But those Google cars are extremely coddled.
1) They're always relying on pre-loaded LIDAR data, so always on routes that Google has mapped ahead of time. It's going to take a very long time to establish a fully comprehensive, national LIDAR map, and even when it's completed, it'll have to be updated constantly.
2) They aren't driving in extreme weather conditions.
3) They're driving in a very limited radius—no one's taking them on a cross-country trip, dealing with unfamiliar and poorly marked roads. So that's 700,000 miles in a highly controlled environment.
4) And 700,000 miles is pretty skimpy, isn't it, when it comes to gauging the problems that arise with cars, and with drivers (human or otherwise)? It's not as though everyone gets into a potentially fatal accident every 500,000 miles. But when you combine all vehicles and all those drivers, and consider the total number of cars on the road, that's where the number of total collisions start to get scary. I have no doubt that robotic cars will reduce crashes, and, if they really become the rule, not the exception, will save a crazy amount of lives (and money). Still, it's not like these things are going to be creeping along at 20 mph. At 65 mph, physics can kick your ass real quick.