http://www.nytimes.com/2015/09/02/technology/personaltech/google-says-its-not-the-driverless-cars-fault-its-other-drivers.html

“The real problem is that the car is too safe,” said Donald Norman, director of the Design Lab at the University of California, San Diego, who studies autonomous vehicles. “They have to learn to be aggressive in the right amount, and the right amount depends on the culture.”

A major fault with the Google car? I seems it's too safe and law abiding, unlike human drivers.