Data that Google submitted recently to the California Department of Motor Vehicles shows that during road tests over a 14-month period, the company's autonomous cars had to hand over control to a human driver on 272 occasions because of potential safety issues.
In addition, a human driver had to disengage autonomous control on 69 other occasions to ensure safe operation of the vehicle, the report noted. In 13 of these instances, the vehicle would have potentially made "contact" with another object if the human driver had not assumed control of the autonomous vehicle, Google said.
Although those numbers might seem substantial, the data in the report covers a 14-month period between September 2014 and November 2015 during which Google tested its self-driving cars in autonomous mode for more than 1.3 million miles. Close to 425,000 of those miles were on public roads in California.
Google classifies autonomous vehicle disengagements into two categories. The first is what the company called "immediate manual control" disengagements, when the software detects a failure or potential failure in the autonomous technology, like a broken wire, a communication glitch or sensor reading anomaly. In these instances, the car automatically turns control over to the human driver.
The second category of disengagement in Google's book is when the driver proactively takes manual control of the vehicle because of doubts over its ability to safely navigate a particular situation in autonomous mode.
According to Google, disengagement rates on its autonomous vehicles dropped sharply as the tests progressed. The number of immediate manual control disengagements, for instance, dropped from 785 miles per disengagement in the fourth quarter of 2014 to 5,318 miles per disengagement last November.
Google, like other companies that want to test autonomous vehicles in California, is required to provide an annual "disengagement report" to the government detailing the number of times its vehicles had to disengage or be disengaged from autonomous mode for safety reasons.
In addition to Google, six other manufactures were required to submit similar reports by Jan. 1, 2016. They include Mercedes Benz, Tesla Motors, Bosch, Nissan and Volkswagen/Audi. Four other manufacturers—Honda, Ford, BMW and Cruise Automation—were authorized to test autonomous vehicles in the state last year, and their disengagement reports will be due next January.
Google's autonomous vehicle safety record tends to draw more attention than that of others by virtue of the fact that the company is one of the most active in this space. But a comparison of Google's numbers with that of the others suggests that Google's autonomous vehicles may have performed relatively better than some of the others, especially when the total number of miles driven is taken into account.
Mercedes Benz, for instance, reported a total of 496 manual disengagements and 471 automatic disengagements on just one of the vehicles it tested over a distance of about 1,300 miles in autonomous mode. Another vehicle it tested experienced a total of 67 disengagements in 400 miles of testing.
Nissan similarly reported a total of 106 disengagements involving four cars that were tested in autonomous mode for a total of about 1,485 miles.
Meanwhile, Tesla claimed that its vehicles did not have to disengage from autonomous mode a single time after it began testing autonomous vehicle technology, but it did not offer any details on how many vehicles were tested or for how many miles.