Four of the nearly 50 self-driving cars now rolling around California have gotten into accidents since September, when the state began issuing permits for companies to test them on public roads.

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 Inc. 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 were not 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 would not 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 16 kilometres per hour (10 miles per hour). The person spoke on condition of anonymity because they were not 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.

Lack of information troubling to critics

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."

Delphi's autonomous car approaches New York City, its final destination of a 5,500-kilometre road trip across the U.S. An October collision involving a similar self-driving Delphi vehicle took place as it waited to make a left turn and another car broadsided it. (Delphi Corp.)

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.

A higher priority so far is teaching them to avoid causing a serious accident that could set public and political acceptance of the technology back years, said Raj Rajkumar, a pioneer of the technology with Carnegie Mellon University.

In the October accident involving Delphi, the front of its 2014 Audi SQ5 was moderately damaged when, as it waited to make a left turn, another car broadsided it, according to an accident report the company shared with AP. The car was not in self-driving mode, Delphi spokeswoman Kristen Kinley said.

Google, which has 23 Lexus SUVs, would not discuss its three accidents in detail.

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.

Hard to compare to human driver collision rate

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 225,000 kilometres (140,000 miles).

The national rate for reported "property-damage-only crashes" is about 0.3 per 160,000 kilometres (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.

"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."