Uber has been aggressively testing autonomous vehicles in Pennsylvania, California and Arizona in recent months and while they’ve racked up some impressive self-driving miles, not everything has been smooth sailing.

According to internal Uber documents leaked to Recode, human drivers have had to take over control of the vehicles quite frequently.

Uber measures how often this happens based on how many miles the vehicle drivers before a human takes over (‘miles per intervention’), how far the car travels before a critical driver takeover to avoid damage or harm, and how many miles the car travels before a so-called ‘bad experience’ (e.g. jerky driving).

Uber’s internal figures reveal that in January, its autonomous vehicles averaged just .9 miles (1.45 km) before a driver had to take control and in February, that figure dropped to .71 miles (1.14 km). However, not all these instances were considered critical. In fact, in Uber’s most recent measurements, a critical intervention is performed on average every 196 miles (315 km).

Uber, like all other companies testing autonomous vehicles in California, has to submit their reports to the DMV. In 2016, Google’s Waymo self-driving company drove more than 635,000 miles (1,021,933 km) and only required human intervention on 124 occasions.

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