As the SUV approached the turn, it detected sandbags near a storm drain blocking its path, forcing it to stop. After waiting for some other vehicles to pass, the car -- still in autonomous mode -- began angling back toward the center of the lane at about 2 mph. That's when it crashed into the side of the bus.
"Our car had detected the approaching bus, but predicted that it would yield to us because we were ahead of it," Google said.
The test driver, who had been watching the bus in the mirror, also expected the bus to slow or stop, Google said, "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," Google said.
This is the first time Google has acknowledged some responsibility for an accident involving one of its autonomous vehicles.
The accident, Google said, 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," the company added.
Google said it has reviewed this incident, as well as thousands of variations on it, in a driving simulator and made refinements to its AV 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 said.
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