Interlocks in YOUR car?

Interlocks Fail

Technology is never perfect. Whether it’s due to human error or device malfunction, gadgets run into problems.

Interlocks are no different, but the potential problems caused by interlock malfunction are serious.

Robert Strassburger, a Vice President at the Alliance of Automobile Manufacturers, presented two scenarios in a 2008 presentation about alcohol detection technology:[2]

In the first, alcohol detectors are assumed to be “Six Sigma” devices. Six Sigma refers to a quality management system in which a product only has 3.4 defective parts per million. In other words, assume 99.99966% of device parts are free of fault.

But even if (and that’s a big “if”) interlocks met this high standard, Strassburger calculated that there would still be almost 4,000 cases of misreadings per day. That’s thousands of people trying to go to work, school, or about their business who could find their car locked down by a faulty interlock.

In Strassburger’s second scenario, interlocks are only “3 Sigma” reliable, which he defines as 2,700 defective parts per million. This would cause almost 3 million misclassifications every day.

These “misclassifications” could go both ways — allowing a drunk person to start the car or locking the engine of a sober driver.

Aside from the inconvenience of having a car that won’t start due to a false positive, there are legal and privacy implications.

Today’s interlocks record each breath sample and can report how many times the driver attempts to start his or her vehicle after drinking. Similarly, NHTSA has argued that universal interlocks should have the same capacity regarding non-convicted drivers:

One concept employs alcohol-vapor sensors installed in vehicles that can communicate their data to police. The data stream would contain vehicle identifiers as well as alcohol concentrations. A low-cost, short-range service such as WiFi Max or similar would be used as the link. Police could use notebook computers or personal digital assistants (PDAs) to receive the data.[2]

This creates a scary situation for the sober driver whose interlock has malfunctioned.


  1. Robert Strassburger Presentation, Lifesavers 2008 Conference
  2. Review of Technology to Prevent Alcohol-Impaired Crashes, page 47