A303 Phase 2 Upgrade

[SouthWest Philip]

Any widening of the Ilminster bypass might also include reconstruction/realignment of the old section of dual carriageway immediately east of South Petherton too.

[Herned]

Maybe I'm overestimating the complexity of it, but it's more than just building another carriageway. The existing one will need to be extensively refurbished - it's three lanes wide, for a start, and has lay-bys on both sides at regular intervals. So a new carriageway will need to be built, yes, but it may or may not need to occupy some of the space used by the existing road, and the bridges are an issue since they appear to need replacement on their existing line, which means either long road closures, or temporary bridges built alongside for the duration of the works, or building new ones alongside and sliding them into place. All possible but fiddly. Plus the existing road is concrete and NH might want to take the opportunity of major works to dig it out and rebuilt it with a tarmac surface, since they now have a policy of getting rid of concrete roads.

In addition to that, the roundabouts at South Petherton and Southfields will be reasonably major GSJs, and South Petherton in particular will have to be built on-line, whereas the other remaining dualling schemes further east will be simpler in terms of junction works. On the Wylye to Mere section, for example, there's only one major junction to consider, for the A350, and that is half built already - a second bridge and a second set of sliproads will finish it without any of the existing junction having to be rebuilt. The existing bridge is even built wide enough to add in a deceleration lane for when it's dualled.

So, I'm not an expert and I'll be interested to see the scheme design when it turns up, but my gut feeling is that Ilminster is the more complex construction project and it'll be good to get it out of the way.

Fair points... but I think given the geography, a new carriageway entirely along the south side of the existing bypass is possible. For the Mere-Chicklade section, there needs to be a totally new alignment past Chicklade and further west, and that it will probably need to cross over and interact with the existing road more. There's also a lot more earth-moving required.

Re the bridges, the B3168 bridge probably wouldn't need replacing as long as the river can be culverted, and the one nearest the South Petherton interchange could be replaced by a direct connection to the roundabout, so that would leave three to be rebuilt which are in open country. They could be replaced by new bridges built offline, which in the case of two of them would mean on the line of the road before the bypass was built.

The South Petherton GSJ will be interesting, I would assume the cemetery is untouchable so there will need to be some other property demolition, unless they come up with something very clever

[jackal]

I would like to suggest a minimalist interpretation of what is required. The carriageway is 3 lanes, so only one extra lane and central reservation are needed. I don't see that an additional carriageway needs to be built alongside, as there is no need for five lanes. It would be more like widening a well aligned S2 to S2+1, or D2 to D3, neither of which typically requires a new carriageway.

More speculatively, it looks to me like almost all the overbridges are wide enough for a D2 using modern techniques (narrow CR with concrete barrier, and more barriers on the outside so the bridge supports can be close to the carriageway). The skewed bridge looks like it maybe isn't wide enough, though it could be the angle is misleading.

I could be wrong of course and they'll go for a more old fashioned replacement of the bridges with full verges etc, but just thought the minimalist possibility was worth flagging. One point supporting a fuller rebuild is that the carriageway is presumably designed for 60mph rather than 70mph, though anecdotally it does feel very dual carriageway-like in terms of how straight the alignment is - possibly it was actually designed for 70mph as a form of future proofing?

As for the GSJs, Southfields is futureproofed by the A358 scheme, so the layout is pretty well understood - the mainline will cut the corner of course. At South Petherton, I imagine the fields to the east of the roundabout will be eaten up to allow a smooth alignment. The alignment may also be straightened west of the roundabout with the Moor Lane bridge replaced to facilitate this. No demolition should be required.

[Herned]

As for the GSJs, Southfields is futureproofed by the A358 scheme, so the layout is pretty well understood - the mainline will cut the corner of course. At South Petherton, I imagine the fields to the east of the roundabout will be eaten up to allow a smooth alignment. The alignment may also be straightened west of the roundabout with the Moor Lane bridge replaced to facilitate this. No demolition should be required.

You're right, I agree, the solution is not to have the GSJ in the same place as the existing roundabout, something like this

[jackal]

A letter from December to a local MP confirms that "the scope of the A303 Phase 2 upgrade scheme identified for initial development as part of the RIS3 Pipeline will be South Petherton to Southfields".

https://www.andrewmurrison.co.uk/sites/ ... letter.pdf

As for the Southfields GSJ, Herned basically has a right. A wonder if a cheap version reusing the quite generously proportioned Moor Lane Bridge would be possible?

[Richardf]

I would like to suggest a minimalist interpretation of what is required. The carriageway is 3 lanes, so only one extra lane and central reservation are needed. I don't see that an additional carriageway needs to be built alongside, as there is no need for five lanes. It would be more like widening a well aligned S2 to S2+1, or D2 to D3, neither of which typically requires a new carriageway.

More speculatively, it looks to me like almost all the overbridges are wide enough for a D2 using modern techniques (narrow CR with concrete barrier, and more barriers on the outside so the bridge supports can be close to the carriageway). The skewed bridge looks like it maybe isn't wide enough, though it could be the angle is misleading.

I could be wrong of course and they'll go for a more old fashioned replacement of the bridges with full verges etc, but just thought the minimalist possibility was worth flagging. One point supporting a fuller rebuild is that the carriageway is presumably designed for 60mph rather than 70mph, though anecdotally it does feel very dual carriageway-like in terms of how straight the alignment is - possibly it was actually designed for 70mph as a form of future proofing?

As for the GSJs, Southfields is futureproofed by the A358 scheme, so the layout is pretty well understood - the mainline will cut the corner of course. At South Petherton, I imagine the fields to the east of the roundabout will be eaten up to allow a smooth alignment. The alignment may also be straightened west of the roundabout with the Moor Lane bridge replaced to facilitate this. No demolition should be required.

Widening like that would be pretty tight, if indeed it's possible (not convinced the bridges are wide enough). Be a pretty low standard D2 if it is possible.

You are assuming dualling would be In the form of putting a second S2 alongside with new bridges where needed, but surely widening of the existing road to D2 standard is the other option. Either symmetrical, where the middle lane becomes the central reservation and you add a lane each side or assymetrical by making 2 lanes one carriageway the 3rd becomes the reservation and you add 2 new lanes alongside. Bridges would need replacing in any case.

[jackal]

You are assuming dualling would be In the form of putting a second S2 alongside with new bridges where needed

In the material you quoted I expressly said the opposite: "The carriageway is 3 lanes, so only one extra lane and central reservation are needed. I don't see that an additional carriageway needs to be built alongside, as there is no need for five lanes. It would be more like widening a well aligned S2 to S2+1, or D2 to D3, neither of which typically requires a new carriageway".

[Richardf]

Having used this road recently i'm not sure why they dont get on with dualling some of the other sections while Stonehenge is still being planned and decided. All would be far simpler to do and would then be in place to give full benefit to the Stonehenge plan once that is done. Would be no different to what happened with the A3 and Hindhead.

[Fluid Dynamics]

Whilst I don't disagree, and the decades long improvements of the A3 from the 60s to the 90s meant that something had to be done at Hinehead as it was Motorway or HQDC either side, you could say that at least the Government is trying do something about Stonehenge rather than leave it to last.

Unfortunately the money sitting against the Stonehenge scheme can't be used twice while it awaits planning determination, nor can money just be allocated to another A303 scheme not in the current RIS tranche without National Highways and in turn the Department of Transport needing to go back to HM Treasury cap in hand. Success in the current environment is likely to be close to zero.

[Richardf]

Whilst I don't disagree, and the decades long improvements of the A3 from the 60s to the 90s meant that something had to be done at Hinehead as it was Motorway or HQDC either side, you could say that at least the Government is trying do something about Stonehenge rather than leave it to last.

Unfortunately the money sitting against the Stonehenge scheme can't be used twice while it awaits planning g determination, nor can money just be allocated to other A303 not in the current RIS without National Highways and in turn the Department of Transport needing to go back to HM Treasury cap in hand. Success in the current environment is likely to be close to zero.

True, but I'd much rather see the likes of Ilminster or Chicklade, or even just Stockton Wood (probably the simplest of all to do) done or at least started before Stonehenge gets going.

Ilminster particularly as I feel that's just as important a scheme as Stonehenge, possibly more so in terms of benefits to the wider network.

[SteveA30]

Especially as it was closed again today. A car stopped in the small hours presumably without lights on, as a lorry hit it at speed.

https://uk.movies.yahoo.com/serious-car ... WFV1Bs3QOH

Reopened 1 pm, just under 12 hours from closure.