Navigating off-road might actually be easier in practice for AI than navigating on roads, since off-road there are generally many fewer other cars to deal with.

As a rough rule of thumb, I'll call a car fully self-driving if, without any changes having been made to the road system to accomodate self-driving cars, I feel comfortable getting in the back seat, telling it to drive me to a certain house in a city a few hundred miles away, and falling asleep.