Jeep Cherokees and Grand Cherokees, customer complaints of sudden acceleration were analyzed to determine if the theory that these events were simply the result of human error.

In our analysis of complaints, we identified 22 vehicles in which at least two different operators had experienced a SAI. There were also instances in which a single operator had experienced more than one SAI, but these events were excluded because it might be argued that these multiple occurrences were related to some specific individual factor (e.g., an operator’s disability or a condition that affected his/her muscular control).

With different operators, it could be reasonably assumed that human error would be random and the frequency of occurrence similar.

However, a statistical analysis indicated that the occurrence of 22 multiple SAIs — with two different operators — is not consistent with the cause being human error. Assuming the cause was random human error, the odds of the occurrence of these 22 multiple SAIs would be the same as the odds of a single person winning the Powerball lottery 8 times in 8 plays.

This analysis served as a solid scientific basis for exclusion of human error as the proximate cause of SAI in this population of vehicles.