Even though self-driving cars offer tremendous promise, the technology is still being perfected. Several companies recently filed reports with the California DMV, showing how often their drivers had to take control. Google's autonomous vehicles had covered 1.3 million miles by November 2015 and 424,331 of them were on roads in California. The vehicles disengaged on 272 occasions due to failure of the autonomous technology. A further 69 events resulted in drivers taking control of the vehicle due to a possible safety threat.
This chart shows the number of test miles and reportable disengagements of self-driving cars from Sept 2014 to Nov 2015.