The accidents are not Google's first: In a briefing with reporters a year ago, the leader of Google's self-driving car program acknowledged three others between when the company first sent cars onto public roads several years ago—without the state's official permission—and May 2014.
In a written statement, Google said that since September, cars driving on streets near its headquarters in Mountain View had "a handful of minor fender-benders, light damage, no injuries, so far caused by human error and inattention."
Google said that while safety is paramount some accidents can be expected, given that its cars have gone "the equivalent of over 15 years of typical human driving," or approximately 140,000 miles.
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The national rate for reported "property-damage-only crashes" is about 0.3 per 100,000 miles driven, according to data from the National Highway Traffic Safety Administration.
In that context, Google's three in about 140,000 miles may seem high. As the company pointed out, however, perhaps 5 million minor accidents are not reported to authorities each year, so it is hard to gauge how typical Google's experience is.
Three other states have passed laws welcoming self-driving cars onto their roads. Regulators in Nevada, Michigan and Florida said they were not aware of any accidents.
As self-driving cars proliferate, others issues will arise that human drivers have dealt with for decades, notably who's liable for an accident. Each test car is required to have $5 million insurance.
Interest in accidents will remain high, especially if the self-driving car is at fault, said Bryant Walker Smith, a law professor at the University of South Carolina who has written extensively on the technology.
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"For a lot of reasons," Smith said, "more might be expected of these test vehicles and of the companies that are deploying them and the drivers that are supervising them than we might expect of a 17-year-old driver in a 10-year-old car."