Great Western Coffee Shop

South Glos have made a strong case for a new junction on the M4 (https://consultations.southglos.gov.uk/consult.ti/M4J18a/consultationHome), just over 2km to the east of the current M32 junction (J19) near where the A432 currently crosses.

There's not really much of a public transport angle to this story; it should take traffic away from the overheated A4174/M32 junction, and one has to assume that the highways engineers have considered the possibility that the traffic jams will just move to the new junction...

The thing that really caught my eye was the cost of this scheme: £428M. Four hundred and twenty-eight million pounds. Goodness me, that would go a long way towards paying for MetroWest...

Interesting to see that there were so many options/suboptions considered. I'd only heard reference to "Western" and "Eastern" options, but it was more than that.

Correction: the proposed junction is at the Westerleigh Road bridge, which is just over 3km east of the M32 junction.

Looking at the documents I see that they plan to shift the M4 north of its current alignment, which goes a little way towards explaining the cost... nonetheless £428M is an astonishing amount of money; just remember that the Borders Railway project came in at around £300M for 30 miles of track; Filton Bank was £33M. Portishead, including its controversial platinum rails, is estimated at £175M...

Why, in this country, do we seem to be incapable of doing any transport project properly and at the time that it is needed ?? This applies equally to road, rail & air.  This idea was first mooted in the late 90s, along with a flyover/under for the ring road where it crosses the M32.  If it had been done then it would have only cost £42.50 !

£428 million !!  A better idea, buy a purpose built fleet of 5 car (which can be extended to 6 in 10/20 yrs time) dmus with full through corridor, 4 abreast comfortable seating, adequate leg room, a good scattering of fixed tables, fold down tables on the airline seating, working air conditioning and a mini buffet ( ;)) for Cardiff - Portsmouth then save the rest for a Junc 18A scheme when the idea is re-floated in 2038.

Nah ! DfT will cascade the 5 car IETs to that route in 2064 and save billions so they can electrify St Erth to St Ives. (HS7)

So what will (would) happen to existing carriageways? Is the plan to actually demolish them and return the land to some other use or simply to build an short parallel section of motorway to serve the new junction, leaving the existing section in use as the main alignment, a little like a smaller-scale version of what happened with original M4 when the "new" Severn Bridge was built?

I don't think the scheme is fully developed yet. As I understand it though the intention is to demolish most of the old alignment, perhaps using some of it as the basis of a local access road.

When the M4 was built passive provision was made for a service station at Pucklechurch and anyone travelling the M4 in either direction can see the blocked off exit and entry slip roads.

DfT or whoever decreed that the service station would not happen but left in limbo that a link to Yate could utilise the infrastructure but were so long winded that any land to make this connection was sold to housing developers and houses were built scuppering that plan. That was the eastern option. A revised eastern option was mooted which would have taken traffic on a circular route round Pucklechurch to join the Ring Road at the Shortwood roundabout but this was rejected fiercely by the Pucklechurch community so this left just the western option.

As this saga has gone on for years land immediately adjoining the M4 itself was sold to, yet again, housing developers. This means that to provide the requisite westbound exit and entry slip roads to be constructed these must use the existing alignment of the M4 so a kink in the motorway proper has to be provided. Authority dithering again.

Yes; you can see the 'passive provision' near Yate here: https://goo.gl/maps/CoUJfga18JA2

...which is not to be confused with the 'road to nowhere', running more or less from 5 o'clock to 11 o'clock in this view: https://goo.gl/maps/quxZWvv2CFq

Isn't the re-alignment more to do with the need to build the new junction without bringing the M4 to a halt for a couple of years, though? Whatever they do it will be necessary to divert traffic around the works, but by building the new junction off the existing line they can presumably minimise the cost of diversions.

No, the current M4 boundary fence is shared with a massive DPD couriers depot and a massive warehouse building currently under construction leaving nowhere to shoehorn in the westbound M4 exit and entry slip roads as I stated. The Lyde Green housing development then continues and shares the M4 boundary fence with a continuation of the road named Jenner Boulevard beyond with its junction with Willow Herb Road.

Everything you say is true, but I don't share your conclusion that this is the result of a planning cockup. If you look at the first option in SOBC_Appendix_B_Scheme_Drawings.pdf, you can see that most of the new junction could be constructed with minimal disruption to the current M4. Having had daily experience of the congestion and delay caused by a much simpler scheme to throw the MetroBus bridge over the M32, I see this as preferable.