People who slow to look at accidents, even just a minor dent. Sometimes it is just someone broken down with a police car present. I sat n a few long tailbacks on Sunday, all just people gawping at the hard shoulder.
Worse when you have been in crawling traffic for 20 minutes and find it so people can have a good look at the other side of the road.
I know people have doubts about autonomous cars, but at least they will not block a motorway while they slow down to look at a parked car. If you like that sort of thing, go to the car park at ASDA, it is full of them. Leave the motorways for people who want to get somewhere.