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Waymo, the self-driving company most people know as the Google car, has accelerated its pace of test driving on public roads.

The company, a subsidiary of Google parent Alphabet, said yesterday that it has passed three million miles of autonomous driving on public roads. It’s been about seven months since Waymo announced the two million-miles mark last October. That means its pace is accelerating, as those second million miles took more than a year, and its first million miles — announced in July 2015 — took several years. It has also driven “billions more in simulation,” Waymo CEO John Krafcik tweeted.

3 million autonomous miles on public roads! Accumulating more real world miles than ever, plus billions more in simulation. https://t.co/bucHDgOw2V — John Krafcik (@johnkrafcik) May 9, 2017

While its sheer number of test miles doesn’t prove how well a robot car can drive — other things matter, too, like road conditions and how often human drivers have to take the wheel — the relative rate at which those cars rack up those miles is a good indicator of whether the cars are progressing.

Because self-driving cars rely on massive amounts of data to improve their algorithms, a key method of teaching a robot car to drive itself is simply to let it drive itself. So it’s good for Waymo that its pace is improving.

What we don’t know: How often humans have been taking over the controls, a statistic called “disengagements.” That’s because while Waymo’s test area includes California, Arizona and Seattle, its public reporting is limited to California. There, the company’s cars drove more than 600,000 miles in 2016 and humans only had to take over 124 times — a big improvement from the prior year.

A few weeks ago, Waymo launched its first public self-driving pilot in Arizona, opening up applications to people who could ostensibly use the autonomous cars every day.

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