Beyond disengagements and miles: how to measure autonomous vehicle safety

Autonomous vehicles (AVs) promise tremendous safety improvements with the elimination of human driver error—but only once the technology has been perfected. Exactly how close are developers to perfecting a safe and reliable self-driving system? That depends on what metrics are used to measure safety.

California publishes statistics on all the self-driving miles accumulated by test vehicles on its roads, as well as the number of times safety drivers have had to take control back from the autonomous systems. For example, Waymo covered 1.45 million miles with its AVs in California last year, with a disengagement rate of 0.076 per 1,000 self-driven miles. Cruise was another big player, racking up 831,040 miles last year with a disengagement rate of 0.082. Lyft covered 42,930 miles in the state but had a much higher disengagement rate of 38.8 per 1,000 miles. But does that suggest Lyft vehicles are less safe than those from Waymo or Cruise?