Marco della Cava

USA TODAY

SAN FRANCISCO - After more than a million miles of autonomous driving over the past six years, Google's self-driving car had never been at fault in the 17 accidents the company reported to the California Department of Motor Vehicles.

Until now.

According to a report to the DMV, on February 14 a self-driving Lexus SUV owned by parent company Alphabet was testing on the streets of Mountain View, Calif., when it struck a bus while traveling at 2 mph. The incident was the result of road conditions that were compromised by sand bags placed in a lane, which caused the Lexus to moved into the left lane. The municipal bus was approaching in that lane at 15 mph.

The Google car anticipated that the bus would slow down, while the bus driver believed the Google car would retreat from its effort to merge. There were no injuries, and the accident resulted in damage to the left front fender and some sensors.

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In all its previous accidents, the majority were the result of human drivers rear-ending the Google cars at slow speeds, typically at intersections where it anticipated the Google car would move ahead.

On Tuesday, Google is due to release its monthly report on its ongoing autonomous car program. But in a statement issued Monday, Google both acknowledged that its computer-driven vehicle made the wrong decision, but also stressed that the incident reflects that kinds of guesswork that goes on between human drivers.

"This is a classic example of the negotiation that’s a normal part of driving — we’re all trying to predict each other’s movements," the statement read. "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."

It added: "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."

Although a growing number of technology and automakers are making fast progress on the road to semi- and fully autonomous cars, only Google is targeting a vehicle that ultimately would not feature a steering wheel or pedals. Google's self-driving car tests are always conducted with legally mandated safety drivers. Sometimes they take over control of the test vehicles - which by law must feature a wheels and pedals - but did not in this particular accident.

Google car leaders have taken exception of late to a proposal by the California DMV to mandate that all self-driving cars have steering wheels and pedals. Google has long maintained that computers linked to sophisticated on-board radar, lasers and cameras will ultimately lead to a reduction in the number of road accidents and deaths. Human error is augmented these days by the prevalence of smartphone-created incidents of distracted driving.

Follow USA TODAY tech reporter Marco della Cava on Twitter: @marcodellacava