From the excellent Driverless: Intelligent Cars and the Road Ahead by Hod Lipson and Melba Kurman:
"At the time this book was written, Google's driverless cars had a total of seventeen minor fender benders and one low-speed collision with a bus. In the seventeen fender benders, the culprit was not the not driverless car, but the other human drivers. On February 14, 2016, however, Google's car had its first significant accident when it "made contact" with the side of a city bus. Unlike the previous seventeen minor collisions, this accident was the fault of the car's software because it erroneously predicted that if the car rolled forward, the bus would stop.
With the exception of the run-in with the bus, the rest of Google's accidents have happened because, ironically, Google's car drive too well. A well-programmed autonomous vehicle follows driving rules to the letter, confusing human drivers who tend to be less meticulous behind the wheel, and not always so law-abiding. The typical accident scenario involves one of Google's obedient driverless cars trying to merge onto a highway or turn right at a busy intersection on a red light. Impatient human drivers, not understanding the car's precise adherence to speed limits or lane-keeping rules, accidentally run into the driverless car."