According to a traffic accident report filed by Google with the Californian Department of Motor Vehicles (DMV), the accident occurred on March 14 in Mountain View, Google's home town in Silicon Valley.
The self-driving Lexus was hugging the right of the lane as it was attempting to make a right-hand turn from El Camino Real onto Castro Street. This way the autonomous Lexus could pass vehicles also in that lane that were stopped at the intersection and awaiting a green signal to proceed straight ahead.
In response to inquiries from Engadget, Google clarified what its car was doing at this intersection: "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."
During the right turn manoeuvre, the Google vehicle had to come to a complete stop because there were sand bags placed around a storm water drain. In order to proceed, the Lexus indicated left and began moving back into the centre of the lane.
Around three seconds later, the Lexus collided with a bus driving up the centre of the right-most lane. In the incident report, Google states that the Lexus was travelling at under 3km/h, while the bus was travelling at around 25km/h.
Google's Lexus suffered damage to the left front fender and wheel, and one of its driver's side sensors. No-one was injured in the accident.
Speaking to the technology website, Google admitted that "we clearly bear some responsibility, because if our car hadn't moved there wouldn't have been a collision".
The tech giant says that both its autonomous driving software and the legally required test driver, who wasn't in control of the car at the time, both saw the bus coming from behind, and both the human and the machine assumed that the bus would slow down and allow them to enter. Clearly, the bus driver thought that the Lexus would stay put and continue waiting.
After reviewing the accident and running through thousands of variations in a simulator, Google's self-driving car code will now assume that buses and other large vehicles "are less likely to yield to us than other types of vehicles".