What is still left to be done? Google has extremely detailed mapping and the self driving car works well in real traffic. (One advantage a car has over planes is that if things go horribly wrong it can just stop.) http://en.wikipedia.org/wiki/Google_driverless_car

There aren't any technical unknowns left. Obviously the algorithms will continue to be tweaked and improved. Importantly it can operate in the existing environment - there is no need for every car to switch to driverless at once.