Roland Perry wrote:
In message , at 20:09:11 on Wed, 19 Jun
2019, Basil Jet remarked:
I currently do have sight problems but that diagram clearly shows
the slip roads from the new alignment being foul of the existing
layout. I'm hoping to go and see the actual documents in the library
tomorrow so may get a better idea then.

The bridges are all in the centres of the junctions, and the roads in
those area are unchanged, unlike the roads on the southern part of the
M4 junction or the northern part of the T5 junction. (I'm not counting
gantries as bridges.)

Talking of gantries; along with lamp-posts, central reservation
barriers, and all the other street furniture, they'd have to be removed
along the affected stretches to make the "set of points, with road cones
swapping the flow overnight" operation postulated up-thread.

Yes, that's true. There would need to be some overnight closures leading up
to the actual switch. Some items could be removed well in advance, during
other works. Removal of overhead gantries would obviously require overnight
closures, but could be done well in advance. Presumably there won't be more
than one overhead gantry in each of the shirt connection zones.

But quite a lot could be done with just lane closures. For example, the
central reservation won't be affected while the northbound carriageway is
moved across in two stages. Later, when it's time to move the southbound
traffic, much of the structure removal and connection work will be done
during closures of the fast lane. The final switchover will require an
overnight closure while the 'points are switched'.