They have, but there's a big list of caveats that go with that. Google's cars are very clearly extremely safe on dry highways in good visibility when humans are ready to take over if there is construction on the highway and so on and so forth. That's where they've got hundreds of thousands of miles.

The cars may well be substantially better than that right now, but they don't have a million miles of experience in more challenging conditions.

Provided the roads are also very well-mapped.

Google (as well as some auto manufacturers) have made impressive progress. I'm actually encouraged that full autonomy (as in I can legally read a book) on a subset of limited access highways in appropriate weather conditions is something we might see within 10 years on a mainstream luxury car.

I am sadly far more skeptical about the general case.

Not just well mapped but also what do they do when there are temporary detours? I can't imagine a situation where an automated driving car copes well with a temporary human based stop/go situation (without signs - I'm talking really temporary here like a construction site with a vehicle backing up). Maybe I'm wrong.

I've been thinking about this. What do humans do when they don't know exactly where they are or there are detours/other off map difficulties? They mindlessly follow the cars in front of them. Waze at at small scale! The autonomous cars just need to a) not be willing to crash and b) follow the car(s) in front of them when in doubt...

The problem can be separated into navigation and situational awareness. The autonomous car won't ever be lost, and I guess it would be easy enough to prompt the passenger for input if it got stuck deciding where to proceed.