https://www.theregister.co.uk/2018/02/02/cali_dmv_driverless_disengagement_reports/

Mercedes' driverless cars need human intervention approximately every 2.08km (1.3 miles), and other makes are totally reliant on frequent switching to manual, according to figures from the Californian Department for Motor Vehicles.

The "disengagement reports" (times an autonomous car was taken over by the human tester) of the major autonomous vehicle companies that test on the Golden State's highways and byways are available on California's DMV's website. Driverless tests take place in several other US states, but there is no equivalent law in these places.