An effort to make its self-driving cars behave more like human drivers may have contributed to a collision between one of Google's self-driving cars and a public-transit bus last month.

Weeks before the Feb. 14 collision, the first in which the company's autonomous technology is at least partially responsible for a crash, Google engineers made software changes that gave self-driving cars new capabilities. This update allowed vehicles making right-hand turns to hug the right side of a lane so that other traffic may pass, much like a human driver might.

"This is the social norm, because a turning vehicle often has to wait for pedestrians," Google said in its monthly status report on its self-driving car project. "... 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."

"We clearly bear some responsibility, because if our car hadn't moved there wouldn't have been a collision." – Google

But in the moments leading up to the accident, which occurred on El Camino Real in Mountain View, California, a well-trafficked artery for the self-driving cars in the company's hometown, the Lexus RX450h autonomobile in question encountered sandbags near a storm drain blocking its eastbound path. It came to a stop.

After waiting for other vehicles to pass, Google says the self-driving car inched back toward the center lane at approximately 2 miles per hour and struck a public-transit bus passing by at 15 miles per hour. Software had projected the bus would slow to let it back into the center of the lane, and the human driver behind the wheel of the Google car also believed the bus would yield.

"Our car had detected the approaching bus, but predicted that it would yield to us because we were ahead of it," Google wrote in its monthly report. "Our test driver, who had been watching the bus in the mirror, also expected the bus to slow or stop. Unfortunately, all these assumptions led us to the same spot in the lane at the same time."

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A spokesperson for the Valley Transportation Authority confirmed one of its buses was involved in the accident, and said an internal investigation had begun. Because the investigation is ongoing, "there has not been any assignment of liability at this point." The accident comes at a time California Department of Motor Vehicles officials are mulling more restrictive regulations for autonomous cars, including whether licensed human operators should be required.

Google says it has reviewed not only the accident, but thousands of variations of it and made updates to its software. What did Google learn? It says its cars will now behave as if buses and other large vehicles are less likely to yield than other types of vehicles.

"We clearly bear some responsibility, because if our car hadn't moved there wouldn't have been a collision," Google 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. ... We hope to handle situations like this more gracefully in the future."

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