Unfortunately the thing to think about here is that as these things become more mainstream, there will be less people who are even skilled enough to control a car. So in the long run, if the system works well, any part of the design that relies upon human intervention is doomed. Ultimately your stopgap would become the equivalent of giving the wheel to the child in the backseat: more dangerous than just letting the computer do its best.
On the other hand, it will be best case a decade or two before these things are out on the road en masse and even then only the youngest drivers wouldn't have gained many hours of driving experience. Meanwhile, in that time both automated car and sensor tech will have advanced quite a bit. It's only been a decade since that first DARPA challenge and five years since google started their project, after all.
For consumer use (as opposed to an experimental vehicle), this is never an option. Even if you assume the person in the car is (present, of-age, licensed, awake, sober), the context switch of promptly having to drive on a busy road after hours/days/months of not having driven would be terrible.
In a driver-less future, all cars communicate their intents to other nearby cars, stopping on a highway will be as safe as pulling over on a shoulder is today. It will feel dangerous, and it will be unsettling knowing cars are passing you by at speed, but the driver-less aspect of all cars ensures a high degree of safety.
That would be the result in the long term. But in this transition period TO driverless cars, I agree that stopping on the highway in your driver-less car is a big no-no.
I believe car-to-car communication and auto-slowing-down based on that will come much sooner than driverless cars (US regulators are already talking about making V2V communications mandatory by 2017), so by the time the first driverless cars are sold, it'll probably be reasonably safe to slow down on the highway.
