Major British interchanges

[Glom]

... and there are many other ways to fully freeflow a stacked roundabout, including adding semi-direct connectors in stages until you have a full stack.

I'd like to see most stackabouts eventually eliminated and I like the idea of gradually reaching full freeflow in stages, but isn't this pathway a wasteful one, leaving you with four unused bridges?

The bridges can becomes pedestrian routes.

[jackal]

... and there are many other ways to fully freeflow a stacked roundabout, including adding semi-direct connectors in stages until you have a full stack.

I'd like to see most stackabouts eventually eliminated and I like the idea of gradually reaching full freeflow in stages, but isn't this pathway a wasteful one, leaving you with four unused bridges?

The general answer is that existing bridges are sunk costs, and as such their continuing use or disuse is irrelevant from the perspective of economic appraisal - all that matters is which option gives the best BCR, which as many schemes have shown us, often involves demolition of perfectly good bridges that aren't pointing in the right direction/aren't at the right elevation/etc.

But regarding this junction in particular (and other stackabouts like it), at least some of the bridges would have to be demolished for any viable freeflow junction, for the simple reason that the bridges are arranged for traffic to move slowly in a circle, rather than quickly through a freeflow junction.

The plans for the M42/A45 stackabout upgrade that were leaked by a parish council showed a windmill/octopus type arrangement that would surely require some existing bridges to be demolished. However, that stackabout has the unusual feature that the roundabout is above the A45 on one side but below it on the other. I believe this makes that site particularly suitable for a windmill type design.

The M25/A3 has the more conventional arrangement with the stackabout sat in the middle of the three levels. IMO this makes it well suited to replacement with two opposing semi-direct right turns at that level that do not cross, in the classic stack-type arrangement. The other two right turns could be provided with semi-direct ramps above or below the existing levels of the junction (making a four-level stack), as loops (making a cloverstack), as cyclic connectors around the edge of the junction (making a three-level cloverturbine), or as some combination of these.

[jackal]

BUMP to add M74/A725, which is now functional as a stackabout. I believe this is the first qualifying interchange in the UK since 2004.

Unfortunately there is no net increase in qualifying interchanges as M4/A470 has been butchered.

[Moyceyy]

Why has the M4/A470 been butchered? Isnt that the largest roundabout in the UK?

[Bryn666]

There's now a direct cut through from the M4 westbound exit onto the A470 northbound, which hasn't eliminated the roundabout but it has added a new set of signals on the A470 mainline.

[Moyceyy]

Thats a terrible change. Surely theres enough space inside that behemoth roundabout for a full length sliproad entry on the mainline A470?

doesnt the new singal on the a470 generate traffic queues on the way out of Cardiff in evening rush hour?

[Gav]

M8/M73 isnt a stack Im afraid as there is no way to go from M8 east bound to M73 north bound. Like wise M73 south bound to M8 west bound is not possible. However traffic flows would not warrant those flows.

Its a limited interchange, with some rather interesting design and one of the shortest signed motorways in the UK. The A8(M).

[jackal]

Re: M8/M73, those movements can be performed via three roundabouts. I think it has to be allowed - M6/M56 was accepted on a similar basis.

Thats a terrible change. Surely theres enough space inside that behemoth roundabout for a full length sliproad entry on the mainline A470?

doesnt the new singal on the a470 generate traffic queues on the way out of Cardiff in evening rush hour?

They could've put in proper M4wb to A470nb freeflow, but at many multiples (prob £100m+) of the cost of the cheap and dirty cut through.

What they've ended up with is an utterly grotesque misuse of the available infrastructure, as they've ended up blocking not just the freeflow out of the city but into it as well. The time lost from bringing tens of thousands of vehicles per day to stationary or near stationary conditions, where they would otherwise be at the speed limit, would amount to a massive monetized loss, so there has either been an oversight calculating the economics of the scheme or (more likely) it was necessitated by dangerous conditions under the previous arrangement (e.g. queues back onto the m4 mainline).

Even so there would have been alternatives to this drastic 'solution', eg a freeflow left turn to A470sb, similar to the one in the opposite corner.

[jackal]

It's also worth noting that two of three options for the Black Cat upgrade would count as 'major interchanges' as defined here.

[Moyceyy]

It's also worth noting that two of three options for the Black Cat upgrade would count as 'major interchanges' as defined here.

What do you mean by this?

[ais523]

I'm not sure which bits of context you're missing, so I'll just give the whole thing.

The backstory is that there's a lack of good routes in the east-west / northeast-southwest directions that avoid the West Midlangs and London. (Think of the idea of a London orbital, but well outside the M25, letting you go round rather than through London if your source and destination are sufficiently far out.) The idea of making a connection in between the two is often known as the "Oxford/Cambridge expressway", because they're both at the right sort of distance from London, and as Oxford and Cambridge are often associated together in the public mind it makes for a good marketing slogan.

Given that there's a lot of demand for for journeys in this general direction, the road-building bodies (I'm pretty sure it's Highways England for this one, although councils might be involved too?) are looking for routes to improve. In some places there aren't any good routes, but where possible, reusing or upgrading existing roads is cheaper. The most common/obvious route for this journey at present is to use the route A34, M40, A43, A421, A1, A428, so that's a natural place to look.

The A421 is a pretty good road already, as is much of the A428. However, the section of the A428 nearest the A1 is much worse, and as such there are plans to build a replacement for the A428 (iikely also numbered A428, with the old road being renumbered) between Wyboston and Caxton Gibbet. Instead of connecting to Wyboston directly (which would force a lot of traffic onto a single rather congested section of the A1), the plan's to connect the new section of road to the A421 directly, thus keeping all the east-west and northeast-southwest traffic away from the A1. Here's the SABRE forums thread on the project.

Of course, if you're building a major new road that you want to be continuous east-west, and it's crossing the A1, a major north-south road, you're going to want to build a pretty powerful junction there. (A caveat; there are rumours that there's a desire to move the A1 to a different alignment in the region, in which case the junction would need to be placed somewhere else, but we think it's unlikely that moving the A1 would happen within the foreseeable future, and thus building a good junction would be worth it in "the interim" or for if the move never happens at all.) The current junction between the A421 and A1 is called the Black Cat roundabout, and, being a simple flat roundabout, is generally acknowledged as underpowered (something that frequently becomes clear in the travel reports on the news!). It already struggles under the current load of traffic, so plugging the A428 into the other side would clearly cause things to break without a major upgrade to the junction. The plans for reworking the junction are already well advanced, and there are strong hints (although I don't think we've had an outright statement) that they're going to build the junction even before the new A428 is ready in order to relieve Black Cat. You can see the new plans for Black Cat here, on page 13 onwards (although if you're interested in the A428 upgrade project, there's a lot of other useful information in that document too).

However, there are three sets of plans, and Highways England are currently asking for comments about which they should use. (Feel free to send them an email, or fill out their form, if you have opinions!) The plans marked Option A and Option C would fulfil the definition in this thread for a major interchange. (Option B wouldn't; the A1 doesn't freeflow northbound in that construction.)

[roadtester]

The other aspect is that by taking east-west traffic off the section of the A1 between the A428 and A421, the A428 upgrade project will, at the margins, itself reduce the pressure for an A1 upgrade.

[KeithW]

The other aspect is that by taking east-west traffic off the section of the A1 between the A428 and A421, the A428 upgrade project will, at the margins, itself reduce the pressure for an A1 upgrade.

Arguable at best, even if there were to be an A1 upgrade similar to the 1993 proposals the gap between Caxton Gibbet and the A421 Bedford bypass would still need to be closed. Similarly the rebuilding of the Black Cat doesnt remove the roundabouts at Bigglewade, Sandy or Buckden or remove the lethal alignment at the Kelpie Marine Boatyard. A major strategic road with a turn off into a marina on a blind bend - what could possibly go wrong !

[Gav]

Re: M8/M73, those movements can be performed via three roundabouts. I think it has to be allowed - M6/M56 was accepted on a similar basis.

Thats a terrible change. Surely theres enough space inside that behemoth roundabout for a full length sliproad entry on the mainline A470?

doesnt the new singal on the a470 generate traffic queues on the way out of Cardiff in evening rush hour?

They could've put in proper M4wb to A470nb freeflow, but at many multiples (prob £100m+) of the cost of the cheap and dirty cut through.

What they've ended up with is an utterly grotesque misuse of the available infrastructure, as they've ended up blocking not just the freeflow out of the city but into it as well. The time lost from bringing tens of thousands of vehicles per day to stationary or near stationary conditions, where they would otherwise be at the speed limit, would amount to a massive monetized loss, so there has either been an oversight calculating the economics of the scheme or (more likely) it was necessitated by dangerous conditions under the previous arrangement (e.g. queues back onto the m4 mainline).

Even so there would have been alternatives to this drastic 'solution', eg a freeflow left turn to A470sb, similar to the one in the opposite corner.

Nope M73 M8 is not a stack even with the new M8 extension it wont be. There is no movement from M8 east to M73 north. or the respective reverse movement. There is significant movement required on local roads to complete that action. So no its not a stack - The M6 M56 ? The Lymm roundabout allows you to complete the required actions, but still its not free flow. Id say that the junction movements need to be in the junction or immediately adjacent with no more than one roundabout to navigate.

[jackal]

Id say that the junction movements need to be in the junction or immediately adjacent with no more than one roundabout to navigate.

All required movements at Baillieston are on roads inside the interchange, unless you think the A8(M) and this bit of Coatbridge Rd are outside it, which seems a stretch. As for a 'no more than one roundabout' rule, this seems arbitrary and would exclude other major interchanges like Bullington Cross and Sandling for no good reason.

The Lymm roundabout allows you to complete the required actions, but still its not free flow.

If we only counted freeflow interchanges, it would be a very different kind of list. Here's one I made earlier

[Gav]

no good reason other than they are not free flow or proper full access junctions...

The new M1 M6 junction - cant be included as does not allow certain movements. You need a defined criteria to enable eligibility, making allowances defeats the purpose.

back to the M8 M73... it doesnt comply.

[jackal]

My criteria are as per the OP: (i) full access and (ii) no break in the central reservation of two mainlines that are at least dual carriageway. I've been given no reason to change them. But by all means, make your own list with your own criteria.

[jackal]

AADT estimates for all British full access interchanges with two unbroken mainlines:

1. M25/M4 Freeflow 365k
2. M25/M40 Freeflow 270k
3. M25/M3 Freeflow 270k
4. M25/A2 Hybrid 250k
5. M60/M62 Freeflow 245k
6. M1/M62 Hybrid 240k
7. M6/M62 Freeflow 235k
8. M60/M62/M66 Stackabout 235k
9. M25/A3 Stackabout 230k
10. M25/M23 Freeflow 220k
11. M6/M56 Hybrid 215k
12. M25/A1(M) Stackabout 205k
13. M25/M11 Freeflow 200k
14. M4/M5 Freeflow 190k
15. M4/A329(M) Freeflow 190k
16. M25/A13 Stackabout 190k
17. M42/A45 Stackabout 190k
18. M6/A38(M) Hybrid 185k
19. A19/A66 Freeflow 170k
20. M25/M20 Hybrid 170k
21. M25/A12 Stackabout 170k
22. M25/A127 Stackabout 170k
23. M1/A52 Stackabout 160k
24. M6/A34 Stackabout 158k
25. M6/M69 Hybrid 155k
26. M20/A229 Stackabout 155k
27. A41/A406 Stackabout 150k
28. A11/A12 Stackabout 150k
29. M8/M73 Hybrid 140k
30. M4/A34 Stackabout 140k
31. M18/A1(M) Stackabout 135k
32. M6/M65 Stackabout 135k
33. A5/A406 Stackabout 130k
34. M1/A41 Stackabout 125k
35. M61/M65 Stackabout 125k
36. M74/A725 Stackabout 125k
37. M57/M62 Hybrid 120k
38. M5/A40 Stackabout 115k
39. A38/B4114 Stackabout 108k
40. A38/A386 Stackabout 105k
41. A45/A46/A444 Stackabout 105k
42. M74/A723 Freeflow 95k
43. A38/A50 Stackabout 95k
44. A34/A303 Stackabout 95k
45. A48/A470 Stackabout 90k
46. M53/A552 Stackabout 83k*
47. M62/A162 Stackabout 76k
48. A441/A448 Freeflow 50k
49. A899/A705 Freeflow 38k*†

*One single carriageway mainline
†No count available for unclassified arm; estimate assumes A705 volumes on this arm

The above figures are arrived at by summing the AADTs of the four main arms of the interchange, and dividing by two (to correct for the AADTs being two-way). No account has been taken of fifth or subsequent arms. AADT data was sourced from SABRE maps.

[Micro The Maniac]

The M25/A3 has the more conventional arrangement with the stackabout sat in the middle of the three levels. IMO this makes it well suited to replacement with two opposing semi-direct right turns at that level that do not cross, in the classic stack-type arrangement. The other two right turns could be provided with semi-direct ramps above or below the existing levels of the junction (making a four-level stack), as loops (making a cloverstack), as cyclic connectors around the edge of the junction (making a three-level cloverturbine), or as some combination of these.

But you have to remember that, at least until they sort out the missing flows at the Ockham junction, and not forgetting the RHS Wisley LILO, the stackabout is an essential u-turn opportunity (adding to the traffic on the junction)

[Glenn A]

The Greymoorhill Interchange is one big bad mother. With the A689 Carlisle NDR and increased traffic on the A689 to Linstock, it is a lot more complex and busier than it was ten years ago and near misses and traffic light jumping are common. Also prior to 1970, I didn't realise the A74 forked away from the A7 in Kingstown and went along Parkhouse Rd where the Asda is now.