Wrong way lights

[Grahamm]

I think the exit is standard width, and is that width so that articulated vehicles can exit with ease. 

Ok, I'd not considered that, but I can see the reasoning.

In that case, they could try what's been done on some roundabouts here, where there's an outer circle of red "bricks" (or something, that's what they look like) which are intended to encourage people to drive around them, rather than cut across.

If those were put at the entrance, there would still be space for the artics to turn whilst giving the visual impression of a narrow entry.

Ok, the Wrong Way signs are probably cheaper, but the less the chances of someone turning up there in the first place, the better.