(Looks like we need a "Truck/Lorry" subcategory)
Sometimes I see a construction vehicle being hauled around on a flatbed trailer, pulled by an ordinary hauler truck. Very often there is a "WIDE LOAD" sign, too.
It seems to me that if the construction vehicle, by itself, is not considered to be
a wide load, then hauling it around should also not qualify as a wide load. Except, normally, the flatbed trailer needs to be taken into account. If it is wide enough for the construction vehicle to safely be driven onto/off it, then that generally makes the trailer wider than the construction vehicle, and "wide load" could indeed apply.
On the other hand, what exactly determines what makes it "safe" for a vehicle to drive onto/off of a flatbed trailer? It seems to me that anything that can guarantee the wheels of the vehicle don't fall off the edge of the trailer, during the onto/off process, would suffice.
Let's try an ASCII sketch:
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If the above represents part of a flatbed trailer oriented left-to-right, with the solid lines being its edges, then the dotted lines represent the movable rails mentioned in the subtitle of this Idea. We move them closer together or farther apart, depending on the distance between the wheels of the vehicle we want to load onto the flatbed trailer.
The height of the rails should be perhaps 1 foot (30 cm), to ensure the wheels can't drive over them; the rails and the space between them should be considered as being "straddled" by the vehicle when it is driven onto the flatbed trailer. This means that the actual width of the flatbed need not be any greater than the width of the vehicle being hauled about.
Tie-downs for the loaded vehicle would still be appropriate, but that is not what this Idea is about.
It means that a "wide load" sign would not be necessary.