Google Inc. said its self-driving vehicles have been involved in 11 minor accidents in the six years they’ve been in testing, and the automobiles were never to blame for the incidents.

All of the accidents during 1.7 million miles of manual and autonomous driving included light damage and no injuries, said Chris Urmson, director of Google’s self-driving car program, in a post on technology news website Backchannel on blog platform Medium. The majority of the collisions happened when the experimental cars were hit from behind, with some on the freeway and some at traffic lights.

Google’s investment in self-driving cars is part of its internal project, dubbed X, that focuses on the long-term opportunities for cutting-edge technologies. The company has touted safety as an upside for self-driving cars — using sensors, mapping and other tools to avoid the mistakes that lead to accidents caused by human drivers.

“We’re thinking a lot about how to measure our progress and our impact on road safety,” Urmson wrote. “We’ll continue to drive thousands of miles so we can all better understand the all-too-common incidents that cause many of us to dislike day- to-day driving?–?and we’ll continue to work hard on developing a self-driving car that can shoulder this burden for us.”

Urmson’s post appeared after the Associated Press reported that three of Google’s self-driving vehicles were involved in accidents in California since September, based on state records.

Google relies on humans in the cars and back-up technology during tests for the self-driving vehicles.

“Even when our software and sensors can detect a sticky situation and take action earlier and faster than an alert human driver, sometimes we won’t be able to overcome the realities of speed and distance,” Urmson said. “Sometimes we’ll get hit just waiting for a light to change.”