All companies testing autonomous vehicles on the state's public roads must provide annual reports to the DMV about "disengagements" that occur when a human backup driver has to take over from the robotic system.

The DMV told eight companies with testing permits to provide clarification about their reports. More than 50 companies have permits to test autonomous vehicles with backup drivers on California roads but not all of them have deployed vehicles.

It turns out that a number of the issues reported are shared across technology from different companies.

Some of the problems had to do with the way the cars sense the environment around them. Others had to do with how the vehicles maneuver on the road. And some had to do with what you might expect from systems made up of networked gadgets: hardware and software failures.

The disengagement reports themselves identify other problems some self-driving vehicles struggle with, for example heavy pedestrian traffic or poorly marked lanes.