One way to measure progress of driverless technology is how often a human safety driver must intervene. For Waymo, this improved rapidly between 2015 and 2018, falling from 0.64 disengagements per 1,000 miles – roughly once every 1,550 miles –  to .09, meaning a car could drive 11,000 miles on its own. But from 2018 to 2019, the rate google moved only slightly, from 0.09 disengagements every 1,000 miles to 0.076.