
They are designed to be used on the road as after all cyclists are not supposed to ride on the pavement, but sadly the UK is not as bike aware/friendly as some other countries where bike trailers are pretty much the norm, and in Holland the cyclist really is king.
I see people with proper bikes (mud guards, bells etc) also trikes with varying types of trailers with shopping dogs kids etc.
Here cyclists really are taking their lives in their hands every time they ride on the road.
If you don't/can't drive what is the alternative, fi some distance is to be covered.
The man may have needed to take the dog to the vet and it wasn't fit to walk that far.