On October 19 this year, an autonomous Waymo vehicle driving at approximately 21 mph through Mountain View, California, struck a motorcyclist, putting them in the hospital.

Now, Waymo has explained what happened and how the car's human safety driver made the mistake, not the car's autonomous system.

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Waymo chief executive John Krafcik said in a blog post this week how the safety driver took the car back into manual mode because they were concerned about a passenger vehicle approaching abruptly from the Waymo's left.

"Our driver responded by quickly moving the vehicle into the right lane," Krafcik said. "Unfortunately, our driver did not see that a motorcyclist had just moved from behind our vehicle into the right lane to pass us."

As a result, the Waymo vehicle's rear right bumper struck the motorcycle, putting the rider in hospital. In steering the Waymo car to the right to avoid one potential accident, another was caused by contacting the overtaking motorcycle, which the driver had not noticed.

A report of the incident filed by Waymo to the California Department of Motor Vehicles can be read here.

Waymo

Krafcik says Waymo ran a simulation of the incident afterwards to work out what happened and claims that, if the car's autonomous system had been left switched on, the car would have spotted both the car to its left and the motorcycle to the right, and slowed accordingly to avoid both. "Our technology correctly anticipated and predicted the future behavior of both the merging vehicle and the motorcyclist", he said.

The Waymo boss added: "Our simulation shows the self-driving system would have responded to the passenger car by reducing our vehicle's speed, and nudging slightly in our own lane, avoiding collision."

Recognizing the fall in trust such an incident could have on Waymo's future customers - and those sharing the roads with its autonomous taxis - Krafcik said: "We're sorry that a member of the community was injured in a collision with one of our cars. We recognize the impact this can have on community trust. We hold ourselves to the highest standard, and we are always working to improve and refine our testing program."

Maintaining consumer trust will be important as Waymo looks to commercialize its driverless cars, charging users to use them as autonomous, on-demand taxis. The company was recently given permission to operate its cars on public roads in California with no safety driver onboard, although fee-paying passengers are not yet riding in these vehicles.