What about his duty to maintain his speed and position to allow
others to merge safely?
Is there such a thing ??
the slip road was clogged up with cars and you could see that it was happening, meaning that they couldn't merge out into the traffic within the number of spaces that were in front of the truck, meaning that it was clearly going to end up with the cars grinding to a halt at the end of the slip road and a dangerous situation resulting.
For that particular case slowing down to allow a bigger gap in front to try to alleviate the problem would have been a sensible thing to do - assuming that the car drivers were astute enough to notice and accelerate out into the gap, which is a long shot.
It would also have avoided the potential of making a mistake when attempting to rapidly change lanes and missing something in your blind spot.