A358 Taunton to Southfields

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SteveM
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Re: A358 Taunton to Southfields

Post by SteveM »

Surely though, looking at it holistically, being so close to the main roundabout the Nexus cross roads can be considered part of the overall 'junction 25' signalisation can't it? As long as there's some intelligence on the lighting stages, drivers will have to stop at least once at J25 + Nexus, so I really don't see what the gripe is. It's certainly too close to J25 itself to allow a GSJ for Nexus alone.
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jackal
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Re: A358 Taunton to Southfields

Post by jackal »

SteveM wrote: Mon May 30, 2022 12:26 Surely though, looking at it holistically, being so close to the main roundabout the Nexus cross roads can be considered part of the overall 'junction 25' signalisation can't it? As long as there's some intelligence on the lighting stages, drivers will have to stop at least once at J25 + Nexus, so I really don't see what the gripe is. It's certainly too close to J25 itself to allow a GSJ for Nexus alone.
The Nexus 25 lights are the only reason that the A358 westbound can't freeflow onto the M5 southbound. A segregated left turn could be provided at the roundabout were it not for the adjacent junction.

Additionally eastbound traffic, having cleared J25, will obviously have lengthened journey times by having to go through another set of lights immediately rather than having a clear run to Southfields.

There are also significant safety issues with the at-grade conflict points at the crossroads, especially given the long run westbound from Southfields. The new design is worse in this respect than the roundabout as there is no deflection, so there is a serious risk of red light running, and higher impact speeds in such cases.

As explained above, the bridge would either have no slips or east-facing only, so no, it is not too close to J25.

It defies reason to spend £500m on a project like this, but introduce significant delays and safety concerns because you won't spend another £10m or so for a basic bridge exactly like those provided everywhere else on the scheme. Classic British roadbuilding false economy.
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Re: A358 Taunton to Southfields

Post by Herned »

jackal wrote: Mon May 30, 2022 12:58 The Nexus 25 lights are the only reason that the A358 westbound can't freeflow onto the M5 southbound. A segregated left turn could be provided at the roundabout were it not for the adjacent junction.

Additionally eastbound traffic, having cleared J25, will obviously have lengthened journey times by having to go through another set of lights immediately rather than having a clear run to Southfields.

There are also significant safety issues with the at-grade conflict points at the crossroads, especially given the long run westbound from Southfields. The new design is worse in this respect than the roundabout as there is no deflection, so there is a serious risk of red light running, and higher impact speeds in such cases.

As explained above, the bridge would either have no slips or east-facing only, so no, it is not too close to J25.

It defies reason to spend £500m on a project like this, but introduce significant delays and safety concerns because you won't spend another £10m or so for a basic bridge exactly like those provided everywhere else on the scheme. Classic British roadbuilding false economy.
:thumbsup:

It's not even a net £10m, as all that tarmac, embankments, drainage etc for the lights won't be cheap either
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Re: A358 Taunton to Southfields

Post by Bryn666 »

Herned wrote: Mon May 30, 2022 14:37
jackal wrote: Mon May 30, 2022 12:58 The Nexus 25 lights are the only reason that the A358 westbound can't freeflow onto the M5 southbound. A segregated left turn could be provided at the roundabout were it not for the adjacent junction.

Additionally eastbound traffic, having cleared J25, will obviously have lengthened journey times by having to go through another set of lights immediately rather than having a clear run to Southfields.

There are also significant safety issues with the at-grade conflict points at the crossroads, especially given the long run westbound from Southfields. The new design is worse in this respect than the roundabout as there is no deflection, so there is a serious risk of red light running, and higher impact speeds in such cases.

As explained above, the bridge would either have no slips or east-facing only, so no, it is not too close to J25.

It defies reason to spend £500m on a project like this, but introduce significant delays and safety concerns because you won't spend another £10m or so for a basic bridge exactly like those provided everywhere else on the scheme. Classic British roadbuilding false economy.
:thumbsup:

It's not even a net £10m, as all that tarmac, embankments, drainage etc for the lights won't be cheap either
Developers presumably complained at the 'extra distance' slip roads would create, whereas this abomination provides direct access onto the A358 in all directions even if it does take 4 months to circle through the entire staging plan for all the turn lanes and pedestrian stages.

Developers can do what they please and NH will always facilitate it, they're basically over a barrel and expected to do it. The real problem lies with the planning system that allows any wealthy party donor to subvert road safety by putting up nebulous 'employment creation' stats for their awful land wasting plans.
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Re: A358 Taunton to Southfields

Post by Herned »

Bryn666 wrote: Mon May 30, 2022 15:05 Developers presumably complained at the 'extra distance' slip roads would create, whereas this abomination provides direct access onto the A358 in all directions even if it does take 4 months to circle through the entire staging plan for all the turn lanes and pedestrian stages.

Developers can do what they please and NH will always facilitate it, they're basically over a barrel and expected to do it. The real problem lies with the planning system that allows any wealthy party donor to subvert road safety by putting up nebulous 'employment creation' stats for their awful land wasting plans.
There aren't any developers. The "business park", which is so far entirely fictional, is promoted by the local district council. They couldn't get a developer interested
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Re: A358 Taunton to Southfields

Post by Bryn666 »

Herned wrote: Mon May 30, 2022 15:18
Bryn666 wrote: Mon May 30, 2022 15:05 Developers presumably complained at the 'extra distance' slip roads would create, whereas this abomination provides direct access onto the A358 in all directions even if it does take 4 months to circle through the entire staging plan for all the turn lanes and pedestrian stages.

Developers can do what they please and NH will always facilitate it, they're basically over a barrel and expected to do it. The real problem lies with the planning system that allows any wealthy party donor to subvert road safety by putting up nebulous 'employment creation' stats for their awful land wasting plans.
There aren't any developers. The "business park", which is so far entirely fictional, is promoted by the local district council. They couldn't get a developer interested
Oh, jeez, that makes it even worse! Why haven't NH just laughed them out of the room then? What's going on?
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Re: A358 Taunton to Southfields

Post by Mapper89062 »

jackal wrote: Mon May 30, 2022 12:58
SteveM wrote: Mon May 30, 2022 12:26 Surely though, looking at it holistically, being so close to the main roundabout the Nexus cross roads can be considered part of the overall 'junction 25' signalisation can't it? As long as there's some intelligence on the lighting stages, drivers will have to stop at least once at J25 + Nexus, so I really don't see what the gripe is. It's certainly too close to J25 itself to allow a GSJ for Nexus alone.
The Nexus 25 lights are the only reason that the A358 westbound can't freeflow onto the M5 southbound. A segregated left turn could be provided at the roundabout were it not for the adjacent junction.

Additionally eastbound traffic, having cleared J25, will obviously have lengthened journey times by having to go through another set of lights immediately rather than having a clear run to Southfields.

There are also significant safety issues with the at-grade conflict points at the crossroads, especially given the long run westbound from Southfields. The new design is worse in this respect than the roundabout as there is no deflection, so there is a serious risk of red light running, and higher impact speeds in such cases.

As explained above, the bridge would either have no slips or east-facing only, so no, it is not too close to J25.

It defies reason to spend £500m on a project like this, but introduce significant delays and safety concerns because you won't spend another £10m or so for a basic bridge exactly like those provided everywhere else on the scheme. Classic British roadbuilding false economy.
Another thing to consider is that if they want everyone going along the holiday route to Devon and Cornwall to go miles out of their way along the A358 instead of taking the shorter but harder to upgrade route through the Blackdowns, if the longer route was freeflowing (in one direction at least) rather than having two congested signalised junctions to pass through, then the detour might be more appealing, especially since many people rely on sat navs to navigate that encourage people away from routes with heavy traffic. Likewise one less busy signalised junction eastbound could be the difference in time between using the two routes in some situations.
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Re: A358 Taunton to Southfields

Post by ChrisH »

I see that the consultation page is modelling a journey time saving of around six minutes in each direction, down from 17 to 11 minutes each way (both peak and off peak journey times are projected to be very similar in 2028). However, the current journey time on the existing road varies from 12 minutes off-peak to 20 minutes in the peak. And the A358 route is currently around 5 minutes slower than the A303 route to Exeter, which means that journey times will need to come down to about 7 minutes in order to be seen as quicker by navigation software. The modelling indicates that this won't happen, which makes me question whether navigation software will choose it for long-distance routes.

EDIT: I've now read the technical traffic note. The signalised junction at Nexus is projected to add up to 45 seconds delay on its own to the A358 journey times, whereas the unsignalised Southfields roundabout adds around 10 seconds of delay. The M5 J25 roundabout itself is not modelled in this report, but was presumably modelled before.
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Re: A358 Taunton to Southfields

Post by Herned »

Bryn666 wrote: Mon May 30, 2022 15:42 Oh, jeez, that makes it even worse! Why haven't NH just laughed them out of the room then? What's going on?
A good question. The recent scheme to widen the roundabout and build the tiny segment of dual carriageway was paid for by Somerset County Council, which included the access to the business park. Presumably the agreement included maintaining that access as and when the A358 dualling happens
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Re: A358 Taunton to Southfields

Post by Herned »

ChrisH wrote: Mon May 30, 2022 17:08 I see that the consultation page is modelling a journey time saving of around six minutes in each direction, down from 17 to 11 minutes each way (both peak and off peak journey times are projected to be very similar in 2028). However, the current journey time on the existing road varies from 12 minutes off-peak to 20 minutes in the peak. And the A358 route is currently around 5 minutes slower than the A303 route to Exeter, which means that journey times will need to come down to about 7 minutes in order to be seen as quicker by navigation software. The modelling indicates that this won't happen, which makes me question whether navigation software will choose it for long-distance routes.
A 40mph speed limit on the A303 through the Blackdowns would solve that problem :evil:
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Re: A358 Taunton to Southfields

Post by DB617 »

Bryn666 wrote: Mon May 30, 2022 15:42
Herned wrote: Mon May 30, 2022 15:18
Bryn666 wrote: Mon May 30, 2022 15:05 Developers presumably complained at the 'extra distance' slip roads would create, whereas this abomination provides direct access onto the A358 in all directions even if it does take 4 months to circle through the entire staging plan for all the turn lanes and pedestrian stages.

Developers can do what they please and NH will always facilitate it, they're basically over a barrel and expected to do it. The real problem lies with the planning system that allows any wealthy party donor to subvert road safety by putting up nebulous 'employment creation' stats for their awful land wasting plans.
There aren't any developers. The "business park", which is so far entirely fictional, is promoted by the local district council. They couldn't get a developer interested
Oh, jeez, that makes it even worse! Why haven't NH just laughed them out of the room then? What's going on?
Would this be a good time to remind you of the existence of M49 J1? NH just don't seem to talk to anyone. They seem to be playing a big expensive game of Cities Skylines.
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Re: A358 Taunton to Southfields

Post by jackal »

The junction serves very little purpose other than access to the imaginary Nexus 25. As the consultation document makes clear, the park and ride would not be accessible from it and rather uses the old A358.

So maybe NH should just build the road past the N25 site with no junction or bridge. Are they really under an obligation to build full size junctions for 'vacant lots' as the Americans say? A maintenance lilo would be sufficient for the site's current needs. They could provide future proofing and match funding for any developer willing to contribute to a bridge.
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Re: A358 Taunton to Southfields

Post by solocle »

Herned wrote: Mon May 30, 2022 17:41
ChrisH wrote: Mon May 30, 2022 17:08 I see that the consultation page is modelling a journey time saving of around six minutes in each direction, down from 17 to 11 minutes each way (both peak and off peak journey times are projected to be very similar in 2028). However, the current journey time on the existing road varies from 12 minutes off-peak to 20 minutes in the peak. And the A358 route is currently around 5 minutes slower than the A303 route to Exeter, which means that journey times will need to come down to about 7 minutes in order to be seen as quicker by navigation software. The modelling indicates that this won't happen, which makes me question whether navigation software will choose it for long-distance routes.
A 40mph speed limit on the A303 through the Blackdowns would solve that problem :evil:
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Re: A358 Taunton to Southfields

Post by SteveM »

jackal wrote: Mon May 30, 2022 12:58
SteveM wrote: Mon May 30, 2022 12:26 Surely though, looking at it holistically, being so close to the main roundabout the Nexus cross roads can be considered part of the overall 'junction 25' signalisation can't it? As long as there's some intelligence on the lighting stages, drivers will have to stop at least once at J25 + Nexus, so I really don't see what the gripe is. It's certainly too close to J25 itself to allow a GSJ for Nexus alone.
The Nexus 25 lights are the only reason that the A358 westbound can't freeflow onto the M5 southbound. A segregated left turn could be provided at the roundabout were it not for the adjacent junction.

Additionally eastbound traffic, having cleared J25, will obviously have lengthened journey times by having to go through another set of lights immediately rather than having a clear run to Southfields.

There are also significant safety issues with the at-grade conflict points at the crossroads, especially given the long run westbound from Southfields. The new design is worse in this respect than the roundabout as there is no deflection, so there is a serious risk of red light running, and higher impact speeds in such cases.

As explained above, the bridge would either have no slips or east-facing only, so no, it is not too close to J25.

It defies reason to spend £500m on a project like this, but introduce significant delays and safety concerns because you won't spend another £10m or so for a basic bridge exactly like those provided everywhere else on the scheme. Classic British roadbuilding false economy.
No - still not getting it.

Surely, as there is no w/b A358 > s/b M5 'jet lane', then meeting a set of lights at Nexus just means you've arrived at J25. The lights for that w/b > s/b flow can and should be co-ordinated, so a green at Nexus should get a green at the roundabout as well, so effectively a free flow. I accept that won't happen in the opposite direction, because either you've come from M5 southbound, or from the roundabout circulatory - one way or the other you're going to get a second red.

My point is that this is really little different from other signalised junctions - I give you M5 J29 as an example where what was once free flow is now signalised, and all that happens on the A30 w/b is the speed limit drops in readiness at Sowton Lane - I assume that's what will happen here as well. So you've arrived at a junction - but you've gained all those lovely miles of DC.
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Re: A358 Taunton to Southfields

Post by Chris5156 »

SteveM wrote: Tue May 31, 2022 16:42My point is that this is really little different from other signalised junctions - I give you M5 J29 as an example where what was once free flow is now signalised, and all that happens on the A30 w/b is the speed limit drops in readiness at Sowton Lane - I assume that's what will happen here as well. So you've arrived at a junction - but you've gained all those lovely miles of DC.
I would be very wary of holding up M5 J29 as an example of good junction design. You can coordinate multiple sets of signals, yes, but the point is that there should be no signals. This is a connection between free-flowing 70mph trunk roads, it simply should not involve passing through multiple sets of traffic lights. The fact that the same happens elsewhere at the junctions of free-flowing 70mph trunk roads is not because that’s best practice but because terrible junction design is endemic on the trunk road network.

As noted upthread, the time penalty caused by the multiple signals at J25 mean that “all those lovely miles of DC” will offer a slim advantage over the A303 at peak times and may offer no advantage all all off-peak. Given that the scheme is specifically intended to tempt A303 traffic over to the M5, and relieve the Blackdown Hills section of its strategic function, that is an absolute failure of design. When the new road is open, it must be reliably faster to get from Southfields Roundabout to Exeter via J25 than via Honiton, otherwise the project has not met one of its objectives.

In many ways, National Highways and their consultants agree with this, because the scheme originally had free-flowing links between the A358 and M5. They appear to have been ditched for budgetary reasons, not because they were a bad idea.
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Re: A358 Taunton to Southfields

Post by Norfolktolancashire »

Chris5156 wrote: Tue May 31, 2022 19:39
SteveM wrote: Tue May 31, 2022 16:42My point is that this is really little different from other signalised junctions - I give you M5 J29 as an example where what was once free flow is now signalised, and all that happens on the A30 w/b is the speed limit drops in readiness at Sowton Lane - I assume that's what will happen here as well. So you've arrived at a junction - but you've gained all those lovely miles of DC.
As noted upthread, the time penalty caused by the multiple signals at J25 mean that “all those lovely miles of DC” will offer a slim advantage over the A303 at peak times and may offer no advantage all all off-peak. Given that the scheme is specifically intended to tempt A303 traffic over to the M5, and relieve the Blackdown Hills section of its strategic function, that is an absolute failure of design. When the new road is open, it must be reliably faster to get from Southfields Roundabout to Exeter via J25 than via Honiton, otherwise the project has not met one of its objectives.
I know what NH will do in this case, relegate the existing A303 between Southfields roundabout and the A30 junction to a B road with 40mph limits and HGV restrictions.

If this doesn't reduce the "through traffic" drastically then removing a section of the existing road would do so! An example is the old A30 halfway along Goss Moor in Cornwall when the new A30 was opened.
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Re: A358 Taunton to Southfields

Post by Herned »

SteveM wrote: Tue May 31, 2022 16:42
Surely, as there is no w/b A358 > s/b M5 'jet lane', then meeting a set of lights at Nexus just means you've arrived at J25. The lights for that w/b > s/b flow can and should be co-ordinated, so a green at Nexus should get a green at the roundabout as well, so effectively a free flow. I accept that won't happen in the opposite direction, because either you've come from M5 southbound, or from the roundabout circulatory - one way or the other you're going to get a second red.

My point is that this is really little different from other signalised junctions - I give you M5 J29 as an example where what was once free flow is now signalised, and all that happens on the A30 w/b is the speed limit drops in readiness at Sowton Lane - I assume that's what will happen here as well. So you've arrived at a junction - but you've gained all those lovely miles of DC.
We should not be accepting traffic lights on a brand new strategic road. NONE of our neighbouring countries would do anything so stupid. It's bad enough that we accept that the re is a roundabout junction with the M5, and the direct slips were unaffordable - the reality is those were the amongst the most valuable sections of the road in terms of time saved. But this idiotic country doesn't work on the basis of what is most rational, so we build massive grade-separated junctions for the 28 people who live in Ashill, and two sets of traffic lights where the new dual carriageway joins the motorway
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Re: A358 Taunton to Southfields

Post by Hdeng16 »

Norfolktolancashire wrote: Tue May 31, 2022 23:19
Chris5156 wrote: Tue May 31, 2022 19:39
SteveM wrote: Tue May 31, 2022 16:42My point is that this is really little different from other signalised junctions - I give you M5 J29 as an example where what was once free flow is now signalised, and all that happens on the A30 w/b is the speed limit drops in readiness at Sowton Lane - I assume that's what will happen here as well. So you've arrived at a junction - but you've gained all those lovely miles of DC.
As noted upthread, the time penalty caused by the multiple signals at J25 mean that “all those lovely miles of DC” will offer a slim advantage over the A303 at peak times and may offer no advantage all all off-peak. Given that the scheme is specifically intended to tempt A303 traffic over to the M5, and relieve the Blackdown Hills section of its strategic function, that is an absolute failure of design. When the new road is open, it must be reliably faster to get from Southfields Roundabout to Exeter via J25 than via Honiton, otherwise the project has not met one of its objectives.
I know what NH will do in this case, relegate the existing A303 between Southfields roundabout and the A30 junction to a B road with 40mph limits and HGV restrictions.

If this doesn't reduce the "through traffic" drastically then removing a section of the existing road would do so! An example is the old A30 halfway along Goss Moor in Cornwall when the new A30 was opened.
That’s a poor example. The old road was dug up thanks to an agreement with an environmental group.
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Re: A358 Taunton to Southfields

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Norfolktolancashire wrote: Tue May 31, 2022 23:19
Chris5156 wrote: Tue May 31, 2022 19:39
SteveM wrote: Tue May 31, 2022 16:42My point is that this is really little different from other signalised junctions - I give you M5 J29 as an example where what was once free flow is now signalised, and all that happens on the A30 w/b is the speed limit drops in readiness at Sowton Lane - I assume that's what will happen here as well. So you've arrived at a junction - but you've gained all those lovely miles of DC.
As noted upthread, the time penalty caused by the multiple signals at J25 mean that “all those lovely miles of DC” will offer a slim advantage over the A303 at peak times and may offer no advantage all all off-peak. Given that the scheme is specifically intended to tempt A303 traffic over to the M5, and relieve the Blackdown Hills section of its strategic function, that is an absolute failure of design. When the new road is open, it must be reliably faster to get from Southfields Roundabout to Exeter via J25 than via Honiton, otherwise the project has not met one of its objectives.
I know what NH will do in this case, relegate the existing A303 between Southfields roundabout and the A30 junction to a B road with 40mph limits and HGV restrictions.

If this doesn't reduce the "through traffic" drastically then removing a section of the existing road would do so! An example is the old A30 halfway along Goss Moor in Cornwall when the new A30 was opened.
The plan is for the existing A303 through the Blackdown Hills to be de-trunked. It will revert to Devon County Council. Their intention is actually to upgrade it to provide a smoother single carriageway road and tidy up some of the more dangerous junctions, which is a level of ambition some way above a reduced speed limit or a closure. However, that only reinforces the need for the A358 scheme to deliver a much, much faster journey if Waze and Google Maps are to be persuaded to direct through traffic via M5 J25.
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Re: A358 Taunton to Southfields

Post by SteveA30 »

Do any of you chaps in the 'business' have any influence or know anyone who could quietly exert some influence behind the scenes, regarding bad junction design? Some LA's and NH seem to be in awe of the developers and unwilling to resist there power. Is all this developer led or Government led? If as stated, no developers are involved in Nexus, why have the LA and NH provided for them?
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