The metric is one way to measure the reliability of any self-driving system, although regulators have not yet determined how safe autonomous vehicles must be before they are introduced in large numbers, a timeframe that could be a decade or more away.
The latest disengagement report showed that Waymo vehicles, in tests conducted from December 2016 through November 2017, on average logged 5,596 miles without Waymo's safety drivers disengaging the system and retaking the wheel. Its next closest contender was Cruise, owned by General Motors, at 1,214 miles on average between disengagements.
Autonomous vehicles are still in the research and development stage and are being tested on public roads, especially in California where most companies competing in the space have tech hubs in Silicon Valley. California law requires that self-driving cars have a person in the driver's seat who can take over control when needed.
Waymo, for example, drove 352,545 miles in the state during the period with only 63 disengagements. Cruise vehicles drove about a third less, at 127,516 miles, and had 105 disengagements.
The third best performance came from Nissan Motor, which drove 5,007 miles and had 24 disengagements, meaning that its vehicles had disengagements on average every 208 miles.
© Thomson Reuters 2018