Basil Jet wrote:
On 20/06/2019 01:04, Recliner wrote:
Basil Jet wrote:
On 20/06/2019 00:09, Recliner wrote:
Basil Jet wrote:
On 19/06/2019 21:07, Recliner wrote:

You first connect the completed new carriageway and its M4 slip road to the
old slip road just before it splits into the east and west bound links. For
the next few months, traffic heading for the M4 will be diverted to the new
northbound carriageway, while through traffic will continue to use the
existing carriageway. During this time, the new carriageway will be built
through the old northbound slip road to connect to the ood carriageway.
Again, there will be and closures for a few weeks and an overnight complete
closure as the final connection is made.

Southbound is easier, but, again, connecting traffic from the M4 might
continue to use the old carriageway for a little while after the through
M25 traffic has been diverted to the new carriageway.


I don't know what the limit is on how close junctions are allowed to be
on motorways, but that might put the T5 junction and the M4 junction too
close together during the interim, leading to dangerous weaving.

Why would they be any closer than they are now?

Because at the moment the traffic for the M4 leaves the main carriageway
a fair distance north of the A4, whereas you would have this traffic
using the new tunnel route (and the through M25 traffic using the old
route) for a few months, which puts the bifurcation point inches north
of the convergence point at the north end of the T5 junction. That's not
going to work. Similar for the southbound. The traffic to and from the
M4 and the traffic to and from Watford has to remain together throughout
the construction to avoid dangerous weaving at the north end of the T5
junction (although obviously the northbound can switch to the new tunnel
months before the southbound does).


I don't think it would be possible for the northbound through and M4
junction traffic to stay together throughout, as the new through route cuts
through the existing slip road to the M4. So there would have to be at
least a short period of a few weeks of separation while the through route
is linked at the northern end, through the current M4 junction slip road.
Maybe there would have to be restrictions on the use of the junctions
during that transition period? For example, T5 to M4 traffic might be
rerouted.

Southbound might be easier, and it might be possible to keep the traffic
flows together. Or, again, M4 to T5 traffic could be temporarily rerouted
for a few weeks.


The M4 to T5 or T5 to M4 isn't the problem, because it keeps left
through the pinch point. The problem northbound is the Gatwick to
Slough traffic cutting from right to left exactly where the T5 to
Watford traffic is cutting from left to right, and southbound the Slough
to Gatwick traffic cutting from left to right exactly where the Watford
to T5 traffic is cutting from right to left.

So there is no way the bifurcation point or merge point south of the M4
junction will be moved to the north end of the T5 junction even for a
one minute period, unless the motorway was down to one lane there, which
is only feasible in the middle of the night.


Perhaps the simplest approach would be to close the T5/M25 northbound
connection for a short period while they work round the clock to connect
the new through carriageway at the northern end, cutting through the
existing slipway. T5 traffic could be diverted via the A3113 or Colnbrook
Bypass.