The company says none of these minor collisions was caused by its cars and that it has avoided many major accidents over 1.7 million miles traveled, much of it with drivers just along for the ride.
The company released the total number after The Associated Press reported Monday that California has received four collision reports involving self-driving cars since it began requiring them last September. Three of those involved Google's cars.
Critics say more transparency is needed to assure the public the technology is safe.
Two accidents happened while the cars were in control; in the other two, the person who still must be behind the wheel was driving, a person familiar with the accident reports told The Associated Press.
Three involved Lexus SUVs that Google outfitted with sensors and computing power in its aggressive effort to develop "autonomous driving," a goal the tech giant shares with traditional automakers. The parts supplier Delphi Automotive had the other accident with one of its two test vehicles.
Google and Delphi said their cars weren't at fault in any accidents, which the companies said were minor.
Since September, any accident must be reported to the state Department of Motor Vehicles. The agency said there have been four but wouldn't comment about fault or anything else, citing California law that collision reports are confidential.
The person familiar with the accident reports said the cars were in self-driving mode in two of the four accidents, all of which involved speeds of less than 10 mph. The person spoke on condition of anonymity because they weren't authorized to discuss the reports publicly.
Five other companies have testing permits. In response to questions from the AP, all said they had no accidents. In all, 48 cars are licensed to test on public roads.
The fact that neither the companies nor the state have revealed the accidents troubles some, who say the public should have information to monitor the rollout of technology that its own developers acknowledge is imperfect.
John Simpson, a longtime critic of Google as privacy project director of the nonprofit Consumer Watchdog, pointed out that the company's ultimate goal is a car without a steering wheel or pedals. That would mean a person has no power to intervene if a car lost control, making it "even more important that the details of any accidents be made public - so people know what the heck's going on."
A chief selling point for self-driving cars is safety. Their cameras, radar and laser sensors give them a far more detailed understanding of their surroundings than humans have. Their reaction times also should be faster. Cars could be programmed to adjust if they sense a crash coming - move a few feet, tighten the seat belts, honk the horn or flash the lights in hope of alerting a distracted driver.
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" since fall. That would be approximately 140,000 miles. Google said its cars have gone over 700,000 miles in self-driving mode since they first hit the road in 2009.
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 aren't 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 weren't aware of any accidents.
As self-driving cars proliferate, other 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.
"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."