Also, Google doesn't drive it's test car on highway speeds with humans inside. Google self-driving car's speed is limited to 25 mph, so even if it hit something, potential that it would be lethal is very slim.
Google has two basic models of self-driving car. One is a modified standard car, and can drive at freeway speeds. If you drive around Mountain View, CA, you'll probably see some of them. The other is a little electric thing with no steering wheel limited to 25MPH. The Computer History Museum in Mountain View has one on display.
