M25 junction 28 improvements

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jackal
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Re: M25 junction 28 improvements

Post by jackal »

Philip_Baum wrote: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).
I agree that a second freeflow right turn will eventually be needed, though probably the cheapest way to do this will be with a loop, given the base price is £64m for a loop compared to £125m for the whirlpool-type turn (option 2) and £150m for the semi-direct connector (option 6). In fact one of the leaked designs seems to combine a loop with a windmill/octopus-type turn.
M25 A12.jpg
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Re: M25 junction 28 improvements

Post by Philip_Baum »

Whirlpool style grade separated freeflow junction from A12 westbound to M25 southbound and M25 northbound; from M25 northbound to A12 eastbound; and from M25 southbound to A12 eastbound. Only 4 over bridges are required.
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Last edited by Philip_Baum on Thu Dec 22, 2016 01:07, edited 1 time in total.
Glom
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Re: M25 junction 28 improvements

Post by Glom »

Still very tight.
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Re: M25 junction 28 improvements

Post by Philip_Baum »

Glom wrote:Still very tight.
Just put a speed limit on it then. The M11/M25 whirlpool jct has a speed limit of 30mph on some of its slip roads! You don't want grade separated junctions taking up loads of land just so cars can drive at 70mph on its slip roads. My proposed grade separated junction is compact and gives all strategic movements free flow i.e. A12 west to M25 clockwise and M25 anticlockwise + M25 clockwise and M25 anticlockwise to A12 east) without needing to use the Brook Street roundabout. The roundabout would then be intended for local traffic to/from Romford via A12 or to/from Brentwood via A1023 and traffic requiring the service area. Also traffic on the A12 west should then be directed into London via M25 anticlockwise and M11 south or M25 clockwise and A13 west as these are grade separated routes whereas the A12 into London is at grade with a lot of traffic jams due to the numerous traffic light controlled junctions.
Last edited by Philip_Baum on Thu Dec 22, 2016 00:51, edited 6 times in total.
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Bryn666
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Re: M25 junction 28 improvements

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Glom wrote:Still very tight.
No worse than the M1/M50 junction in Dublin.
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Re: M25 junction 28 improvements

Post by Peter Freeman »

Philip_Baum wrote: Just put a speed limit on it then.
Exactly. I can't understand the UK obsession that a free-flow link must have a design speed equal (or nearly equal) to its parent roads. This, and a couple of other UK obsessions such as M-status and LARs, has always been an obstruction to pragmatic value-for-money improvements.
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jackal
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Re: M25 junction 28 improvements

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Peter Freeman wrote:
Philip_Baum wrote: Just put a speed limit on it then.
Exactly. I can't understand the UK obsession that a free-flow link must have a design speed equal (or nearly equal) to its parent roads. This, and a couple of other UK obsessions such as M-status and LARs, has always been an obstruction to pragmatic value-for-money improvements.
As posted in another thread, these are the minimum advisory speeds at some UK full access freeflow interchanges:

M25/M23 50mph
M25/M3 50mph
M25/M40 50mph
M25/M4 40mph
M60/M62/M602 40mph
M6/M62 40mph
M4/A329(M) 30mph
M25/M11 30mph
A19/A66 30mph

The only remaining full access freeflow interchanges are the two cloverleafs, the Motherwell octopus and Almondsbury stack, all of which are rather tighter than the likes of M25/M23 or M23/M3.

In short, most, if not all, complex freeflow interchanges in the UK are considered safe for ramp speeds of 30-50mph, which doesn't strike me as unreasonably high.
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Stevie D
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Re: M25 junction 28 improvements

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jackal wrote:As posted in another thread, these are the minimum advisory speeds at some UK full access freeflow interchanges:

M25/M23 50mph
M25/M3 50mph
M25/M40 50mph
M25/M4 40mph
M60/M62/M602 40mph
M6/M62 40mph
M4/A329(M) 30mph
M25/M11 30mph
A19/A66 30mph

The only remaining full access freeflow interchanges are the two cloverleafs, the Motherwell octopus and Almondsbury stack, all of which are rather tighter than the likes of M25/M23 or M23/M3.

In short, most, if not all, complex freeflow interchanges in the UK are considered safe for ramp speeds of 30-50mph, which doesn't strike me as unreasonably high.
Yes, but that was then and this is now. Those junctions were all built long before the current view that free-flow slip roads should have a high design speed came in. Interestingly, the A1(M)/M62 junction has mandatory 50 limits on the slip roads, even though many of them can be taken quite comfortably at NSL. I can't see why that junction would be different from others so that does suggest that there is a move away from advisory limits in that kind of situation.
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jackal
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Re: M25 junction 28 improvements

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Stevie D wrote:
jackal wrote:As posted in another thread, these are the minimum advisory speeds at some UK full access freeflow interchanges:

M25/M23 50mph
M25/M3 50mph
M25/M40 50mph
M25/M4 40mph
M60/M62/M602 40mph
M6/M62 40mph
M4/A329(M) 30mph
M25/M11 30mph
A19/A66 30mph

The only remaining full access freeflow interchanges are the two cloverleafs, the Motherwell octopus and Almondsbury stack, all of which are rather tighter than the likes of M25/M23 or M23/M3.

In short, most, if not all, complex freeflow interchanges in the UK are considered safe for ramp speeds of 30-50mph, which doesn't strike me as unreasonably high.
Yes, but that was then and this is now. Those junctions were all built long before the current view that free-flow slip roads should have a high design speed came in. Interestingly, the A1(M)/M62 junction has mandatory 50 limits on the slip roads, even though many of them can be taken quite comfortably at NSL. I can't see why that junction would be different from others so that does suggest that there is a move away from advisory limits in that kind of situation.
There is no fixed policy of high design speed - it's just a question of what gives best value for money, which will sometimes mean a high speed, sometimes a low speed. This is nicely illustrated by the M25 J28 scheme itself, which examined both 85kph semi-direct/cyclic options and 75kph loop options, and found the best value came from 75kph, so that's what will get built.
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Re: M25 junction 28 improvements

Post by Derek »

jackal wrote: As posted in another thread, these are the minimum advisory speeds at some UK full access freeflow interchanges:

M25/M23 50mph
M25/M3 50mph
M25/M40 50mph
M25/M4 40mph
M60/M62/M602 40mph
M6/M62 40mph
M4/A329(M) 30mph
M25/M11 30mph
A19/A66 30mph
*Minimum* advisory speeds?

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jackal
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Re: M25 junction 28 improvements

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Derek wrote:
jackal wrote: As posted in another thread, these are the minimum advisory speeds at some UK full access freeflow interchanges:

M25/M23 50mph
M25/M3 50mph
M25/M40 50mph
M25/M4 40mph
M60/M62/M602 40mph
M6/M62 40mph
M4/A329(M) 30mph
M25/M11 30mph
A19/A66 30mph
*Minimum* advisory speeds?

Derek
Yes. For instance, if some slips have 50mph advisories, some 40mph, only 40mph gets listed.
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Re: M25 junction 28 improvements

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You mean lowest advisory speed limit. Calling it a minimum implies traffic needs to be going at least that fast.
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jackal
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Re: M25 junction 28 improvements

Post by jackal »

Glom wrote:You mean lowest advisory speed limit.
No, I mean precisely what I said.
Calling it a minimum implies traffic needs to be going at least that fast.
No it doesn't.
Last edited by jackal on Tue Aug 22, 2017 10:46, edited 2 times in total.
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jackal
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Re: M25 junction 28 improvements

Post by jackal »

Option 5F has been selected. While it's the only option with two lanes on the new loop, the start of the loop looks too tight to me, though apparently it's the same design speed. It will be a good improvement anyway.

http://roads.highways.gov.uk/projects/m ... rovements/
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Re: M25 junction 28 improvements

Post by Truvelo »

Curves are supposed to have easements so they start and finish with larger radii and tighten up in the middle. This one has a really sharp bend soon after leaving the M25.
How would you like your grade separations, Sir?
Big and complex.
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Re: M25 junction 28 improvements

Post by multiraider2 »

Good to see this getting nearer to at least some kind of resolution.
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Re: M25 junction 28 improvements

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Unless there is some obstruction that cannot be worked around, I can see that being tweaked a bit during design.
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Re: M25 junction 28 improvements

Post by Glom »

They went with the fancier design. Let's appreciate this while it lasts.
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Re: M25 junction 28 improvements

Post by Bryn666 »

That's a horrible horizontal alignment and I fully expect it to be savaged at a safety audit - as said unless there is something subtle we can't see that loop could be much smoother.
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Lewis1997
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Re: M25 junction 28 improvements

Post by Lewis1997 »

It seems a little bit of a long detour free-flow link but much appreciated either way. However, I feel a free-flow link from the A12(S) ---> M25 Clockwise is also needed as I feel a lot of traffic is moving back and forth in this direction as the A12 is essentially the gateway to East Anglia from the Dartford Crossing and the near continent. However, HE have made there decision and it is expensive at some £80m but much needed to some degree.
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