M25 junction 28 improvements

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Bryn666
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Re: M25 junction 28 improvements

Post by Bryn666 »

I've worked at removing Brook Street from the roundabout in my proposal... this has the added bonus of allowing the A12 to become an Expressway open to motorised vehicles only then as prohibited vehicles still have an escape.
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Bryn
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Re: M25 junction 28 improvements

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WHBM wrote:I thought 270-degree loops were now out of favour because of their liability to overturning accidents with HGVs.
This must be an urban myth.
There's nothing more HGV unfriendly than a lozenge shaped (as they usually are) 'two bridge roundabout junction' connecting two multi laned roads. With it's gentle entry curve, long straight & then sharp tight curve, they are terrible.
Granted Girton didn't have a great record but I think that was a design fault (constant curve - encouraged you enter too fast). The M6 nth to M62 east compact free-flow(Croft?) is fine, an initial tight curve that eases as you progress.
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Re: M25 junction 28 improvements

Post by jackal »

85CF380 wrote:
WHBM wrote:I thought 270-degree loops were now out of favour because of their liability to overturning accidents with HGVs.
This must be an urban myth.
There's nothing more HGV unfriendly than a lozenge shaped (as they usually are) 'two bridge roundabout junction' connecting two multi laned roads. With it's gentle entry curve, long straight & then sharp tight curve, they are terrible.
Granted Girton didn't have a great record but I think that was a design fault (constant curve - encouraged you enter too fast). The M6 nth to M62 east compact free-flow(Croft?) is fine, an initial tight curve that eases as you progress.
Croft is a preposterously large junction, much bigger than your typical Texan stack for instance. Not all loops can be supersized like that. Here it is to scale with M4/A329, which is the same design:

https://www.google.co.uk/maps/@53.42620 ... 570307,15z
https://www.google.co.uk/maps/@51.43192 ... 524236,15z

I do agree that there's nothing inherently wrong with a loop though. The radii on many non-loop freeflow junctions are similarly tight. Here, for instance, is M60/M62/M602 at the same scale:

https://www.google.co.uk/maps/@53.48749 ... 784342,15z
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Re: M25 junction 28 improvements

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jackal wrote:Croft is a preposterously large junction, much bigger than your typical Texan stack for instance.
It really is. I found it fascinating when I lived nearby. My favourite comparison is that its footprint is substantially larger than the whole of Warrington town centre.
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Re: M25 junction 28 improvements

Post by Bryn666 »

The reason being that Drake et al had wanted to make sure the primary movement - which to them was from the south towards Liverpool and vice-versa - was doable at a high a speed as possible. It's not a lie to say you can do 70 mph on the M62 e/b to M6 s/b connector. Even the loops are possible to take at 50, but it's not comfortable; which is why there's an advisory 40 on both loops.

Alas today it's clear that the dominant turn movements are to and from Manchester as evidenced by the numerous bits of tinkering has shown, including the future proposal to put ramp metering on the whole thing. That'll be fun at 70.
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Re: M25 junction 28 improvements

Post by Truvelo »

It might not be the case that flows from the south towards Manchester are any higher than to Liverpool but the loop clearly gives a false impression of increased traffic due to its lower design standard than the Liverpool slips. You also have to factor in traffic already on the M62 where the loop joins.

This false impression can be seen on recently completed schemes like Catthorpe where the queue at the end of the M6 has magically disappeared since the new junction opened.
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jackal
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Re: M25 junction 28 improvements

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Bryn666 wrote:The reason being that Drake et al had wanted to make sure the primary movement - which to them was from the south towards Liverpool and vice-versa - was doable at a high a speed as possible. It's not a lie to say you can do 70 mph on the M62 e/b to M6 s/b connector. Even the loops are possible to take at 50, but it's not comfortable; which is why there's an advisory 40 on both loops.

Alas today it's clear that the dominant turn movements are to and from Manchester as evidenced by the numerous bits of tinkering has shown, including the future proposal to put ramp metering on the whole thing. That'll be fun at 70.
The current project's metering is just on the connectors for M6nb and M6sb to M62eb, which certainly supports the notion that congestion is worse for the Manchester turns, though as Truvelo says there will be several factors in that congestion. Is there a more expansive future metering scheme at Croft?

PS - metering scheme thread here: http://www.sabre-roads.org.uk/forum/vie ... 62#p856142
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Re: M25 junction 28 improvements

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We are getting some way off topic here, but...
jackal wrote:The current project's metering is just on the connectors for M6nb and M6sb to M62eb, which certainly supports the notion that congestion is worse for the Manchester turns, though as Truvelo says there will be several factors in that congestion. Is there a more expansive future metering scheme at Croft?

PS - metering scheme thread here: http://www.sabre-roads.org.uk/forum/vie ... 62#p856142
Anecdotally (and having lived very close by for several years, admittedly a decade ago now) the M62 between M6 and M60 is busy all day and one long traffic jam in the morning rush hour, while the M62 between M6 and Liverpool tends to flow much more freely. I would expect the turns towards Manchester are more heavily used than those towards Liverpool.

The traffic counts on SABRE Maps suggest the M6 south of there carries up to 150,000 AADT; the M6 to the north just over 100,000; the M62 to the west below 100,000; and the M62 to the east up to 120,000. That would support the idea that the busiest turning movement is between the south and east arms of the junction.
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Re: M25 junction 28 improvements

Post by Glom »

There's a new video on the HE YouTube channel. It explains the three options, which are the loop, the bigger loop and the biggest loop. Can't see how making the loop bigger makes things any better.
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Re: M25 junction 28 improvements

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The full Technical Appraisal Report is available here: https://highwaysengland.citizenspace.co ... %20v01.pdf

There were seven broad options:

1. hamburger
2. cyclic ramp [variously referred to as a 'northern loop', 'northern hook' or 'northern link']
3. compact cyclic ramp ['compact northern loop', 'compact northern hook', or 'compact northern link']
4. satellite roundabout [not sure what this means?]
5. loop ['single cloverleaf']
6. semi-direct ramp ['southern link']
7. do-maximum ['for example a double cloverleaf'].

Options 2, 3, 5 and 6 were taken forward to option development. Only option 6 and various versions of option 5 had benefits greater than costs, and option 6 would have cost around £150m, beyond the scheme budget. It seems the main reason why 5C and 5F have the highest BCRs of the consultation options is the greater construction impacts of 5B's 'bolt on' approach. All three are designed for 75kph.

This bit is also interesting given that option 5 ('single cloverleaf') was selected:
The single cloverleaf option offers flexibility and scalability, and importantly could form
the first phase of a longer term scheme similar to the Do-maximum
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Re: M25 junction 28 improvements

Post by jackal »

Turning volumes here are interesting. Around 1/3 of M25 traffic turns off, showing the importance of the junction. The turns to/from Chelmsford are equally busy, and much more so than the turns to/from London. Also of note is that the A12 through the junction has twice the traffic of any turning movement. This shows the advantage of adding to the basic stackabout rather than sacrificing straight-on movements for more turning freeflow as some have suggested.
M25 J28 volumes.PNG
Last edited by jackal on Mon Dec 12, 2016 19:22, edited 2 times in total.
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Re: M25 junction 28 improvements

Post by roadtester »

jackal wrote: 4. satellite roundabout [not sure what this means?]
Could it be something like a Longbridge Island sort of arrangement, possibly with a flyover on the second roundabout as well?
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Re: M25 junction 28 improvements

Post by Chris5156 »

jackal wrote:Turning volumes here are interesting. Around 1/3 of M25 traffic turns off, showing the importance of the junction. The turns to/from Chelmsford are equally busy, and much more so than the turns to/from London. Also of note is that the A12 through the junction has twice the traffic of any turning movement. This shows the advantage of adding to the basic stackabout rather than sacrificing straight-on movements for more turning freeflow as some have suggested.M25 J28 volumes.PNG
That's really interesting. What also struck me is that, of the two really busy right turns, the one proposed to be relieved with a loop is less busy than the other. It must be easier to build the loop there than in the south west quadrant.
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jackal
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Re: M25 junction 28 improvements

Post by jackal »

Chris5156 wrote:That's really interesting. What also struck me is that, of the two really busy right turns, the one proposed to be relieved with a loop is less busy than the other. It must be easier to build the loop there than in the south west quadrant.
From the Technical Appraisal Report:
The RIS statement for the 2015 to 2020 road period sets out what Government expects in
terms of improvements for Junction 28. This strategy also incorporates the following objectives:
Improvements to Junction 28 to alleviate and manage current and future congestion
through the junction, by the introduction of a new free flow link for right turning traffic
between the M25 motorway anticlockwise and the A12 east
The TAR accordingly only examines ways of providing the M25ac->A12e movement.

In any case, the volumes for those two movements are very close, and HE have come up with some excellent value options.
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Re: M25 junction 28 improvements

Post by Philip_Baum »

This junction layout would allow free flow for all strategic traffic movements between the A12 and M25. Traffic can still access the service area and Brook Street.
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Re: M25 junction 28 improvements

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Philip_Baum wrote:This junction layout would allow free flow for all strategic traffic movements between the A12 and M25. Traffic can still access the service area and Brook Street.
It would, but you'd be making a five-level junction. The bridge structures and approach embankments would be colossal and the length of the slips before they could reach the level of the existing roads would be extraordinary.
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Re: M25 junction 28 improvements

Post by Glom »

Yeah, there's no way that could be done in the space shown. It's barely bigger than the roundabout. I believe that is a textbook example of crayonisting.
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Re: M25 junction 28 improvements

Post by Philip_Baum »

Chris5156 wrote:
Philip_Baum wrote:This junction layout would allow free flow for all strategic traffic movements between the A12 and M25. Traffic can still access the service area and Brook Street.
It would, but you'd be making a five-level junction. The bridge structures and approach embankments would be colossal and the length of the slips before they could reach the level of the existing roads would be extraordinary.
In hindsight you're right about the slips being to short to provide enough clearance but I was trying to come up with something a bit like the A2 jct with the M25 although providing free flow to and from M25 north and south rather than just to M25 north and from M25 south as is the case with the A2. I can see the new slips between the M25 and A2 have longer approaches and the junction is on four levels. Perhaps the attached ideas are a bit more realistic?
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Re: M25 junction 28 improvements

Post by jackal »

^ The M25nb->A12eb movements there are similar to the rejected options 2 and 6. But as you can see, you still have much tighter curves and more sudden (in some cases, impossible...) changes in vertical alignment. The below are designed for 85kph, so would be signed for 50mph, which is not really overengineered.
Option 2.PNG
Option 6.PNG
This is the overall assessment of the options. Option 2 is a non-starter as costs outweigh benefits but option 6 would have been quite a sensible improvement, just not quite as good value as the loop options taken to consultation (5B, 5C, and 5F).
Brook Street summary.PNG
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Re: M25 junction 28 improvements

Post by Philip_Baum »

jackal wrote:^ The M25nb->A12eb movements there are similar to the rejected options 2 and 6. But as you can see, you still have much tighter curves and more sudden (in some cases, impossible...) changes in vertical alignment. The below are designed for 85kph, so would be signed for 50mph, which is not really overengineered.
Option 2.PNG
Option 6.PNG
This is the overall assessment of the options. Option 2 is a non-starter as costs outweigh benefits but option 6 would have been quite a sensible improvement, just not quite as good value as the loop options taken to consultation (5B, 5C, and 5F).
Brook Street summary.PNG
Your right; they are quite similar to the two options I proposed although Highways England only propose free flow from M25 northbound to A12 eastbound. I think there should really be free flow to/from the A12 to/from East Anglia to/from the M25 north and southbound given it is better for A12 westbound traffic heading into London to use the M25 northbound to leap over to follow the M11 or use the M25 southbound to follow the A13. Then I would expect my first option (similar to option 2) to be cheaper than my second option (similar to option 6) given only 4 over bridges are required as oppose to 6 in my second option (6 not 7 given the two carriageways over Brook Street could run over the same bridge). That option basically forms an incomplete whirlpool (incomplete given it does not need to provide free flow to/from the A12 to/from Romford to/from the M25).
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