In related article http://www.reuters.com/article/us-google-selfdrivingcar-idUS...

"Google said in the filing the autonomous vehicle was traveling at less than 2 miles per hour, while the bus was moving at about 15 miles per hour."

Google said in a statement on Monday that "we clearly bear some responsibility, because if our car hadn’t moved, there wouldn’t have been a collision. That said, our test driver believed the bus was going to slow or stop to allow us to merge into the traffic, and that there would be sufficient space to do that."