Yandex claims that its autonomous cars have driven 2 million miles to date, double the figure it reported in October and 4 times the number announced in August. The Russian tech titan revealed the latest milestone in its Q4 2019 financials earlier today.

Additionally, Yandex shared for the first time that it has invested $35 million in its self-driving program since its inception, $24 million last year and $9 million in the fourth quarter alone.

By way of a brief recap, Yandex’s on-demand transport subsidiary, Yandex.Taxi, unveiled its self-driving car program back in May 2017, shortly before it began piloting the vehicles on Moscow roads. In the intervening years, the company has expanded its fleet across Russia and Israel (Tel Aviv) and to the U.S. (Las Vegas). It also laid claim to being the first public autonomous ride-hailing service when it launched a pilot in the Russian town of Innopolis back in 2018.

Doubling its self-driving miles to 2 million in just four months, after taking two years to reach its first million, is quite a growth trajectory. However, a major contributing factor has been the rapid expansion of its fleet, which grew nearly 10 times to 110 driverless vehicles in all markets last year.

Metrics

“Miles-driven” has emerged as a key metric of progress and safety in the race to put autonomous vehicles on roads at scale, with Alphabet’s self-driving car unit Waymo recently announcing it had passed 20 million miles. However, many have questioned the wisdom of relying on this metric alone. For example, driving 1 million miles on the same 100-mile stretch of road won’t teach software how to navigate diverse conditions.

The context in which miles are driven is integral to establishing reliable safety metrics, which is partly why Yandex has expanded its program to cover different locales around the world. However, the company doesn’t break out its “disengagement” rate, defined as times the safety driver has to take over. This number can be useful because the lower the disengagement rate, the better the software likely is at managing tricky “edge cases” on the road. Disengagement rate isn’t a perfect safety indicator either, as the context of environmental and road conditions is relevant — but it’s still a useful signal for assessing the progress of self-driving car systems.

So Yandex’s latest milestone should be taken with a grain of salt. But while the industry awaits better progress and safety metrics, “miles-driven” helps highlight the resources a company is putting into its autonomous vehicle programs. For Yandex, this translates into roughly 500,000 miles every two months.