So they just drove over the same "few thousand miles of roadway" again and again and again and again? Until they got to 700,000 miles?
I think you meant this as sarcasm, but that one is mostly correct. These cars are not doing cross country trips, so claiming a few thousand miles of roadway is probably an overestimate. They drive the same roads and areas over and over and over again.
As it should. Because you don't know if that piece of paper is covering a rock or a pothole or whatever.
I have been tempted to carry a bucket of chaff and just see how well a Google car handles it, but then again rain and snow are problems so the experiment is really not needed.
The point here is that a human can notice things that a current auto driving car can not. Not all humans pay attention, but for the percentage that do you can tell when a paper bag is blowing around on the freeway. Human reaction to those things is generally measured and controlled much better than a google car. In time, I am sure it will get better but you need to discuss what is there today, not what we wish it had and are working for.
So they cannot deal with new stop LIGHTS but they can deal with new stop SIGNS. WTF?
I'm not sure how much you drive around California, but if you ever do you will see why this one is an issue. Many traffic lights in Mountain view for example are angled downward, so you have to be at a certain distance to see the color. There is one by Shoreline and Central that you can't see until you are about 40-50 feet away (for those interested, east bound traffic at the fire station).
Compare that issue with scanning for a red octagon pattern, and is should become obvious why stop signs are much easier to do. Traffic lights would be easy if they broadcast a signal, but they don't.
Overall, I'm not against self driving cars as long as we can choose between modes of operation. I think we are a long way off in terms of technology to make them safe in all environments, that does not imply even decades. I am mostly concerned with the health impact of all those radars and sensors broadcasting everywhere, but that's mostly due to my own ignorance (I have not taken any time to study since they are extremely rare).