A428 Black Cat - Caxton Gibbet improvement

[JackieRoads]

If that junction was completed as an GSJ, it would look a little bit weird due to the fact that this:

A1

Roundabout

A421/A428

[M4Simon]

If that junction was completed as an GSJ, it would look a little bit weird due to the fact that this:

A1

Roundabout

A421/A428

Why is that weird? It is a widely used layout, variants of which can be seen at M62/M60/M66; M62/M1; M62/A162 (former A1); M25/A3; M25/A1(M); M25/A2; M25/M20; A1(M)/M18; M4/A470, which is huge, and A406/A41 which is a very compact version. There's an offset version where the A34 crosses the M4.

There are questions over its capacity compared with a full free-flow junction, but given that it also needs to link into the local road network, it is not a bad solution, and definitely better than an upgrade of the Black Cat where only one of the through routes is grade-separated.

Simon

[thatapanydude]

Supplementary consultation is open. From a quick look nothing much has changed from a roads perspective.

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

I'm interested in this from an A1 perspective - the E/W route is not a route that I would be likely to use as it runs perpendicular to any route I am likely to want to take. Anything that removes the Black Cat roundabout has got to be a good thing, and a fully grade separated interchange with the A421/A428 corridor achieves that. The sharp bend by the boatyard is retained, which is not ideal, but won't be the bottleneck on the A1. Turning flows at the Buckden roundabout are relatively low, though I have been caught in congestion there. This leaves the Sandy roundabout and the two Biggleswade roundabouts.

The geometry of the Biggleswade South roundabout is terrible, though very little traffic would turn right out of Biggleswade onto the northbound A1, or would turn from the southbound A1 to the little lane heading west, so its not too much of a problem for A1 northbound traffic in my experience (perhaps it is because I am usually heading north early in the day). The Biggleswade north roundabout has better geometry and I don't recall that being too bad when I've used it.

This leaves the Sandy roundabout as the next bottleneck on the A1. An on-line improvement is very difficult without demolition of property and I can see that pressure to 'sort out' the A1 here will grow. Nevertheless, the A421/A428 scheme will bring much needed relief to the A1 as well as bringing benefits to the east-west corridor.

Simon

At Biggleswade south, you are right it is free flowing no major issues! Though with the new housing at Biggleswade and the retail park I would expect turning movements to increase a lot. It is also now flared on the approach so easy to add a bridge over the roundabout to GSJ.

Biggleswade north is a real problem, usually queues on the NB approach in the evening along with Sandy that has significant daily queues SB in the morning rush hour. Both need to be fixed via a bypass.

The only gripes I have is why a new alignment for the A1 NB at the marina was not incorporated into the Black Cat scheme here, likewise a new Buckden bypass when the A1/A14 works were going on earlier - that would have solved most of the A1's issues in a swoop with minimal extra costs onto the respective schemes.

[Roadiecambs]

As a local, the consultation information dropped through my letterbox this morning.

Generally reasonably impressed with the quality of the scheme, and very pleased particularly that they haven't gone for a nasty cheapo option at Black Cat (like grade separation for the A428/A421 only). From the new documents, I see that there will be a wide central carriageway on the A1 as it goes under the stackabout. Presumably this is for visibility as the A1 is on a severe curve at that point, but I do wonder if it's also potential future proofing for upgrade to D3ALR if anything north or south of Black Cat ever gets approved? Maybe I'll try and find that out at the consultation.

I also do wonder how well Caxton Gibbet will cope once 2350 new houses are built in West Cambourne, which has now started construction just to the south. Of course, all the A1198 SB to A428WB traffic will now also be funneled through here, as there is no junction onto the new road from the B1040 at Eltisley.

[KeithW]

As a local, the consultation information dropped through my letterbox this morning.

Generally reasonably impressed with the quality of the scheme, and very pleased particularly that they haven't gone for a nasty cheapo option at Black Cat (like grade separation for the A428/A421 only). From the new documents, I see that there will be a wide central carriageway on the A1 as it goes under the stackabout. Presumably this is for visibility as the A1 is on a severe curve at that point, but I do wonder if it's also potential future proofing for upgrade to D3ALR if anything north or south of Black Cat ever gets approved? Maybe I'll try and find that out at the consultation.

I also do wonder how well Caxton Gibbet will cope once 2350 new houses are built in West Cambourne, which has now started construction just to the south. Of course, all the A1198 SB to A428WB traffic will now also be funneled through here, as there is no junction onto the new road from the B1040 at Eltisley.

Its not ideal but I think it should cope once the big roundabout is gone and the 428 flows through. There are far busier junctions of that sort that work perfectly well.

[jackal]

There are questions over its capacity compared with a full free-flow junction, but given that it also needs to link into the local road network, it is not a bad solution

Too often this has been the excuse for inadequate stackabouts (e.g. M25/A1, M25/A2, M62/M57) when there are better alternatives that cater for local movements AND key strategic flows (see option A from the original Black Cat consultation). I expect this will be okay for a while, but with high growth in the area and other road improvements down the line (A1, revived Oxbridge Expressway?) it's likely to end up as another pinch point needing hundreds of millions to fix when it could have been done for free on opening. Option A was no more structurally complex than C (the stackabout).

The only gripes I have is why a new alignment for the A1 NB at the marina was not incorporated into the Black Cat scheme here, likewise a new Buckden bypass when the A1/A14 works were going on earlier - that would have solved most of the A1's issues in a swoop with minimal extra costs onto the respective schemes.

It is mission creep for them to sort out the marina to the extent they are. A whole new Buckden bypass is a hundred million at least. If these improvements were national priorities they would be in RIS. It would hardly be sensible for a scheme like Simister, for instance, to drop out because the money is needed to gold plate other schemes. Already essential schemes like Chain Bar have been dropped due to spending elsewhere.

[KeithW]

The only gripes I have is why a new alignment for the A1 NB at the marina was not incorporated into the Black Cat scheme here, likewise a new Buckden bypass when the A1/A14 works were going on earlier - that would have solved most of the A1's issues in a swoop with minimal extra costs onto the respective schemes.

1) Money.

2) It was not in scope - the project aims were clearly defined.

Quicker, safer and more reliable journeys
A new 10 mile dual carriageway will cut some journeys
by more than a third at peak times, making journeys
safer and more reliable. Together with new and better
junctions, the scheme will save motorists as much as
1.5 hours in a working week as they travel daily between
the Black Cat and Caxton Gibbet roundabouts.
A three tier free-flowing junction at Black Cat will allow traffic to
move freely north-south and east-west and then travel on a new
70 mph dual carriageway with over twice the existing capacity
past St Neots, to the Caxton Gibbet junction where drivers
can pick up the existing dual carriageway to Cambridge.

Any project manager is paranoid about scope creep. That can leave the project late and over budget which is not good for your future career. They have stretched the boundaries a little here and there as it is. Any new alignment at the marina would almost certainly have to be part of a larger scheme.

One of the reasons we have been stuck with the Black Cat for so long was that there was always the lure of a future all singing all dancing 3 lane motorway waiting in the wings.

[JackieRoads]

If that junction was completed as an GSJ, it would look a little bit weird due to the fact that this:

A1

Roundabout

A421/A428

Why is that weird? It is a widely used layout, variants of which can be seen at M62/M60/M66; M62/M1; M62/A162 (former A1); M25/A3; M25/A1(M); M25/A2; M25/M20; A1(M)/M18; M4/A470, which is huge, and A406/A41 which is a very compact version. There's an offset version where the A34 crosses the M4.

There are questions over its capacity compared with a full free-flow junction, but given that it also needs to link into the local road network, it is not a bad solution, and definitely better than an upgrade of the Black Cat where only one of the through routes is grade-separated.

Simon

I never meant that.

Slightly, this would have meant this:

Layer 1: A1

Layer 2: The Roundabout

Layer 3: A421/A428

This shows like that's so damn strange.

[Roadiecambs]

There are questions over its capacity compared with a full free-flow junction, but given that it also needs to link into the local road network, it is not a bad solution

Too often this has been the excuse for inadequate stackabouts (e.g. M25/A1, M25/A2, M62/M57) when there are better alternatives that cater for local movements AND key strategic flows (see option A from the original Black Cat consultation). I expect this will be okay for a while, but with high growth in the area and other road improvements down the line (A1, revived Oxbridge Expressway?) it's likely to end up as another pinch point needing hundreds of millions to fix when it could have been done for free on opening. Option A was no more structurally complex than C (the stackabout).

The only gripes I have is why a new alignment for the A1 NB at the marina was not incorporated into the Black Cat scheme here, likewise a new Buckden bypass when the A1/A14 works were going on earlier - that would have solved most of the A1's issues in a swoop with minimal extra costs onto the respective schemes.

It is mission creep for them to sort out the marina to the extent they are. A whole new Buckden bypass is a hundred million at least. If these improvements were national priorities they would be in RIS. It would hardly be sensible for a scheme like Simister, for instance, to drop out because the money is needed to gold plate other schemes. Already essential schemes like Chain Bar have been dropped due to spending elsewhere.

I am not really clear though that Option A was any better. Sure, it provided a freeflow link for A1 SB to A421 WB, a very busy movement, but at the expense of all the other movements, which would have had very circuitous routes via two small dumbbell roundabouts to get through the junction. Bear in mind that, e.g. A428 WB to A1 SB (and A1NB to A428 EB) is likely to have increased movement with all the Wintringham development on the southern edge of St Neots. Plus it is true that A1 SB to A428 WB will become a less dominant movement once the A428 is rerouted. I agree that something more ambitious (4 level stack etc) would have been the gold-plated solution, but given the limited budget and the need to provide local access I actually think the stackabout is the best compromise here. I doubt there will be any major traffic problems until after Buckden roundabout is removed, for which there are no plans at all.

[c2R]

I agree that there's little point in a fully free-flowing solution when the A1 may not eventually be on this alignment. Although whether or not that will be in my lifetime, who knows!

The two things I would like to have seen as part of this scheme that aren't included are:
* replacement of the Tempsford Bridge with a new N/B bridge for A1 traffic - while I appreciate that this isn't on the direct line of the scheme, when considering the marina and local accesses relating to all of this, there is surely a case for the retention of the bridge and road for local traffic - Roxton could be linked to the Tempsford junction via the bridge, meaning that access wouldn't have to be provided from the Black Cat, and the marina access would then not be an issue as it would no longer be on the main line of the A1.
* improvement to the access for the Caxton Gibbet fast food tin sheds,which should never have been allowed permission so close to the existing roundabout in the first place.

[trickstat]

If that junction was completed as an GSJ, it would look a little bit weird due to the fact that this:

A1

Roundabout

A421/A428

Why is that weird? It is a widely used layout, variants of which can be seen at M62/M60/M66; M62/M1; M62/A162 (former A1); M25/A3; M25/A1(M); M25/A2; M25/M20; A1(M)/M18; M4/A470, which is huge, and A406/A41 which is a very compact version. There's an offset version where the A34 crosses the M4.

There are questions over its capacity compared with a full free-flow junction, but given that it also needs to link into the local road network, it is not a bad solution, and definitely better than an upgrade of the Black Cat where only one of the through routes is grade-separated.

Simon

I never meant that.

Slightly, this would have meant this:

Layer 1: A1

Layer 2: The Roundabout

Layer 3: A421/A428

This shows like that's so damn strange.

Many of the examples given (e.g.M25/A1(M) and A406/A41) have a roundabout above one of the routes with another route on a flyover above so I do not see how it is "strange".

[KeithW]

I agree that there's little point in a fully free-flowing solution when the A1 may not eventually be on this alignment. Although whether or not that will be in my lifetime, who knows!

The two things I would like to have seen as part of this scheme that aren't included are:
* replacement of the Tempsford Bridge with a new N/B bridge for A1 traffic - while I appreciate that this isn't on the direct line of the scheme, when considering the marina and local accesses relating to all of this, there is surely a case for the retention of the bridge and road for local traffic - Roxton could be linked to the Tempsford junction via the bridge, meaning that access wouldn't have to be provided from the Black Cat, and the marina access would then not be an issue as it would no longer be on the main line of the A1.
* improvement to the access for the Caxton Gibbet fast food tin sheds,which should never have been allowed permission so close to the existing roundabout in the first place.

I see no problem at Caxton Gibbet, you come off the A428 on the slip road and use the dumbbells just as at many other A Road service areas. The A1198 isnt that busy.

While the outlets there are new there have been roadside facilities there for a VERY long time. The last iteration was a Chinese restaurant in what had been the Caxton Gibbet Inn
https://www.huntspost.co.uk/lifestyle/g ... t-1-416483

West of the roundabout were two filling stations - one that is now a car wash and a Little Chef that became a convenience store.

https://www.google.co.uk/maps/@52.22867 ... 312!8i6656

The new setup will be much better.

[c2R]

I don't disagree that the new arrangement will help, but the problem is currently vehicles coming off the roundabout and users exiting the development pulling out in front of them, resulting in minor shunts and collisions - while of course the accesses that were used for the restaurant were re-used, the number of vehicles now using the site at peak hours is much higher than the number of vehicles using the entrance in days past as a result of the site being larger and geared towards drive through coffee and junk food stops - so it's not a real comparison with what used to be on the site. There is also a new access road to the south of the development that appears to be used unofficially mainly for car sharing to Cambridge. It would have made sense to access the development from this, or from a direct access from the new dumbbell roundabout, rather than continuing with the existing arrangement.

[jackal]

There are questions over its capacity compared with a full free-flow junction, but given that it also needs to link into the local road network, it is not a bad solution

Too often this has been the excuse for inadequate stackabouts (e.g. M25/A1, M25/A2, M62/M57) when there are better alternatives that cater for local movements AND key strategic flows (see option A from the original Black Cat consultation). I expect this will be okay for a while, but with high growth in the area and other road improvements down the line (A1, revived Oxbridge Expressway?) it's likely to end up as another pinch point needing hundreds of millions to fix when it could have been done for free on opening. Option A was no more structurally complex than C (the stackabout).

I am not really clear though that Option A was any better. Sure, it provided a freeflow link for A1 SB to A421 WB, a very busy movement, but at the expense of all the other movements, which would have had very circuitous routes via two small dumbbell roundabouts to get through the junction. Bear in mind that, e.g. A428 WB to A1 SB (and A1NB to A428 EB) is likely to have increased movement with all the Wintringham development on the southern edge of St Neots. Plus it is true that A1 SB to A428 WB will become a less dominant movement once the A428 is rerouted. I agree that something more ambitious (4 level stack etc) would have been the gold-plated solution, but given the limited budget and the need to provide local access I actually think the stackabout is the best compromise here. I doubt there will be any major traffic problems until after Buckden roundabout is removed, for which there are no plans at all.

Option A is better for movements on the roundabouts as well. Going around a couple of uncongested dumbells is easy compared to navigating a huge, congested gyratory that's signalised from day 1 as this is confirmed to be (see general arrangement). If you're right that WB to SB and NB to EB will become busier movements the roundabout will require further signalisation, and probably extra 'stacking space', as these movements conflict with SB to WB. As Option A freeflows SB to WB there is far less traffic on the roundabouts and signalisation is unnecessary.

[ais523]

Stackabouts work best in situations where only one of the four right-turn movements is busy (in which case there are no major conflicts and the whole thing can be sorted out quite easily with traffic light timings). Presumably that is what lead to a stackabout being selected here (combined with the dubious guidance that stackabouts should be the "default" form of junction if both straight-on movements need freeflow).

Whether or not the current junction design will be adequate will therefore depend on how many of the turning movements become widely used in future, which is something that I don't feel qualified to predict.

[KeithW]

I don't disagree that the new arrangement will help, but the problem is currently vehicles coming off the roundabout and users exiting the development pulling out in front of them, resulting in minor shunts and collisions - while of course the accesses that were used for the restaurant were re-used, the number of vehicles now using the site at peak hours is much higher than the number of vehicles using the entrance in days past as a result of the site being larger and geared towards drive through coffee and junk food stops - so it's not a real comparison with what used to be on the site. There is also a new access road to the south of the development that appears to be used unofficially mainly for car sharing to Cambridge. It would have made sense to access the development from this, or from a direct access from the new dumbbell roundabout, rather than continuing with the existing arrangement.

The road to the south is an access road to a self storage business. It was originally a farm access road I believe.

Frankly the access from the A1198 is the least of the problems on the existing A428, while bumps do happen they are typically low speed damage only.

This junction is literally lethal. Traffic along the B1040 (including me) had to make a right turn across a busy road with vehicles doing 60 mph.
https://www.google.co.uk/maps/@52.22196 ... 312!8i6656

Not to mention the Croxton junction
https://www.google.co.uk/maps/@52.22483 ... 312!8i6656

[skiddaw05]

Anyone know where they're going to put the black cat?

[KeithW]

Anyone know where they're going to put the black cat?

Lacquer it to the surface of the new road

[jervi]

Anyone know where they're going to put the black cat?

Suspend it from the A421 so it hovers at roundabout level. I don't care what any safety audit says, that's what I want.

[Roadiecambs]

Too often this has been the excuse for inadequate stackabouts (e.g. M25/A1, M25/A2, M62/M57) when there are better alternatives that cater for local movements AND key strategic flows (see option A from the original Black Cat consultation). I expect this will be okay for a while, but with high growth in the area and other road improvements down the line (A1, revived Oxbridge Expressway?) it's likely to end up as another pinch point needing hundreds of millions to fix when it could have been done for free on opening. Option A was no more structurally complex than C (the stackabout).

I am not really clear though that Option A was any better. Sure, it provided a freeflow link for A1 SB to A421 WB, a very busy movement, but at the expense of all the other movements, which would have had very circuitous routes via two small dumbbell roundabouts to get through the junction. Bear in mind that, e.g. A428 WB to A1 SB (and A1NB to A428 EB) is likely to have increased movement with all the Wintringham development on the southern edge of St Neots. Plus it is true that A1 SB to A428 WB will become a less dominant movement once the A428 is rerouted. I agree that something more ambitious (4 level stack etc) would have been the gold-plated solution, but given the limited budget and the need to provide local access I actually think the stackabout is the best compromise here. I doubt there will be any major traffic problems until after Buckden roundabout is removed, for which there are no plans at all.

Option A is better for movements on the roundabouts as well. Going around a couple of uncongested dumbells is easy compared to navigating a huge, congested gyratory that's signalised from day 1 as this is confirmed to be (see general arrangement). If you're right that WB to SB and NB to EB will become busier movements the roundabout will require further signalisation, and probably extra 'stacking space', as these movements conflict with SB to WB. As Option A freeflows SB to WB there is far less traffic on the roundabouts and signalisation is unnecessary.

Sorry, but I don't see how putting all the movements apart from A1SB-A421WB on two tight roundabouts is in any way an improvement over a large stackabout (bear in mind that A421EB to A1NB has a freeflow lane in option C). There are a large number of conflicting movements in the dumbbell arrangement, and far less queueing space, so I have no doubt that the dumbbell roundabouts would also have to be signalised from day one too.

For example, in Option A, on the southern roundabout, A1NB to A428EB conflicts with A1SB to A428EB. On the northern roundabout, A421EB to A1SB (admittedly a small movement) conflicts with A1NB to A428EB and A1Sb to A428EB. At least in the stackabout there is plenty of queueing space, and the roundabout is large and wide, which certainly can't be said for the dumbbell option.