16.5m or 18.75m in length

Can someone clarify for me what differentiates the two?

I understand 16.5m is for artics and 18.75m for wagon and drag but what would, for example, a car transporter be if connected with a hitch and not a fifth wheel? Is that then a w+d as it uses the same coupling?

Would a fifth wheel car transporter be 18m because it has a low loader or does that only apply to a single deck?

You mean a Estate with a Glider Plane on a trailer?

The way I’ve understood it is, if its on a fifth wheel above the axle its an artic, if its a hitch behind the axle its a wagon and drag.

So your example of a car transporter on a fifth wheel would need to be 16m.

If the towing vehicle is carrying the trailer then its an artic if not its a wagon and drag, there are car transporters with a fifth wheel that are a wagon and drag but thats another story to do with demounts.

Ian G:
Can someone clarify for me what differentiates the two?

2.25 metres differentiates the two.