He explained that each vehicle contains "layers of computer code that may be added from one model year to next" that control nearly every system, from acceleration to braking to stability. Rizzoni said this software is rigorously tested, but he added: "It is well-known in our community that there is no scientific, firm way of actually completely verifying and validating software."
Here's an example everyone is familiar with: You're working at your computer in Windows software and an error message pops up. It asks whether you want to report the error to Microsoft. Microsoft has exhaustively tested this version of Windows before its release, but it cannot completely predict how it will operate out in the world, subject to user demands. That's why it gathers error reports and uses them to fix the software on a rolling basis.
If you put a lot of parts together to form a complex electromechanical machine and make it talk to itself via software, it can behave, sometimes, in ways you cannot anticipate. It can fail for reasons you cannot anticipate.
Ahrens ends the piece with a quote from a 2009 LA Times interview with a psychologist:
"Richard Schmidt, a former UCLA psychology professor and now an auto industry consultant specializing in human motor skills, said the problem almost always lies with drivers who step on the wrong pedal.
'When the driver says they have their foot on the brake, they are just plain wrong,' Schmidt said. 'The human motor system is not perfect, and it doesn't always do what it is told.' "