There was a bunch of research on this a while back, essentially saying the driver needs to be paying driving level attention the whole time or the car needs to handle itself completely; you can't just expect someone to go from not driving to driving at 60mph without warning.
I mean, you can probably design autonomous "disengage" modes- hitting the emergency blinkers and heading for the breakdown lane is the extreme default. On a lesser level, the thing could just drive like I do. If a merge is too tricky? Just keep going straight, and recalculate on the next exit.
This is helped by that fact that modern cars seem to have pretty good "Just don't run into something" sensors already, and from my own experience as a bad driver with a decent accident record, not running into other things is most of the battle.
So yeah, I could totally see autonomous cars evolving the ability to safely get themselves off the road. Of course, you're still gonna need to do that a lot less often than every three thousand miles, but you don't have to get it to zero, just around the point where normal cars break down mechanically.