Self-driving cars may ultimately prove safer than having a human behind the wheel, but that doesn't mean they can avoid accidents entirely. The Associated Press is reporting that four of the 48 self-driving vehicles licensed to operate on California roads have gotten into minor fender-benders since the state began issuing permits in September. Two of those accidents allegedly happened with the vehicles in self-driving mode; the others are being blamed on human error.

Three of the cars were Lexus SUVs that are part of Google's intensive self-driving tests near the company's Mountain View headquarters. Delphi Automotive owns the vehicle involved in the fourth accident. The specific details around each incident aren't being revealed, since California law allows for collision reports to remain private. But according to the AP's source, none of them were severe crashes and each occurred at speeds of under 10 mph.

Google blames these incidents on "human error and inattention."

Delphi's self-driving Audi was "moderately damaged" after it was suddenly broadsided by another vehicle while waiting to turn left at a light. It was not in self-driving mode at the time. And in a statement, Google admitted that its Lexus SUV fleet has so far been involved in "a handful of minor fender-benders, light damage, no injuries, so far caused by human error and inattention." Google operates 23 Lexus vehicles that have been retrofitted with its autonomous driving sensors and other technology. Each contains security features like a kill switch that instantly disables the self-driving system. They are far more conventional than Google's own design for a self-driving car.

But since this exciting concept remains so new to many, critics believe that companies pushing the cause of self-driving cars should share full details around any accident with the public and lawmakers. Google has now been involved in a total of six crashes when you include the three it quietly reported last year, spanning from when it began testing autonomous driving through May 2014. (The company told us most of those happened when its own cars were rear-ended by inattentive drivers.) Five other companies with similar permits told the AP they've so far avoided accidents on the road, and other states that allow self-driving tests (Nevada, Michigan, and Florida) reported no incidents. Google seems to believe that the occasional accident is inevitable with all the driving its cars are doing; so far Google's fleet has traveled "the equivalent of over 15 years of typical human driving," according to a statement it gave the AP.