Unless you’ve been in a cave, you know that Toyota has been in the news lately for a problem involving “sudden unintended acceleration,” or SUA. This occurs when Toyotas apparently zoom off on their own, without the driver depressing the accelerator pedal. The official definition, according to the NTSB, is

“Sudden acceleration incidents” (SAI) are defined for the purpose of this report as unintended, unexpected, high-power accelerations from a stationary position or a very low initial speed accompanied by an apparent loss of braking effectiveness. In a typical scenario, the incident begins at the moment of shifting to “Drive” or “Reverse” from “Park”

The definition limitation “from a stationary position or a very low initial speed” has drawn criticism. Newer reports involve acceleration from highway speeds, with deadly results.

For facts-and-data people like us, there are a few questions that need to be asked:

  1. Have SUA incidents been increasing in recent years?
  2. Do Toyotas have a higher incidence than other vehicles? Does the Prius?
  3. Has the cause of SAS ever been investigated by an objective researcher? If so, was a cause identified?

Let’s examine these one by one.


The data do appear to show a couple of spikes (see Figure 1). However, it is interesting that the spikes immediately follow news coverage of the problem. This is the second media feeding frenzy about this issue. The first was the Audi 5000, the target of a 1986 episode of “60 Minutes.” In other words, drivers have been complaining about sudden unintended acceleration events for a quarter of a century and continue to lodge these complaints with manufacturers and NHTSA. If the news reports were the result of covering the problem, rather than fanning the flames of public hysteria, one would expect the spikes to come before rather than after the reports. Another interesting fact is that a Google search on the term sudden unintended acceleration include a large number of legal firms in the search results. Could they be fanning the flames?

Figure 1

Are Toyotas The Worst of the Lot?

But does the focus of the news, the Toyota Prius, stand out as particularly dangerous? Not according to the web site AllAboutPrius. The site reports that an analysis of all complaints to the NTSB related to vehicle speed control for the 2004-2009 Toyota Prius revealed that it ranked only 18th in reported injuries. That’s roughly consistent with the number of Priuses sold as a proportion of overall new cars.

Root Cause

An investigation of the problem in the Audi 5000 in the mid-1980s by the NTSB concluded that the problem was driver error. Other NTSB investigations implicate floor mats. It is possible that SUA is related to the cruise control, a hypothesis that isn’t hard to test. Steve Wozniak, the cofounder of Apple, speculated that his Prius had the problem and speculated that software might be the issue. In short, the debate over the root cause of SUA is still unknown


Considering that it has been the object of intense interest for over 24 years, it seems unlikely that a consensus on the cause will be reached any time soon. There are many constituencies with an interest in prolonging the publicity around SUA. And there is no doubt that the public’s interest is intense. People have been killed and injured in accidents caused by SUA and millions of Toyota drivers (including me and my wife) are concerned about the safety of our vehicles. The news media is in the business of attracting an audience for their product, and this story does the trick. Product liability and personal injury lawyers stand to collect millions in fees and the longer the story is big news the greater the market for attorneys.

But one thing seems clear: the problem is not linked to the Toyota Production System, widely known as Lean. This has never been asserted by serious investigators or journalists, but it has been suggested by some who don’t especially care for Toyota. Lean is a methodology that assures that standard procedures are developed and followed to assure both efficiency and quality. It is almost certain that, once the cause is known, Lean will be part of the long-term solution by incorporating the revised design into the Toyota Production System.

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