Just with what they showed in this video, imagine someone spends a month going down the same highway every day at the same time inventorying vulnerable vehicles to generate a pattern.I was going to mention that. Many cars are already on a network and can be controlled in some manner. Not usually steering and throttle (maybe Tesla?) but at the least, immobilized. That would be havoc enough.
Then on attack day, cranks the radio, hits the washer fluid and shuts down the engine on those vehicles. It wouldn't take many on the same highway, same time, close enough in proximity to cause absolute pandemonium.