Google's self-driving cars have got into quite a few accidents, but they've been caused by human-driven cars crashing into them, not the other way around.

"Our self-driving cars are being hit surprisingly often by other drivers who are distracted and not paying attention to the road," head of Google's driverless car program Chris Urmson wrote in a blog post Thursday.

He describes an incident in which a human-driven car (can you imagine: In a few years we may have to indicate that a car is human-driven more often than not) slammed into the back of Google's self-driving Lexus.

Google's car, explains Urmson, stopped (together with two human-driven cars in front) at a green light to avoid being stuck at the middle of an intersection due to heavy traffic ahead — just like a human would.

However, the human-driven car behind just kept going, hitting the Google car at 17 mph. The human in it did not hit the brakes at all, probably due to distraction. You can see the incident in Google's video, below.

These types of accidents started happening more often after Google's cars went from freeways to surface streets. "This is exactly where you’d expect a lot of minor, usually unreported collisions to happen," Urmson writes.

So far, Google's stats for self-driving cars are looking good. Since the beginning of the project in 2009, other drivers hit Google's self-driving cars 14 times. The number of self-driving cars causing a collision? Zero.

"We’ll take all this as a signal that we’re starting to compare favorably with human drivers," writes Urmson.