I don't know where you live, but where I live the highways have lots of stopping during busy times. And of course drivers always have to be aware of stopped cars as they do happen. I suspect highways are where the automated cars have the easiest time, and it is far more likely side roads and suburban areas that are more confusing. Abrupt stops are more likely there (eg kids/pets running across the road). In the US I do believe that running into the back of another car under almost all circumstances makes you at fault.
Where I live a panic stop in heavy traffic on anything called a "highway" could get you killed. I imagine a lot of the stops we see on the highway that do not result in accidents are made safer by the network effects of lots of cars doing the same thing over time and communicating visually. It's different in nature than somebody (or some machine) getting confused and stopping suddenly in the middle of the road.
Don't get me wrong: things aren't so grim for the "just stop" approach of dealing with a problem. There's no reason an automated vehicle won't be able to communicate with other cars, warn everyone of an emergency stop (at a minimum), even find the best path off the road, depending on how serious the problem is.
