A358 Taunton to Southfields

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jackal
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Re: A358 Taunton to Southfields

Post by jackal »

^ Preparatory works for the main scheme perhaps?
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Re: A358 Taunton to Southfields

Post by Herned »

HE's design contractor is doing some exploratory work before the preferred route announcement
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Re: A358 Taunton to Southfields

Post by Chris5156 »

Herned wrote: Mon May 24, 2021 21:59 HE's design contractor is doing some exploratory work before the preferred route announcement
I have some drawings of how to connect an expressway to a motorway without passing through half a dozen congested roundabouts, if they'd like to explore that at all :wink:
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Re: A358 Taunton to Southfields

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Chris5156 wrote: Tue May 25, 2021 00:30
Herned wrote: Mon May 24, 2021 21:59 HE's design contractor is doing some exploratory work before the preferred route announcement
I have some drawings of how to connect an expressway to a motorway without passing through half a dozen congested roundabouts, if they'd like to explore that at all :wink:
Yes, but that's your collection of Bad Junctions, isn't it? 8-)
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Re: A358 Taunton to Southfields

Post by Herned »

Chris5156 wrote: Tue May 25, 2021 00:30 I have some drawings of how to connect an expressway to a motorway without passing through half a dozen congested roundabouts, if they'd like to explore that at all :wink:
Sadly I suspect they will remain as drawings
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Re: A358 Taunton to Southfields

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Apparently this scheme won't be submitted to the Planning Inspectorate until next summer. However, the scoping report appendices include drawings with far more detail than the PRA, including a dumbbell at the A378, diamond with priority junctions at Ashill, and interestingly, distinct flaring at Southfields - presumably future proofing for a GSJ. But what of the promised improvements at J25? Nothing at all - predictably the scheme just stops at the crappy new Nexus 25 roundabout.

https://infrastructure.planninginspecto ... %206.3.pdf
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Re: A358 Taunton to Southfields

Post by JonB2028 »

jackal wrote: Mon Aug 09, 2021 17:03 and interestingly, distinct flaring at Southfields - presumably future proofing for a GSJ.
That would be an uncommon application of common sense. It does look like that, for a nice sweeping radius linking to the Ilminster bypass. But, I wonder can that be done, can the land be acquired based on a non-confirmed future scheme, that is still at the aspirational stage (stage 0)?
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Re: A358 Taunton to Southfields

Post by Herned »

jackal wrote: Mon Aug 09, 2021 17:03 Apparently this scheme won't be submitted to the Planning Inspectorate until next summer. However, the scoping report appendices include drawings with far more detail than the PRA, including a dumbbell at the A378, diamond with priority junctions at Ashill, and interestingly, distinct flaring at Southfields - presumably future proofing for a GSJ. But what of the promised improvements at J25? Nothing at all - predictably the scheme just stops at the crappy new Nexus 25 roundabout.

https://infrastructure.planninginspecto ... %206.3.pdf
Thanks for sharing, some 'interesting' choices with the local road network which I can't see getting through without objections

The flaring at Southfields implies that the GSJ will end up similar to the A31-A331 junction which is a better outcome than I expected, but it's a shame there is no future proofing at the M5 end.
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Re: A358 Taunton to Southfields

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Herned wrote: Mon Aug 09, 2021 18:00
jackal wrote: Mon Aug 09, 2021 17:03 Apparently this scheme won't be submitted to the Planning Inspectorate until next summer. However, the scoping report appendices include drawings with far more detail than the PRA, including a dumbbell at the A378, diamond with priority junctions at Ashill, and interestingly, distinct flaring at Southfields - presumably future proofing for a GSJ. But what of the promised improvements at J25? Nothing at all - predictably the scheme just stops at the crappy new Nexus 25 roundabout.

https://infrastructure.planninginspecto ... %206.3.pdf
Thanks for sharing, some 'interesting' choices with the local road network which I can't see getting through without objections

The flaring at Southfields implies that the GSJ will end up similar to the A31-A331 junction which is a better outcome than I expected, but it's a shame there is no future proofing at the M5 end.
So how would a GSJ at Southfields work then? How would it prioritise the A303 A358 mainline?
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Re: A358 Taunton to Southfields

Post by Herned »

Richardf wrote: Mon Aug 09, 2021 19:15
So how would a GSJ at Southfields work then? How would it prioritise the A303 A358 mainline?
Presumably something like this, with the green being the mainline and a new eastbound slip in blue:

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Re: A358 Taunton to Southfields

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Herned wrote: Mon Aug 09, 2021 19:21
Richardf wrote: Mon Aug 09, 2021 19:15
So how would a GSJ at Southfields work then? How would it prioritise the A303 A358 mainline?
Presumably something like this, with the green being the mainline and a new eastbound slip in blue:

Image
Thanks I see now.
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Re: A358 Taunton to Southfields

Post by A320Driver »

I don’t share the optimism that the Southfields end is future-proofed. The flaring looks more to do with the radius of the bend, I have noticed plenty of examples across the country which are similar.
That said, it looks possible to get at least one carriageway through the gap and over the southbound A358 towards A303 eastbound.
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Re: A358 Taunton to Southfields

Post by Truvelo »

This was the original proposal. The alignment of the through route appears the same with just the slip road having a smoother line in the current proposals. I agree with the flaring on the bend northwest of this being for sightline purposes.
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Re: A358 Taunton to Southfields

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I don't see how this could be for sightline purposes. The rest of the scheme does of course have some sections of wider central reservation for sightlines but on a much smaller scale.

Furthermore, the curve into Southfields is only as sharp as it is because the eastbound carriageway takes such a wide line, so this theory would require that the flaring is to improve sightlines, which is only needed because of the flaring!

Also, a subtle point, but the curve into Southfield slightly "overshoots" (i.e., it actually turns gently left before it joins the existing roundabout approach), which makes no sense for slightlines purposes, but plenty of sense if you're trying to reduce the skew on a future bridge (Truvelo's picture of the previous plan shows a much more extreme version of the same skew-reducing overshoot).

The future proofing theory fits with the alignment much better than the sightlines theory and is very straightforward given "A303 Phase 2 upgrade" is in the RIS pipeline.
Last edited by jackal on Tue Aug 10, 2021 14:23, edited 1 time in total.
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Re: A358 Taunton to Southfields

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jackal wrote: Tue Aug 10, 2021 11:08 I don't see how this could be for sightline purposes. The rest of the scheme does of course have some sections of wider central reservation for sightlines but on a much smaller scale.

Furthermore, the curve into Southfields is only as sharp as it is because the eastbound carriageway takes such a wide line, so this theory would require that the flaring is to improve sightlines, which is only needed because of the flaring!

Also, a subtle point, but the curve into Southfield slightly "overshoots" (i.e., it actually turns gently left before it joins the existing roundabout approach), which makes no sense for slightlines purposes, but plenty of sense if you're trying to reduce the skew on a future bridge (Truvelo's picture of the previous plan shows a much more extreme version of the same skew-reducing overshoot).

The future proofing theory fits with the alignment much better than the sightlines theory and is very straightforward given "A303 Phase 2 upgrade" is in the RIS pipeline.
Although from a cost and buildability point of view the future bridge would be better served by being squarer as the original proposal. As the future slip road will eventually become a low speed approach to Southfelds roundabout with only local traffic (so possibly only one lane unless resilience/maintenance requirements dictate otherwise) why bother safeguarding at all for an expensive skewed bridge but just rebuild the slip square to the future mainline?
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Re: A358 Taunton to Southfields

Post by AAndy »

If you forget coldra's (jcn24 m4) grade seperated bit (m4) the roundabout is much busier (than ilminster) but works well. The 2 unsignalled roads users might complain a bit, but even at rush hour there is rarely much of a queue that I've seen. put signalling into ilminster, a slip eastbound and a slip though the middle for westbound A303 ...the space is there. Coldra is a showcase for how signalling can improve a busy roundabout.
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Re: A358 Taunton to Southfields

Post by Chris5156 »

jackal wrote: Mon Aug 09, 2021 17:03 Apparently this scheme won't be submitted to the Planning Inspectorate until next summer. However, the scoping report appendices include drawings with far more detail than the PRA, including a dumbbell at the A378, diamond with priority junctions at Ashill, and interestingly, distinct flaring at Southfields - presumably future proofing for a GSJ. But what of the promised improvements at J25? Nothing at all - predictably the scheme just stops at the crappy new Nexus 25 roundabout.

https://infrastructure.planninginspecto ... %206.3.pdf
Three out of four isn’t bad I suppose. Designing suitable junctions where one high speed road terminates on another is not HE’s strongest suit so this was perhaps inevitable, and certainly no big surprise given what we’ve seen of this scheme up until now.

Still, it gives us something to look forward to, which is seeing the plans for free-flow sliproads when the M5 interchange improvement is announced as part of RIS4 or RIS5 :laugh:
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Re: A358 Taunton to Southfields

Post by Truvelo »

Regarding the M5 junction the 2002 proposals had a freeflow link to the M5 south but not the reverse even though there is room for it and it would avoid having to pass through four arms of the roundabout. Even with the park & ride and other development taking place the 2002 scheme would still be possible now.
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Re: A358 Taunton to Southfields

Post by ikcdab »

Back in 2002 (approx) there was a suggestion of a second M5 junction which would have taken the A358. That now seems to have disappeared.
I struggle to see how the current plans will improve things. The A358 is currently clogged and at a standstill for most of the day. So what is different now? The new link is clearly designed to take traffic away from the A303 West of ilminster, so even more traffic will be barrelling along the improved A358 and just come to a grinding halt at the j25 signal controlled roundabouts. I see nothing here to ease the standstills on the A358. Am I missing something?
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Re: A358 Taunton to Southfields

Post by Herned »

ikcdab wrote: Tue Aug 24, 2021 18:19 Back in 2002 (approx) there was a suggestion of a second M5 junction which would have taken the A358. That now seems to have disappeared.
I struggle to see how the current plans will improve things. The A358 is currently clogged and at a standstill for most of the day. So what is different now? The new link is clearly designed to take traffic away from the A303 West of ilminster, so even more traffic will be barrelling along the improved A358 and just come to a grinding halt at the j25 signal controlled roundabouts. I see nothing here to ease the standstills on the A358. Am I missing something?
Yes, there will be two lanes for traffic to queue in instead of one! It will also make life more bearable for the residents of Henlade, and no doubt provide lots of lovely development opportunities.

On a more serious note, the works to J25 seem to have reduced the queueing issues a fair bit, even during the very busy holiday season so it might not be too bad, at first. I hope they are safeguarding the land for a direct link to the M5, but I wouldn't count on it
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