One of Google's self-driving cars hit a public bus last month while it was in autonomous mode on a Mountain View, California, road.

The collision occurred on Valentine's Day and Google reported it to California's Department of Motor Vehicles in an accident report that the agency posted Monday. Though not the first time one of Google's self-driving cars has been involved in an accident, it is the first time one of Google's cars has caused the accident.

The DMV report doesn't address fault, but Google has said it is at least partially responsible for the incident.

At the time of the accident, Google's car was rolling at 2 mph and the bus at 15 mph. No one was injured. Google's car "sustained body damage to the left front fender, the left front wheel, and one of its drivers side sensors," according to the DMV report.

The Santa Clara Valley Transportation Authority said none of the 15 passengers or driver on its bus was injured. The bus had minor damage.

"An internal VTA investigation is still going on, there are several pieces of information that need to be examined, so no determination of liability has been made," spokeswoman Stacey Hendler Ross said in a written statement.

Google plans to detail the crash tomorrow in its regular monthly report on the self-driving cars. In a copy of the upcoming report provided to Mashable, the company says "we clearly bear some responsibility," though it attributes the crash to the "type of misunderstanding happens between human drivers on the road every day."

Google wrote that its car was trying to get around some sandbags on a street in Mountain View, California, when its left front struck the right side of the bus. All of Google's self-driving cars have human drivers who are able to take control of the vehicle, if necessary. In this case, the human driver didn't intervene because he thought the bus would yield to Google's car.

You can read Google's description of the accident below.

Our self-driving cars spend a lot of time on El Camino Real, a wide boulevard of three lanes in each direction that runs through Google’s hometown of Mountain View and up the peninsula along San Francisco Bay. With hundreds of sets of traffic lights and hundreds more intersections, this busy and historic artery has helped us learn a lot over the years. And on Valentine’s Day we ran into a tricky set of circumstances on El Camino that’s helped us improve an important skill for navigating similar roads. El Camino has quite a few right-hand lanes wide enough to allow two lines of traffic. Most of the time it makes sense to drive in the middle of a lane. But when you’re teeing up a right-hand turn in a lane wide enough to handle two streams of traffic, annoyed traffic stacks up behind you. So several weeks ago we began giving the self-driving car the capabilities it needs to do what human drivers do: hug the rightmost side of the lane. This is the social norm because a turning vehicle often has to pause and wait for pedestrians; hugging the curb allows other drivers to continue on their way by passing on the left. It’s vital for us to develop advanced skills that respect not just the letter of the traffic code but the spirit of the road. On February 14, our vehicle was driving autonomously and had pulled toward the right-hand curb to prepare for a right turn. It then detected sandbags near a storm drain blocking its path, so it needed to come to a stop. After waiting for some other vehicles to pass, our vehicle, still in autonomous mode, began angling back toward the center of the lane at around 2 mph — and made contact with the side of a passing bus traveling at 15 mph. Our car had detected the approaching bus, but predicted that it would yield to us because we were ahead of it. (You can read the details below in the report we submitted to the CA DMV.) Our test driver, who had been watching the bus in the mirror, also expected the bus to slow or stop. And we can imagine the bus driver assumed we were going to stay put. Unfortunately, all these assumptions led us to the same spot in the lane at the same time. This type of misunderstanding happens between human drivers on the road every day. This is a classic example of the negotiation that’s a normal part of driving — we’re all trying to predict each other’s movements. In this case, we clearly bear some responsibility, because if our car hadn’t moved there wouldn’t have been a collision. That said, our test driver believed the bus was going to slow or stop to allow us to merge into the traffic, and that there would be sufficient space to do that. We’ve now 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.

Google's self-driving cars have been involved in more than a dozen accidents since 2009 but, until now, those accidents were the result of other drivers rear-ending the cars.

In this case, however, there may never be a formal determination of liability, especially if damage was negligible and neither Google nor the transit authority pushes its side. Still, the Feb. 14 collision could be the first time a Google car in autonomous mode caused an accident.

Additional reporting by the Associated Press.