Why Toyota’s Firmware Was Not a Killer

As some readers may remember, in 2013 an Oklahoma jury found that Toyota's embedded software was to blame for unintended acceleration that resulted in a fatal accident. The embedded software expert who convinced the jury that software was to blame made his trial testimony and slides available to the public and invited us to “judge for ourselves.” A flurry of articles in technical publications followed, most of which (to the best of my recollection) described the expert's testimony in favorable terms, accepting the conclusion that software caused the accident. Many of these articles had attention-grabbing titles such as Toyota’s killer firmware and Toyota Case: Single Bit Flip That Killed.

Having developed embedded software for many years, and having written an opinion piece in the Los Angeles Times  years before the trial about the possibility that Toyota's software was to blame for reported incidents of unintended acceleration, I read the expert's testimony with great interest and excitement. I was expecting to find a convincing argument based on the evidence that the software was indeed to blame for the accident in question.

As I delved deeper and deeper into the testimony, however, my excitement turned to disappointment. It became clear that there was no credible theory based on the evidence. I felt it was important to set the record straight, and so I submitted an article with my technical analysis to the IEEE Technology and Society Magazine . That article was peer reviewed and then accepted for publication in their most recent issue (December, 2016). (You can also find a pre-publication version of the article on my company's website, as well as two video interviews here and here providing additional context.)

In this column, I summarize some of my findings (please refer to the IEEE article for a more complete discussion that includes all the technical details).

As discussed in the IEEE article, the plaintiffs convinced the jury that Toyota's embedded software was responsible for the accident by employing the following approach:

  1. First, they bombarded the non-technical jury with criticisms of the quality of Toyota's software from two different software experts. The first expert did not see any of Toyota's source code, but nonetheless his entire testimony, which is also publicly available (see Part 1 and Part 2), was directed toward criticizing the quality of Toyota's software. As anyone with extensive experience developing real-world software knows, software quality assessments can be highly subjective.
  2. The second of the two experts, who did examine Toyota's source code, also criticized the quality of Toyota's software. Then he told the jury that “to a reasonable degree of engineering certainty, it was more likely than not”  that the death of a task running on the engine control processor (referred to at trial as “Task X”) was responsible for the accident, despite the fact that the evidence presented at trial did not support that conclusion.

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