An autonomous car does need to be 100%. Otherwise it isn't autonomous. Just cool drivers assist. Whole different ball game.
If it is fully autonomous for 90% of the roads, then I would say that it is autonomous depending on the situation... don't take the car on some back country roads and you will be fine. And the key piece of interstate driving, if they can make that work at 100% then that would be enough to say from a legal sense that if you are on the interstate and you have the car in auto-pilot/whatever then you can do whatever you want because the car is capable of being fully in control. As the example was given, it will work like in "I, Robot" where you drove the car yourself until you hit the special highway in which you could turn the driving over to the car. I would also hope at some point this enables them to let us raise the speed limits on the highway since the computer will be able to react faster than a human.
I would think the second wave of autonomous driving will be it working 100% in major metropolitan areas... which as I said, 70% of the US Population lives in a major metropolitan area. So unless you go to visit your grandparents farm in nowhere Kansas... you will be fine, and it will be 100% autonomous. This will enable a new brand of "zip-2" or "lyft" where you hail an autonomous taxi that will be able to take you where you want to go within a certain limit.
The last and final hurdle, which may very well never be fully overcome, is unmarked country roads. But again, that doesn't mean you aren't 100% autonomous under certain conditions. I again go back to my example of how 20% of the US population still doesn't even have the internet. Yet, almost the entire US market relies on the internet in order to function. Autonomous cars will be the same way, it will work in most areas, but not all... so if you want to venture into the Australian outback, I'm sorry, you will just have to drive yourself out there...