MOUNTAIN VIEW (CBS SF) — Three Google employees went to the hospital after getting rear-ended in a self-driving car.

The July 1 incident is the first reported injury accident involving the self-driving car.

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As employee Chris Urmson reports in a blog, a Google Lexus vehicle was driving autonomously toward an intersection in Mountain View during rush hour when a car slammed into the back of the car at 17 mph.

The employees were treated for whiplash and allowed to go back to work.

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“As you can see from the video above, our braking was normal and natural, and the vehicle behind us had plenty of stopping distance — but it never decelerated,” Urmson wrote. “This certainly seems like the driver was distracted and not watching the road ahead … But we’re now driving enough — and getting hit enough — that we can start to make some assumptions about that real crashes-per-miles-driven rate; it’s looking higher than we thought.”

Urmson said other drivers have hit Google cars 14 times since the start of their project in 2009 — 11 of those were rear-enders.

“Not once has the self-driving car been the cause of the collision,” Urmson said. “Instead, the clear theme is human error and inattention. We’ll take all this as a signal that we’re starting to compare favorably with human drivers.”

In all, 48 vehicles are being tested on California’s roads. As of May 15, the most recent data provided by the DMV shows Google is testing the most, with 23 cars being tested.

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California rules state that a person must be behind the wheel when a self-driving car is being tested on public roads.