[_ph_]

Sorry, a car can go slow for many good reasons not visible to the following traffic (and be it only, that something in front of that car is slow, but you cannot see it). You have to be prepared to that. A car can even be entirely stopped and you should be able to stop without hitting it. Failing to do so is entirely the fault of the following traffic.

It is an entirely different matter, that one isn't allowed to drive unnecessarily slow to not block the following traffic. This is about the traffic flow, not of safety.