Well, I do use a car that is able to drive (almost) autonomous on a highway, so I know that the tech to drive on highways exist since several years.

All the difficult stuff – slow traffic, parking cars, crossings, pedestrians… – does not exist on highways.

The only problem that still remains is the problem you mention: what to do in case of trouble?

Of course you have to stop on a highway to prevent an accident or in case of an emergency. That’s exactly what humans do. But then humans get out of the car, set up warning signs, get help &c. Cars cannot do this. The result is reported in this article.