The first automated car needs to handle human drivers well. Therefore all later cars will just inherit the ability to share a road with humans ( and very likely

The first automated car needs to handle human drivers well. Therefore all later cars will just inherit the ability to share a road with humans ( and very likely there will be laws which mandate this until all cars are automated). About your second question, at least at high speeds the capacity of an break is more important for the distance needed to stop the car, than the reaction time. So the situation does not change much.