Unusual Stacked Interchanges in Brisbane, Australia

[Peter Freeman]

Some discussion of these 3-level features has occurred in Sabre over the last year. Their merits may have been overlooked, so here they'll have a topic to themselves. I've posted in the International Roads forum, since the examples that I know are in Australia, and I'm 99% certain that you'll never see one in the UK. Stacked Diamonds are, however, not uncommon in USA; and a Stacked SPUI exists in each of Malaysia and Singapore.

In Brisbane, Australia, the Stacked Diamond is here -
https://www.google.com.au/maps/@-27.503 ... ?entry=ttu
and the Stacked SPUI is here -
https://www.google.com.au/maps/@-27.411 ... ?entry=ttu

A clearer Stacked Diamond (without any attached local roads/streets) is here -
https://www.google.co.uk/maps/@39.93659 ... ?entry=ttu (USA)

A clearer and un-cluttered Stacked SPUI is here -
https://www.google.com/maps/place/1%C2% ... &entry=ttu (Malaysia)

To appreciate these two interchange types, a prior understanding of these will help -
1. SPUI (a 2-level service interchange)
2. Signalised Diamond (also a 2-level service interchange)
3. 3-level Stacked Roundabout (a system interchange, eg. Lofthouse M1/M62)
4. 4-level Stack (a system interchange). Visualise it in the classic form, where the turning connectors make a Maltese Cross shape (eg. Almondsbury M4/M5).

What is a Stacked Diamond? It's similar to a Stacked Roundabout, but the circle has been changed into a square. An on-ramp and an off-ramp connect to each corner of the square, and each of those corners has traffic signals.

Its advantages over a 'Stackabout' are a smaller footprint and reduced cost. Also, in my opinion, higher capacity, easier to drive and safer (debatable).

What is a Stacked SPUI? Imagine a 4-level Stack, but with the two turning-connector layers combined into one layer, making it 3-level. But, you say, the turning vehicles will crash into each other! No, because that central point is 2-stage signalised. That's why it's related to a SPUI: vehicles pass each other passenger-side to passenger-side, first one pair of turns, then the other pair. Exactly like a SPUI.

Its advantages over a 4-level Stack are reduced cost, smaller footprint and lower height.

Comments are invited. Some earlier discussions are here -
viewtopic.php?t=42026&start=20 (post about stacked SPUI on 01-12-2020)
viewtopic.php?t=44817&start=20 (post about volleyball on 25-11-2023)

[Bryn666]

Out of the two, the stacked SPUI is far more efficient in terms of signal staging and potential provision for non-motorised users.

Quite honestly the stacked diamond is rubbish; it has all the problems of a stacked roundabout for a kick off, and worse geometry to boot.

[nowster]

Quite honestly the stacked diamond is rubbish; it has all the problems of a stacked roundabout for a kick off, and worse geometry to boot.

There's something very Simister about it.

[Peter Freeman]

Quite honestly the stacked diamond is rubbish; it has all the problems of a stacked roundabout for a kick off, and worse geometry to boot.

There's something very Simister about it.

Yes, I've thought that too - the squareness. (and I get the joke ... ).

Simister is perhaps my least-disliked stackabout (South Mimms the most), even though it needs to be, and is to be, relieved. The spiralisation works, creatively aided by the coloured asphalt. On each occasion I've used it, I've just sailed through with no problem. But then, I do have the luxury of avoiding peak-hour travel.

[Vierwielen]

Quite honestly the stacked diamond is rubbish; it has all the problems of a stacked roundabout for a kick off, and worse geometry to boot.

There's something very Simister about it.

Look up the Eglish translation for the Latin word "sinister", and all of the UK will become "sinister".

[Peter Freeman]

Look up the Eglish translation for the Latin word "sinister", and all of the UK will become "sinister".

Well, there's nothing left to say then.

[ChrisH]

Look up the Eglish translation for the Latin word "sinister", and all of the UK will become "sinister".

Well, there's nothing left to say then.

That doesn't sound right to me.

***

Back on the topic, the SPUI solution seems like a decent idea where you have a lot of right-turning traffic. The trick is to clear the pillars of the flyover from the centre of the junction, as the example in Brisbane does by offsetting it. A3/M25 would be a good candidate for this treatment if we are insistent on not doing the job properly, but the pillars of the A3 overpass would block where the turns would need to go. This junction in Krakow is similar, and suffers from the roundabout locking up because it's used almost exclusively by left-turning vehicles.

[nowster]

Simister is perhaps my least-disliked stackabout (South Mimms the most), even though it needs to be, and is to be, relieved. The spiralisation works, creatively aided by the coloured asphalt. On each occasion I've used it, I've just sailed through with no problem. But then, I do have the luxury of avoiding peak-hour travel.

It's effectively a stacked diamond nowadays with all left turns bypassed. Many years ago we were speculating here on whether it could be converted into a stacked SPUI.

[Peter Freeman]

Back on the topic,
This junction in Krakow is similar ...

To me, that is a stacked diamond ('stackamond'), but retaining a very strong hint of stacked roundabout ('stackabout'). They are, after all, close cousins.

Edit: I suspected that this was a conversion (a 'squaring') from a normal stackabout, but that's not so. GE shows that the whole intersection was previously a simple at-grade roundabout. When the double-grade-separation was introduced in 2011, the Stackamond was implemented as we see it now: squarish. So, I'm impressed, but I would have been more impressed if they'd been brave enought to make it square. Of course, I'd have been even more impressed with a stacked SPUI or a full freeflow, but they (especially the latter) are not justified as both these roads are semi-motorways (scan each way to see).

... and suffers from the roundabout locking up because it's used almost exclusively by left-turning vehicles.

Traffic symmetry surely implies that it's used by just as many right-turning vehicles? It doesn't appear, from G.Maps traffic layer, to experience congestion. The E-W road on the flyover above it does, but that's caused by its downstream junctions in each direction and/or its insufficient width.

I think it's nicely done, and built on a generous scale: a smaller footprint than AU's, but more lanes. One of the stackamond's good points is that it's easy to up-scale (ie. up-lane) to gain capacity. If you go far with up-laning a roundabout or a stackabout, it becomes unwieldy and intimidating/confusing to drivers, and prone to lane-choice error and resultant side-swipes.

Bryn, notice that this one serves pedestrians, etc, well. With a bit of effort, I believe that all signalised interchanges can be nmu-friendly - something that is difficult to arrange at roundabouts.

Edit: added 'or a stackabout'

[ChrisH]

Back on the topic,
This junction in Krakow is similar ...

To me, that is a stacked diamond ('stackamond'), but retaining a very strong hint of stacked roundabout ('stackabout'). They are, after all, close cousins.

... and suffers from the roundabout locking up because it's used almost exclusively by left-turning vehicles.

Traffic symmetry surely implies that it's used by just as many right-turning vehicles? It doesn't appear, from G.Maps traffic layer, to experience congestion. The E-W road on the flyover above it does, but that's caused by its downstream junctions in each direction and/or its insufficient width.

The right-turners effectively bypass the roundabout on slip lanes, and of course the straight-on movements are 90% covered by the grade separation - that's what I meant by the preponderance of left-turners. I've driven through here before and the roundabout has been effectively gridlocked; a SPUI solution would unlock that significantly.

[Peter Freeman]

... I've driven through here before and the roundabout has been effectively gridlocked; a SPUI solution would unlock that significantly.

Indeed, of the two interchange types being discussed here, the stacked SPUI is superior. However, I firmly believe that the stacked diamond is superior to the stacked roundabout. Others disagree.

There is room for them all, because in real life, site-specific considerations and constraints often have as much influence as the traffic requirements. For example, in the case of upgrade sites, it's quite difficult to move concrete pillars,

[Peter Freeman]

FYI, here is a list of Stacked Diamonds, compiled by Jackal (and transferred to this dedicated topic for relevance).

Dusseldorf, Germany: https://www.google.co.uk/maps/@51.2529768,6.7716335,16z
Johannesburg, South Africa: https://www.google.co.uk/maps/@-26.2666 ... ?entry=ttu
Brisbane, Australia: https://www.google.com.au/maps/@-27.503 ... ?entry=ttu
Krakow, Poland (hybrid): https://www.google.com.au/maps/place/Kl ... ?entry=ttu

USA -
Philadelphia:
https://www.google.co.uk/maps/@39.93649 ... ?entry=ttu
Detroit:
https://www.google.co.uk/maps/@42.44634 ... 955476,16z?
https://www.google.co.uk/maps/@42.44635 ... ?entry=ttu
https://www.google.co.uk/maps/@42.44469 ... ?entry=ttu
Texas:
https://www.google.co.uk/maps/@29.59028 ... ?entry=ttu
https://www.google.co.uk/maps/@29.78487 ... ?entry=ttu
https://www.google.co.uk/maps/@29.71215 ... ?entry=ttu
https://www.google.co.uk/maps/@32.89551 ... 723334,16z
https://www.google.co.uk/maps/@29.55421 ... ?entry=ttu
As a step in an upgrade trajectory leading to a stack:
https://www.google.co.uk/maps/@30.19470 ... ?entry=ttu
https://www.google.co.uk/maps/@29.78487 ... ?entry=ttu
https://www.google.co.uk/maps/@30.12943 ... ?entry=ttu
https://www.google.co.uk/maps/@30.33839 ... 002598,16z
https://www.google.co.uk/maps/@30.32466 ... 727187,16z

[booshank]

Clearly it's substandard compared with a freeflow interchange, but an advantage over a stackabout is that it's probably easier to navigate, especially if you're not used to large multi-lane roundabouts. Large, multilane roundabouts aren't unique to the UK but they're generally quite rare elsewhere. Even if you're familiar with them in general, they often have non-standard lane arrangements that make them confusing for unfamiliar drivers.

[Peter Freeman]

... an advantage over a stackabout is that it's probably easier to navigate, especially if you're not used to large multi-lane roundabouts. Large, multilane roundabouts aren't unique to the UK but they're generally quite rare elsewhere. Even if you're familiar with them in general, they often have non-standard lane arrangements that make them confusing for unfamiliar drivers.

Large multi-lane roundabouts are indeed problematic. It's not just me who thinks so: I've read that British learner drivers fequently cite them as their most feared encounter on the road, and I've seen similar statements by participants on a UK TV program that I used to watch, about learning-to-drive. I think the uncertainty/confusion/hesitancy is not confined to learners.

Here's an example, near my brother's home in Nottingham.
https://www.google.com.au/maps/place/No ... ?entry=ttu
It's not a motorway interchange, but it illustrates the complexity that can quickly arise from the rotary insistance. I've gradually got used to it, but it foxed me several times, early on, into taking the wrong exit. Local familiarity shouldn't be a prerequisite for navigation.

Quite honestly the stacked diamond is rubbish; it has all the problems of a stacked roundabout for a kick off, and worse geometry to boot.

Well, it has 4 sets of lights like a stackabout. The lights are linked. It has 4 corners and four straights, instead of a circle. Most people can handle corners and straights competently, when gantries assign the lanes, and road markings guide your path.

This interchange** is a 2-level square-about in Australia. It's not a stackamond, but it's related. It provides the same functionality as a standard UK 2-bridge GSR.
https://www.google.com.au/maps/@-26.660 ... ?entry=ttu
If you follow the google car through here, down the off-ramps and around the square, you'll see how simple it is to navigate. You'd have to be very dumb to get this wrong and end up in the wrong lane or on the wrong exit.

This is a typical gantry sign on a modern AU road intersection (near to me - unfortunately GMaps satellite has not caught up yet). A stackamond could have four, possibly eight, of these signs -
https://www.google.com.au/maps/@-38.062 ... ?entry=ttu
The intersection is not connected to the stackamond subject, but it illustrates how clear and explicit this type of sign can be. Increased number of lanes doesn't affect the clarity. AU perhaps makes more use of gantry signs than UK does.

** Note: I've posted about this interchange before. It was going to be a GSR, but when it came to road-dualling time, they changed the plan to create this grade-separated square-about. See post 04-08-2021 here -
viewtopic.php?t=41867&start=20

[jackal]

Thanks for making the thread Peter, which I've only seen now, and for inserting my examples.

However, I firmly believe that the stacked diamond is superior to the stacked roundabout. Others disagree.

Indeed. For any new readers, here are some grounds I gave in the other thread for favouring the stackabout over the stackamond:

A roundabout basically points traffic around the circulatory carriageway, reducing impact angles, whereas a stacked diamond has a series of near-90 degree turns. The rbt avoids high impact T-bone collisions, which signalised crossroads are infamous for. This advantage is there for 1 and 2 level signalised rbts too, but is outweighed, on my view, by the inefficiency of such configurations, which has its own safety implications. But at 3 levels there is no downside of the roundabout compared to the squareabout. I therefore think it is no accident that the stackabout/stackamond family caught on in roundabout loving Britain but not crossroads loving America, and that the stackabout is much more common internationally.

I think you're overselling the geometric benefits of the stackamond. It can not simply assume all the ease and familiarity of crossroads or diamonds as they're quite a different layout with all the roads one-way. This means, for instance, that whatever reason there is not to paint a right turn arrow on the road at a stackabout would apply even more at a (left hand traffic) stackamond, as the prohibited right turn is less tight for the latter - indeed, the angle is the same as for the legal left turn. The stackabout, by contrast, is arranged so traffic is pointing at the places it should be going, which is a capacity and safety benefit. I concede there is maybe something of a benefit in terms of ease of adding stacking space at the stackamond, but this is outweighed by the deficiencies of a functionally rotary junction that isn't arranged as one.

Look at it this way: you only have 360 degrees to play with, so why give as much of that to an illegal turn as to the legal turn (i.e., 90 degrees left and 90 degrees right) as a stackamond does? A roundabout corrects that by giving the illegal turns acute angles every time they occur, opening up gentler angles for legal movements.

... Again, I can't see what reason you can really have (aside from future proofing and maybe familiarity) for not wanting a junction to curve in the direction of travel where there's the space for it. It's a pretty fundamental aspect of road design that you don't try to "catch out" the user with unnecessarily sharp turns where there's the space for gentle ones.

We of course agree that the stacked SPUI is better than both.

[booshank]

... an advantage over a stackabout is that it's probably easier to navigate, especially if you're not used to large multi-lane roundabouts. Large, multilane roundabouts aren't unique to the UK but they're generally quite rare elsewhere. Even if you're familiar with them in general, they often have non-standard lane arrangements that make them confusing for unfamiliar drivers.

Large multi-lane roundabouts are indeed problematic. It's not just me who thinks so: I've read that British learner drivers fequently cite them as their most feared encounter on the road, and I've seen similar statements by participants on a UK TV program that I used to watch, about learning-to-drive. I think the uncertainty/confusion/hesitancy is not confined to learners.

Oh definitely, not just learners, I think most drivers dislike encountering an unfamiliar large, multi-lane roundabout. Drivers will soon learn to navigate the examples that they often use, but it doesn't help with unfamiliar ones as they are usually non-standard.

[Peter Freeman]

I found another grade separated squareabout, this one in Cape Town, South Africa -
https://www.google.com.au/maps/place/Be ... ?entry=ttu

Again, it's not a stackamond - it's only two-level - but it's obviously designed for that dvelopment to occur in the future.

[Chris5156]

I found another grade separated squareabout, this one in Cape Town, South Africa -
https://www.google.com.au/maps/place/Be ... ?entry=ttu

Again, it's not a stackamond - it's only two-level - but it's obviously designed for that dvelopment to occur in the future.

Looks like the boundaries are pushed well back to allow for other developments too, especially in the north western quadrant - left turn slips at least, but there would be room for other upgrades too.

[Peter Freeman]

I found another grade separated squareabout, this one in Cape Town, South Africa -
https://www.google.com.au/maps/place/Be ... ?entry=ttu

Again, it's not a stackamond - it's only two-level - but it's obviously designed for that development to occur in the future.

Looks like the boundaries are pushed well back to allow for other developments too, especially in the north western quadrant - left turn slips at least, but there would be room for other upgrades too.

Perhaps staged as -
1. Add the missing SE left turn slip
2. Improve the NW left turn slip
3. Add the N-S flyover
4. Add freeflow right turns
None of these developments appear to be imminently required.

[jackal]

I found another grade separated squareabout, this one in Cape Town, South Africa -
https://www.google.com.au/maps/place/Be ... ?entry=ttu

Again, it's not a stackamond - it's only two-level - but it's obviously designed for that development to occur in the future.

Looks like the boundaries are pushed well back to allow for other developments too, especially in the north western quadrant - left turn slips at least, but there would be room for other upgrades too.

Perhaps staged as -
1. Add the missing SE left turn slip
2. Improve the NW left turn slip
3. Add the N-S flyover
4. Add freeflow right turns
None of these developments appear to be imminently required.

It's only really the NW quadrant that has much space beyond what's required for the left turn. And the river is why that space is there. So I doubt there were plans for anything beyond the central flyover plus left turns.

Indeed, if you look at the next junction along, which also seems to be futureproofed for a stackamond, one left turn takes up a vast amount of space: https://www.google.co.uk/maps/@-33.9607 ... a=!3m1!1e3

There's yet another futureproofed stackamond in CT which seems to be in the Kliprivier Dr format with C/D lanes for the next junction:
https://www.google.co.uk/maps/@-33.8857 ... a=!3m1!1e3