If you’re following a vehicle with proper distance and it slams the brakes you should be able to stop in time as you’ve calculated their trajectory and a safe speed behind.
You dont need to calculate their trajectory. It’s enough to know your own.
If a heavy box falls off a truck and stops dead in front of you, you need to be able to stop. That box has no trajectory, so it’s an error to include other vehicles trajectories in the safe distance calculation.
Traffic can move through an intersection closely by calculating a safe distance, which may be smaller than the legal definition, but still large enough to stop for anything suddenly appearing on the road. The only thing needed is that the distance is calculated based on your own speed and a visually confirmed position of other things. It can absolutely be done regardless of the speed or direction of other vehicles.
Anyway. A backwards facing truck is a weird thing to misinterpret. Trucks sometimes face backwards for whatever reasons.
It would be interesting to know how the self driving car would react to a ghost driver.