Self-driving cars and ride-sharing vehicles are hitting the road in increasing numbers. Sometimes the car is driving smoothly enough (with a human twiddling their thumbs in the driver's seat) that other drivers don't even notice.

We've still got a long way to go before humans can lounge in the back seat on fully autonomous roads, though, and much of the neccessary progress has to come from improving self-driving AI software through exhaustive testing. That means getting more self-driving cars on the road that are driving a lot more without human assistance to fine-tune their systems. As such, the plethora of autonomous vehicle companies are all busy logging miles.

One of the biggest testbeds for self-driving cars is California. The state's Department of Motor Vehicles logged disengagement reports from December 2017 to November 2018 for 48 companies (28 actively testing on public roads) and 496 total self-driving vehicles. The data, aggreggated and published by website The Last License Holder, found that the companies' collective vehicles drove more than 2 million miles in autonomous mode over that span.

The California DMV also logged 143,720 disengagements, meaning when a human safety driver has to take back control, either due to a system error or to avoid a dangerous situation the autonomous vehicle is not yet trained to navigate. So when evaluating all the self-driving carmakers on the road in California, we have the benefit of comparing how many total miles were driven against how often a human operator had to intervene.

By both measures, Alphabet's Waymo self-driving car unit is far in the lead. Waymo vehicles drove more than 1.2 million miles with over 11,000 miles per disengagement. GM's Cruise subsidiary is in second at over 447,000 miles and more than 5,200 miles per disengagement.

At a distant third in self-driving distance is Apple at almost 80,000 miles but only 1.1 miles per disengagement. Apple recently laid off more than 200 employees from its Project Titan self-driving car initiative, reportedly as part of a strategy shift away from building physical cars and more toward developing autonomous driving software.

Uber's California testing has been similarly fraught. The ride-hailing company drove 26,899 autonomous miles over the span measured but logged a paltry 0.4 miles per disengagement. Uber has experienced numerous other issues with its self-driving-car testing, including suspending tests in Arizona after a fatal incident and shutting down its autonomous truck unit.

Among the other companies testing self-driving cars in California are automakers such as Mercedes-Benz and Nissan, tech companies including Nvidia and China's Baidu, and numerous startups incuding Zoox, Nuro, and Pony.AI.