I don't see "Google needs to pre-map the route" as a hurdle at all. Even if Google had never been in my area, I would not mind one bit having to do my daily commute manually for a week while the car builds more and more detailed maps of the route, if it saved me the hassle of the commute for months thereafter.
Plus, every time the automatic car goes over the road, it's presumably checking its prior maps and flagging any anomalies from its preset mission. It still needs to recognize a new stop sign, but if a stop sign disappears, it will stop anyway while waiting for an answer on "was it was taken out on purpose, or did some kids steal it?".
This is the basis of the SLAM algorithms (SLAM = simultaneous localization and mapping). As you say, if we all had these cars, we'd quickly know about changes. Of course, the hard problem is finding the new lights, but if this was to become mainstream I think a lot of things would change. For example, if a town decides to install a red light, they don't just install one, they register it in a national database.
If the car could map the route automatically, then it wouldn't need the pre-map in the first place. The problem is that creating the map routes currently requires expert human attention. Driving your daily commute manually for a week won't do any good.
Plus, every time the automatic car goes over the road, it's presumably checking its prior maps and flagging any anomalies from its preset mission. It still needs to recognize a new stop sign, but if a stop sign disappears, it will stop anyway while waiting for an answer on "was it was taken out on purpose, or did some kids steal it?".