PS: i guess the insurance on self diving cars is a little bit different and the accident rate is only part of the picture. Just an example: humans don't get hacked.
If someone holds a gun to your head and tells you to run a red light, who is liable? Humans can get persuaded to do illegal things by criminals just as well.
I imagine Google has enough money that they can self-insure. Particularly since it's a very low accident rate.