Black Cat - Cambourne rumour?

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Peter Freeman
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Re: Black Cat - Cambourne rumour?

Post by Peter Freeman » Thu Aug 12, 2021 04:36

Design of the new Black Cat interchange has been extensively discussed, up to about three pages ago, in this thread. Despite better options being available, most are more expensive, and we all seem to have pragmatically if reluctantly accepted that a stackabout will do the job. And it's too late for a change-of-mind now.

An advantage of the stackabout solution is that it's a familiar layout for UK drivers. Also, it offers the handy opportunity to plug in a nuisance minor road as a fifth arm, as is the case here. Ironically however, what at first seems an advantage can, in the fullness of time, contribute to the inevitable capacity problem. In this case, Bedford Road could be handled outside of the roundabout, as seen in at least one of HE's designs.

Anyway, thinking on from a recent discussion in one of the Australian topics, I suggest that, since this roundabout will be fully signalised from the word go, a signalised 3-level diamond could be used instead. It would have a similar cost, but arguably higher capacity since it can more easily accommodate multiple approach lanes and left-turn bypasses. Rather surprisingly, it can also accommodate a 5th arm (and 6th, 7th and 8th!) by dualling a short length of off-ramp. This can be seen on two arms of this example in Brisbane:
https://www.google.com.au/maps/@-27.503 ... 705,16.74z

I wonder whether a UK designer would ever be tempted to pilot such a solution?

(edited to correct 'on-ramp' to 'off-ramp').
Last edited by Peter Freeman on Fri Aug 13, 2021 00:34, edited 1 time in total.

KeithW
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Re: Black Cat - Cambourne rumour?

Post by KeithW » Thu Aug 12, 2021 06:58

Peter Freeman wrote:
Thu Aug 12, 2021 04:36
Design of the new Black Cat interchange has been extensively discussed, up to about three pages ago, in this thread. Despite better options being available, most are more expensive, and we all seem to have pragmatically if reluctantly accepted that a stackabout will do the job. And it's probably too late for a change-of-mind now.

An advantage of the stackabout solution is that it's a familiar layout for UK drivers. Also, there's the handy opportunity to plug in a nuisance minor road as a fifth arm, as is the case here. Ironically however, what at first seems an advantage can, in the fullness of time, contribute to the inevitable capacity problem. There are ways that Bedford Road could be handled outside of the roundabout, as seen in at least one of HE's designs.

Anyway, thinking on from a recent discussion in one of the Australian topics, I suggest that, since this roundabout will be fully-signalised from the word go, a signalised 3-level diamond could be used instead. It would have a similar cost, but arguably higher capacity since it can more easily accommodate multiple approach lanes and left-turn bypasses. Rather surprisingly, it can also accommodate a 5th arm (and 6th, 7th and 8th!) by dualling a short length of on-ramp. This can be seen on two arms of this example in Brisbane:
https://www.google.com.au/maps/@-27.503 ... 705,16.74z

I wonder whether a UK designer would ever be tempted to pilot such a solution?
Bedford road is a minor movement basically handling local traffic for Roxton and Great Barford. AADF is around 3k I believe.

One of the reasons we were stuck with the Black Cat for far too long was that there was always supposed to be a better solution for the A1 between Baldock and Buckden but the closest it got was the 1993 scheme for a new offline motorway to the west of the A1 which was cancelled. As of when this thread kicked off Highways England stated there were no plans to upgrade the A1 and it was quite clear that as traffic grows along the A421/A428 axis the roundabout would simply not cope. A three level stackabout was pretty much as cheap a solution as would work and it requires a relatively small foot print. This means it could be viewed as temporary solution that could be discarded if ever the A1(M) upgrade happens. At least it allows freeflow traffic on the A1 and A421/A428 routes.

I have come across three level diamond interchanges in the USA and was never particularly impressed and they seemed to take up a lot of land which is fine in Ohio or Texas but not I suspect at the Black Cat.

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Re: Black Cat - Cambourne rumour?

Post by Peter Freeman » Thu Aug 12, 2021 08:04

Seems to be plenty of room (judging only from google maps/earth). Not much extra required, and mainly on N-S axis. Buildings already being removed ...?

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Re: Black Cat - Cambourne rumour?

Post by KeithW » Thu Aug 12, 2021 08:43

Peter Freeman wrote:
Thu Aug 12, 2021 08:04
Seems to be plenty of room (judging only from google maps/earth). Not much extra required, and mainly on N-S axis. Buildings already being removed ...?
Let us be realistic here, start changing the design and it wont happen for another decade, the DfT is not going to let HE start the whole process again. The challenge here is going to be keeping the traffic flowing during construction. Take a look at the video here and you will see an extended take on the east west axis and temporary routes along the North South axis .
https://highwaysengland.co.uk/our-work/ ... he-scheme/

In any case with contracts already having been issued its much too late to start changing the basic design now. Consultations started 5 years ago in 2017.

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https://highwaysengland.citizenspace.co ... -con-2020/

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jackal
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Re: Black Cat - Cambourne rumour?

Post by jackal » Thu Aug 12, 2021 10:04

The difference between a fully signalised four arm stackabout and a three level diamond is really just that the former is more rounded. In the UK it is considered that the associated deflection has safety advantages. So it seems unlikely that a three level diamond would ever be used in a case like this one.

The use case I can think of in the UK would be where there's very little space, as the straight roads on a diamond would potentially allow you to build an extremely compact design, with the slips all tight and parallel to the mainlines, using retaining walls or viaducts to achieve the necessary grade separation. This would of course not be much use in a rural location like this, and it would also be very different to the Brisbane example.

What I find quite obnoxious about three level diamonds is that the long turns (right turns in the UK or Australia) all have to go through three sets of lights. A stackabout does at least have the option of omitting lights for less busy conflict points, using the roundabout as a roundabout, as is indeed the case with the Black Cat design. Of course, a fully signalized stackabout has the same obnoxious feature as a three level diamond. It screams "you should have built something else". It should only be a last resort where you can't fit in anything better, or a stopping point on the way to a freeflow interchange, as in Texas.

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Re: Black Cat - Cambourne rumour?

Post by Peter Freeman » Fri Aug 13, 2021 00:40

KeithW wrote:
Thu Aug 12, 2021 08:43
Let us be realistic here, start changing the design and it wont happen for another decade, the DfT is not going to let HE start the whole process again.
...
In any case with contracts already having been issued its much too late to start changing the basic design now.
Yes, I acknowledged this - I said "too late now". I was offering food for thought - perhaps more-considered thought in the future.

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Re: Black Cat - Cambourne rumour?

Post by KeithW » Fri Aug 13, 2021 07:17

Peter Freeman wrote:
Fri Aug 13, 2021 00:40
KeithW wrote:
Thu Aug 12, 2021 08:43
Let us be realistic here, start changing the design and it wont happen for another decade, the DfT is not going to let HE start the whole process again.
...
In any case with contracts already having been issued its much too late to start changing the basic design now.
Yes, I acknowledged this - I said "too late now". I was offering food for thought - perhaps more-considered thought in the future.
In which case a new thread about the merits of a 3 level divided diamond might be a better way to start a discussion on the matter rather than adding to a thread about an existing road scheme.

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Re: Black Cat - Cambourne rumour?

Post by Peter Freeman » Fri Aug 13, 2021 08:30

I agree, and I'll continue elsewhere.

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Re: Black Cat - Cambourne rumour?

Post by c2R » Thu Sep 16, 2021 15:07

From National Highways


A 10-mile section of new road to be delivered as part of a proposed major upgrade in Bedfordshire and Cambridgeshire is set to be named A421, National Highways has announced today (16 September 2021) as the examination process continues to progress.



When the A428 Black Cat to Caxton Gibbet improvements scheme opens in 2025/2026, the new dual carriageway will become an extension of the existing A421, which currently stops at the Black Cat Roundabout, through to Caxton Gibbet junction.

The existing A428 will also be renumbered in two sections as it changes from part of the strategic road network to become two local roads. The existing A428 will become the A1428 between Great North Road in St Neots and Cambridge Road junction. From Cambridge Road junction to Caxton Gibbet the existing A428 will become the B1428, continuing the existing B1428 which passes through St Neots further west.
It doesn't say whether the bit from Caxton to Cambridge will also be renumbered...
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jackal
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Re: Black Cat - Cambourne rumour?

Post by jackal » Thu Sep 16, 2021 15:20

c2R wrote:
Thu Sep 16, 2021 15:07
From National Highways

It doesn't say whether the bit from Caxton to Cambridge will also be renumbered...
Surely it will.

Looks like a surprisingly sensible round of renumbering. It could be argued that the A428 from Cambridge Road could be left with the number, but I guess signs have to swapped for white ones anyway and the change helps to indicate the hierarchy.
Last edited by jackal on Thu Sep 16, 2021 15:23, edited 1 time in total.

B1040
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Re: Black Cat - Cambourne rumour?

Post by B1040 » Thu Sep 16, 2021 15:23

Beaten to it!
I thought that the former A428 /45 near Hardwick was already unclassified. I also thought that nothing new was being build East of the Gibbet except tie ins.

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Peter350
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Re: Black Cat - Cambourne rumour?

Post by Peter350 » Thu Sep 16, 2021 16:11

Like others have said, a very sensible decision on renumbering. I initially thought the B1428 would be a brand new number but it turns out it is already allocated to the old A428 through St Neots, so this would really just be an extension of it.

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Re: Black Cat - Cambourne rumour?

Post by thatapanydude » Thu Sep 16, 2021 16:54

A pity with the number, I would have preferred the use of the A1303 to link up the section in Cambridge to St Neots.
A1/A1(M) >>> M1

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jackal
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Re: Black Cat - Cambourne rumour?

Post by jackal » Thu Sep 16, 2021 17:14

thatapanydude wrote:
Thu Sep 16, 2021 16:54
A pity with the number, I would have preferred the use of the A1303 to link up the section in Cambridge to St Neots.
Not really a bad suggestion, but it'd be an 11 mile multiplex for the sake of 3 miles of new A1303 - a bit of a faff compared to a new number. (Incidentally also a reason why a new number is preferable to retaining the stub of A428, which would be connected by a similarly long multiplex in the other direction.)

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Re: Black Cat - Cambourne rumour?

Post by jervi » Thu Sep 16, 2021 17:19

Glad that NH/HE/HA decided to number it as an A421 extension.
Its just common sense and would be another bit of road numbering I hate otherwise.

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Re: Black Cat - Cambourne rumour?

Post by skiddaw05 » Thu Sep 16, 2021 17:58

Just a pity they'll have to patch the new signs at Girton

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Re: Black Cat - Cambourne rumour?

Post by KeithW » Thu Sep 16, 2021 18:26

thatapanydude wrote:
Thu Sep 16, 2021 16:54
A pity with the number, I would have preferred the use of the A1303 to link up the section in Cambridge to St Neots.
It does via the junction with the A428 at Madingley Mulch. The existing D2 section and the new route of the A428 will be renumbered as A421 giving a contiguous road from Cambridge to the M1 at Brogborough.

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Re: Black Cat - Cambourne rumour?

Post by B1040 » Thu Sep 16, 2021 18:39

I think the long distance use of numbers, especially A numbers can lead people to think that the old number is a coherent route.
I suspect that the A1307 going through Huntingdon could lead to a lot of congestion in Huntingdon.

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Re: Black Cat - Cambourne rumour?

Post by Chris56000 » Thu Sep 16, 2021 23:51

Hi!

Yes, it might sound a daft question, but didn't anybody think of renumbering the whole of the existing A428 from the A1 to Caxton Gibbet Interchange (when open) to B1428 and the existing B1428 thro' St Neots and along Cambridge Road to the new Cambridge Road Interchange downgraded to unclassified once the new dual carriageway A421 opens, or perhaps more usefully, simply renumbering the old A428 as non–primary (red) A1428 all the way from Wyboston A1 interchange to Caxton Gibbet Interchange ?

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Re: Black Cat - Cambourne rumour?

Post by A303Chris » Fri Sep 17, 2021 08:46

Why I fully agree with the main route being the A421, the old road number seems a bit weird especially this
The existing A428 will become the A1428 between Great North Road in St Neots and Cambridge Road junction
So we have the existing St Neots by pass becoming the A1428 ending on the B4128. Personally I would have renumbered the bypass the B4128 and declassified the B4128 through the town. I just thing A roads ending on 'B' roads unless on the coast or end of a valley in a mountainous area just looks wrong.
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