While the accident report, which was written by Google, does not assign fault, the company said its Lexus was in autonomous mode when it was trying to avoid some sandbags on a street in Mountain View, California, at which point it struck the left side of the bus at 2 mph. No injuries were reported.
The report said the Google test driver who was in the car “saw the bus approaching in the left side mirror but believed the bus would stop or slow" to allow Google's car into traffic. Three seconds later, as the car moved to the center of the lane, it hit the side of the bus, which was traveling at 15 mph, according to the report.
According to California law, the test driver must sit in the front seat and grab the wheel when needed.
DMV spokeswoman Jessica Gonzalez told the Associated Press the agency planned to discuss the incident with Google today to determine what went wrong. Google's autonomous vehicles have been involved in at least a dozen accidents, with many being caused by human drivers rear-ending the autonomous vehicles.
In Google's monthly report, the company pointed out the incident 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," the report said. "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."
With 360-degrees of awareness, the self-driving cars are gaining new insights into dangerous driving behaviors, including drifting between lanes and running red lights -- both of which can contribute to accidents. Google said it has logged more than 2 million miles to date on the road. Company representatives did not immediately respond to ABC News' request for comment.