A428 Black Cat - Caxton Gibbet improvement

[J6onM27]

I put this in a historic posting....
http://www.sabre-roads.org.uk/forum/dow ... &mode=view
http://www.sabre-roads.org.uk/forum/dow ... &mode=view

[KeithW]

Given that and history the lack of even a formal study for this A428 improvement I see little chance that construction could start by 2019/20. Talk of of improvements to the Black Cat roundabout only raises my suspicions further. The only realistic option IMHO is a new road running south of the to the end of the A421 and this involves major works incuding new crossings of the Great Ouse and East Coast Main Line and replacing the Black Cat roundabout with a proper free flow GSJ. I would be pleasantly surprised if the planning and approvals required could be completed and signed off in under 5 years.

Upthread Haydn has already indicated that the plan is an off-line route from Black Cat, south of Little Barford, rejoining the line of the existing A428 east of St Neots and an upgrade from there to Caxton Gibbet.

Replacing Black Cat with a free-flowing GSJ will be a waste of time and money, for all the reasons already discussed: the A1 is likely to be moved onto a new line when it's upgraded so Black Cat will cease to be the point where the two major routes intersect sooner or later. And even in the interim a free-flowing GSJ is wasted because to both the north and south the A1 is seriously substandard.

The effect of which will be to make the delays at the Black Cat even worse and seriously affect A1 traffic while doing nothing to improve A428 through flow. Right now at peak you come down the Wyboston slip and join the queue for the Black Cat. The result is an increasing amount of traffic rat running through Little Barford and Great Barford along the Tempsford Road, the very problem the A421 upgrade was supposed to fix ! Given the spending on replacing roundabouts with junctions elsewhere on the A1 this is beginning to sound more like a mantra than a plan.

[A303Paul]

Given that and history the lack of even a formal study for this A428 improvement I see little chance that construction could start by 2019/20. Talk of of improvements to the Black Cat roundabout only raises my suspicions further. The only realistic option IMHO is a new road running south of the to the end of the A421 and this involves major works incuding new crossings of the Great Ouse and East Coast Main Line and replacing the Black Cat roundabout with a proper free flow GSJ. I would be pleasantly surprised if the planning and approvals required could be completed and signed off in under 5 years.

Upthread Haydn has already indicated that the plan is an off-line route from Black Cat, south of Little Barford, rejoining the line of the existing A428 east of St Neots and an upgrade from there to Caxton Gibbet.

Replacing Black Cat with a free-flowing GSJ will be a waste of time and money, for all the reasons already discussed: the A1 is likely to be moved onto a new line when it's upgraded so Black Cat will cease to be the point where the two major routes intersect sooner or later. And even in the interim a free-flowing GSJ is wasted because to both the north and south the A1 is seriously substandard.

The effect of which will be to make the delays at the Black Cat even worse and seriously affect A1 traffic while doing nothing to improve A428 through flow. Right now at peak you come down the Wyboston slip and join the queue for the Black Cat. The result is an increasing amount of traffic rat running through Little Barford and Great Barford along the Tempsford Road, the very problem the A421 upgrade was supposed to fix ! Given the spending on replacing roundabouts with junctions elsewhere on the A1 this is beginning to sound more like a mantra than a plan.

Not sure how an expressway D2 linking Camboourne to the A421 with GSJ at each end is going to make A428 through flow worse??

However I do agree that forcing the A1 traffic through two dumbells will be an issue (even allowing for less turning traffic on/off the A421. A proper two bridge roundabout would seem in order. A three level stack would be a huge waste of money given that as and when that substandard bit of the A1 is replaced it will almost certainly be offline.

[KeithW]

Nice to see that the A421 upgrade is coming at last, but what I don't get is why the A1 is taking the main line, not the A421. I know that the A1 is busier currently, but if the A1(M) is built then its unlikely to go via black cat meaning you're left with the A421 at a GSJ, which would have to be altered. It would save more money if the A421 took the mainline and when eventually the A1(M) is built, the dumbbell junction can be easily changed for a LAR assessing or even getting rid of the junction with out the need to completely destroy it because the A421 avoid its.

I think it's generally understood, further up the thread, that the A421 will be taking the mainline through the junction, and the A1 will stop for a roundabout or two underneath it until such time as it's upgraded off line.

The trouble is what is 'generally understood' is often not what appears on the ground. Until we see a plan or at least a detailed study its all just pie in the sky.

[Chris5156]

The effect of which will be to make the delays at the Black Cat even worse and seriously affect A1 traffic while doing nothing to improve A428 through flow. Right now at peak you come down the Wyboston slip and join the queue for the Black Cat. The result is an increasing amount of traffic rat running through Little Barford and Great Barford along the Tempsford Road, the very problem the A421 upgrade was supposed to fix ! Given the spending on replacing roundabouts with junctions elsewhere on the A1 this is beginning to sound more like a mantra than a plan.

You're comparing apples and oranges. The reason Black Cat is such a problem now is the huge volume of traffic - about 50% - that comes south on the A1 and turns west onto the A421, blocking the flow of northbound traffic onto the junction. This is why it's been signalised. The fact that A421/428 traffic has to share a length of the A1 and then negotiate the roundabout and the lights is why the on-slip at Wyboston is such a problem.

The new road will remove both those problems because east-west traffic will not follow any part of the A1 and therefore the flow of traffic turning south-to-west at Black Cat will be vastly reduced.

Without the A1 to the north of Black Cat being so overloaded, and without that very significant volume of turning traffic to accommodate, the traffic flows at Black Cat become much simpler, and if the A1 must still pass through a roundabout it will do so more easily and with fewer delays than now. I suspect even a dumbbell setup that sees the A1 pass through two roundabouts, as long as it's only an interim measure, will still involve fewer delays for north-south journeys than at present.

[roadtester]

The reason Black Cat is such a problem now is the huge volume of traffic - about 50% - that comes south on the A1 and turns west onto the A421, blocking the flow of northbound traffic onto the junction. This is why it's been signalised. The fact that A421/428 traffic has to share a length of the A1 and then negotiate the roundabout and the lights is why the on-slip at Wyboston is such a problem.

The new road will remove both those problems because east-west traffic will not follow any part of the A1 and therefore the flow of traffic turning south-to-west at Black Cat will be vastly reduced.

Without the A1 to the north of Black Cat being so overloaded, and without that very significant volume of turning traffic to accommodate, the traffic flows at Black Cat become much simpler, and if the A1 must still pass through a roundabout it will do so more easily and with fewer delays than now. I suspect even a dumbbell setup that sees the A1 pass through two roundabouts, as long as it's only an interim measure, will still involve fewer delays for north-south journeys than at present.

Yes, I think you've captured the problem exactly - it's not so much the current absolute volumes of traffic as the awkward way in which the main east > west flow has to negotiate the area, fouling everything up in the process, that makes this such an important fix.

I'd agree interrupting the A1 flow in the way described is probably the least bad option as even if the A1 is upgraded off-line, the existing A1 will probably live on at the very least as a single carriageway LAR so the junction would be appropriate for that, meaning any investment in it wouldn't be wasted.

[KeithW]

Yes, I think you've captured the problem exactly - it's not so much the current absolute volumes of traffic as the awkward way in which the main east > west flow has to negotiate the area, fouling everything up in the process, that makes this such an important fix.

I'd agree interrupting the A1 flow in the way described is probably the least bad option as even if the A1 is upgraded off-line, the existing A1 will probably live on at the very least as a single carriageway LAR so the junction would be appropriate for that, meaning any investment in it wouldn't be wasted.

The assumption seems to be that the majority of the turning traffic originates from the A428 but I have seen no evidence to suggest this is true. The reality is the A421 dumps a lot of traffic on to the A1 at the Black Cat and observation suggests a significant amount either goes south or continues past the A428 junction.

[roadtester]

Yes, I think you've captured the problem exactly - it's not so much the current absolute volumes of traffic as the awkward way in which the main east > west flow has to negotiate the area, fouling everything up in the process, that makes this such an important fix.

I'd agree interrupting the A1 flow in the way described is probably the least bad option as even if the A1 is upgraded off-line, the existing A1 will probably live on at the very least as a single carriageway LAR so the junction would be appropriate for that, meaning any investment in it wouldn't be wasted.

The assumption seems to be that the majority of the turning traffic originates from the A428 but I have seen no evidence to suggest this is true. The reality is the A421 dumps a lot of traffic on to the A1 at the Black Cat and observation suggests a significant amount either goes south or continues past the A428 junction.

Someone - I think jackal or wrinkly - posted a lot of detail on the numbers in a previous thread.

[darkcape]

Looking at a few recent road schemes and early plans etc it seems dumbbell GSJs are being favoured not only because they only need one bridge but because their simple layout means future upgrades can happen fairly easily. Any form of GSJ at Black Cat will vastly improve things. And anyway it's not like we don't have plenty of redundant GSJs caused by bypasses already.

[A303Paul]

I would imagine the dumbell would need to be signalised because there will still be a significant amount of traffic coming down from Peterborough way and turning right - enough to cause northbound problems on the A1. You will also have northbound traffic queuing to turn right as well once it is a fast route to Cambridge.

Signallising the southern bit of the dumbbell also gives a problem that southbound traffic queuing to turn right onto the A421 would tail back beyond the other half of the dumbell which would cause chaos.

I think they need a proper two bridge roundabout. I can't imagine that being much more expensive than a dumbell.

or if they wanted to be brave and novel they could build a cloverleaf and GSJ the A1 without needing more than one bridge......

[jackal]

Yes, I think you've captured the problem exactly - it's not so much the current absolute volumes of traffic as the awkward way in which the main east > west flow has to negotiate the area, fouling everything up in the process, that makes this such an important fix.

I'd agree interrupting the A1 flow in the way described is probably the least bad option as even if the A1 is upgraded off-line, the existing A1 will probably live on at the very least as a single carriageway LAR so the junction would be appropriate for that, meaning any investment in it wouldn't be wasted.

The assumption seems to be that the majority of the turning traffic originates from the A428 but I have seen no evidence to suggest this is true. The reality is the A421 dumps a lot of traffic on to the A1 at the Black Cat and observation suggests a significant amount either goes south or continues past the A428 junction.

Someone - I think jackal or wrinkly - posted a lot of detail on the numbers in a previous thread.

It was I The 2014 figures (heading N to S) are:

38,579 A1 Brampton
42,555 A1 North of A428
(17,364 A428 East of A1)
53,267 A1 North of Black Cat
(27,784 A421 West of Black Cat)
24,977 A1 South of Black Cat
34,282 North of A603
35,228 A1 South of A603
38,787 A1 South of Sandy
40,192 A1 South of Biggleswade

As I mentioned before, I can't really explain the low reading south of Black Cat. The next count point to the south is 10,000 AADT higher, and there are no major junctions between the two. The low count may be an artefact.

The A428 only has 17,364AADT total, so once you consider that much (possibly most) of this traffic is local or heading to/from the A1 it is clear that a majority of the 27,784 on the A421 has ultimately come from the A1 as roadtester said.

The numbers also suggest that there is more of a case for at least GSJing the A1 than there is for improving the A421/A428 route. That said, I can see the point of delaying the A1 improvement given the current study 'will look at the case for improving the non-motorway section linking the two parts of the A1(M) to motorway standard', with all the national-level benefits that would bring to the M1, M11, etc.

[jackal]

I would imagine the dumbell would need to be signalised because there will still be a significant amount of traffic coming down from Peterborough way and turning right - enough to cause northbound problems on the A1. You will also have northbound traffic queuing to turn right as well once it is a fast route to Cambridge.

Signallising the southern bit of the dumbbell also gives a problem that southbound traffic queuing to turn right onto the A421 would tail back beyond the other half of the dumbell which would cause chaos.

I think they need a proper two bridge roundabout. I can't imagine that being much more expensive than a dumbell.

or if they wanted to be brave and novel they could build a cloverleaf and GSJ the A1 without needing more than one bridge......

A cloverleaf is a rather expensive and hard to justify design in a country like the UK with high land costs (especially in SE England) and strong environmental protections. Deliberately building one as a temporary measure is basically impossible!

But they could quite easily fold the more southerly slips into the SE corner of the dumbell so the A1 flow doesn't conflict with A1Sb->A421Wb, which is the main right turn. If A1Nb->A428Eb is also predicted to be a major movement they could fold the northerly slips in the NW corner, again preventing a conflict between this turn and the A1 flow.

[Chris5156]

The A428 only has 17,364AADT total, so once you consider that much (possibly most) of this traffic is local or heading to/from the A1 it is clear that a majority of the 27,784 on the A421 has ultimately come from the A1 as roadtester said.

Even given that assumption, I expect a considerable proportion of those right turners came from the A428 and removing them from the roundabout will improve the junction, if only by allowing considerably more green time for A1 northbound traffic to enter the junction. It may well remove the need for signalisation of any kind, so the A1 would then pass through an unsignalised roundabout instead of a signalised one in the interim period.

[jackal]

The A428 only has 17,364AADT total, so once you consider that much (possibly most) of this traffic is local or heading to/from the A1 it is clear that a majority of the 27,784 on the A421 has ultimately come from the A1 as roadtester said.

Even given that assumption, I expect a considerable proportion of those right turners came from the A428 and removing them from the roundabout will improve the junction, if only by allowing considerably more green time for A1 northbound traffic to enter the junction. It may well remove the need for signalisation of any kind, so the A1 would then pass through an unsignalised roundabout instead of a signalised one in the interim period.

Yes, there are of course major benefits to grade separating a major flow even if it is not actually the main flow. Nor is it sensible to provide grade separation for the main flow (the A1) given the potential for conflicts with future upgrades to that route.

[c2R]

Are there any of those old temporary bridges from East London left lying around?

Seriously though, I'd have thought the simplest solution for now would be to put a bridge to take the A1 northbound over the top of the A421/8 and remove the conflict.

This could be re-used in the final plan to save money to carry the local access road over the A421/8.

A single route number would also be nice!

[A303Paul]

I would imagine the dumbell would need to be signalised because there will still be a significant amount of traffic coming down from Peterborough way and turning right - enough to cause northbound problems on the A1. You will also have northbound traffic queuing to turn right as well once it is a fast route to Cambridge.

Signallising the southern bit of the dumbbell also gives a problem that southbound traffic queuing to turn right onto the A421 would tail back beyond the other half of the dumbell which would cause chaos.

I think they need a proper two bridge roundabout. I can't imagine that being much more expensive than a dumbell.

or if they wanted to be brave and novel they could build a cloverleaf and GSJ the A1 without needing more than one bridge......

A cloverleaf is a rather expensive and hard to justify design in a country like the UK with high land costs (especially in SE England) and strong environmental protections. Deliberately building one as a temporary measure is basically impossible!

But they could quite easily fold the more southerly slips into the SE corner of the dumbell so the A1 flow doesn't conflict with A1Sb->A421Wb, which is the main right turn. If A1Nb->A428Eb is also predicted to be a major movement they could fold the northerly slips in the NW corner, again preventing a conflict between this turn and the A1 flow.

I wasn't really thinking of a full blown cloeverleaf, rather two roads crossing with four sets of two way "slip roads" between them

Basically something like this

http://www.scarm.info/blog/wp-content/u ... g-5-3D.jpg

[A303Paul]

Are there any of those old temporary bridges from East London left lying around?

Seriously though, I'd have thought the simplest solution for now would be to put a bridge to take the A1 northbound over the top of the A421/8 and remove the conflict.
bc1.png
This could be re-used in the final plan to save money to carry the local access road over the A421/8.
bc2.png

A single route number would also be nice!

A34 - the duplicate A34 in Manchester can become A3400

[KeithW]

Are there any of those old temporary bridges from East London left lying around?

Seriously though, I'd have thought the simplest solution for now would be to put a bridge to take the A1 northbound over the top of the A421/8 and remove the conflict.

Erm there is also congestion on the southbound side especially during the morning rush hour. Not to mention the queues that tail back up the A421 eastbound. It was to partly to alleviate that problem they installed traffic signals. In fact the northbound congestion is now WORSE than before they started tinkering as there was previously a short section of slip road directly from the A1 North to the old A421 (Bedford Road).

At a minimum When the A421 was dualled the A1 should have gone over the existing junction on a flyover with the roundabout remaining below. I expect we will see several more rounds of tinkering before they do it right.

[si404]

A34 - the duplicate A34 in Manchester can become A3400

Yuck.

A421 is easy (though hopefully the MK-Oxford section doesn't use the A421 corridor), but A42 wouldn't be much more effort (though obviously it means doing something with the Measham-M1 road).

[6637]

A34 - the duplicate A34 in Manchester can become A3400

Agreed - A34 is a short number and good for an important long-distance route (Winchester to Cambridge). It is also a number that people are already familiar with.

You probably don't even bother renumbering the Manchester A34; it's already a separate road for all intents and purposes and people are unlikely to confuse the two.