(Bloomberg) — Google Inc. said its self-driving vehicles havebeen involved in 11 minor accidents in the six years they've beenin testing, and the automobiles were never to blame for theincidents.

Hit from behind

All of the accidents during 1.7 million miles of manual andautonomous driving included light damage and no injuries, saidChris Urmson, director of Google's self-driving car program, in apost on technology news website Backchannel on blog platformMedium. The majority of the collisions happened when theexperimental cars were hit from behind, with some on the freewayand some at traffic lights.

Google's investment in self-driving cars is part of its internalproject, dubbed X, that focuses on the long-term opportunities forcutting-edge technologies. The company has touted safety as anupside for self-driving cars — using sensors, mapping and othertools to avoid the mistakes that lead to accidents caused by humandrivers.

"We're thinking a lot about how to measure our progress and ourimpact on road safety," Urmson wrote. "We'll continue to drivethousands of miles so we can all better understand theall-too-common incidents that cause many of us to dislike day-to-day driving–and we'll continue to work hard on developing aself-driving car that can shoulder this burden for us."

Realities of speed and distance

Urmson's post appeared after the Associated Press reported thatthree of Google's self-driving vehicles were involved in accidentsin California since September, based on state records.

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

"Even when our software and sensors can detect a stickysituation and take action earlier and faster than an alert humandriver, sometimes we won't be able to overcome the realities ofspeed and distance," Urmson said. "Sometimes we'll get hit justwaiting for a light to change."