A Google self-driving car struck a municipal bus in a minor crash earlier this month in what may be the first case of one of its autonomous cars hitting another vehicle.
In a Feb. 23 report filed with California regulators, Google said the crash took place in Mountain View, California on Feb. 14 when a self-driving Lexus RX450h sought to get around some sandbags in a wide lane.
Google said in the filing the autonomous vehicle was traveling at less than two miles per hour, while the bus was moving at about 15 miles per hour.
The vehicle and the test driver "believed the bus would slow or allow the Google (autonomous vehicle) to continue," it said.
But three seconds later, as the Google car in autonomous mode re-entered the center of the lane, it struck the side of the bus, causing damage to the left front fender, front wheel and a driver side sensor. No one was injured.
Google said it has since replaced software on the car.
"We clearly bear some responsibility, because if our car hadn't moved, there wouldn't have been a collision."
The company also said it has reviewed this incident "and thousands of variations on it in our simulator in detail and made refinements to our software. From now on, our cars will more deeply understand that buses (and other large vehicles) are less likely to yield to us than other types of vehicles, and we hope to handle situations like this more gracefully in the future."
It had previously said that its autonomous vehicles had never been at fault in any crashes.
The Mountain View Police Department said that no police report was filed in the incident.
Stacey Hendler Ross, spokeswoman for the Santa Clara Valley Transportation Authority, which operates municipal buses in Mountain View and other cities in the area, confirmed the incident occurred, but said she did not know any details.
The California Department of Motor Vehicles said on Monday that manufacturers of autonomous vehicles must report crashes, but "the DMV is not responsible for determining fault."
A spokesman for the U.S. National Highway Traffic Safety Administration declined to comment.
The crash comes as Google has been making the case that it should be able to test vehicles without steering wheels and other controls.
In December, Google criticized California for proposing regulations that would require autonomous cars to have a steering wheel, throttle and brake pedals when operating on public roads. A licensed driver would need to be ready to take over if something went wrong.
Google said in November that in six years of its self-driving project, it has been involved in 17 minor accidents during more than two million miles of autonomous and manual driving combined.
"Not once was the self-driving car the cause of the accident," Google said at the time.