Diverging Diamonds

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Bryn666
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Re: Diverging Diamonds

Post by Bryn666 »

Peter Freeman wrote: Thu Apr 14, 2022 05:19 I like Jackal's GSJ hierarchies (subject always of course to local considerations). Comparing slots 3, 4 and 5 in each version is quite startling. I know which camp I'm in!

However, I would reverse the order of slots 1 and 2. A signalised diamond can handle far more traffic than a dumbbell - it's a function of how wide you flare the off ramps at the signal line (over-simplification).

Also, I'd like an un-signalised diamond as an extra entry at the start of that list -

1. Unsignalised diamond
2. Dumbbell
3. Diamond=parclo (four ramp)
4. DDI
5. SPUI
6. Parclo (six ramp)
7. Full freeflow

I know un-signalised diamonds are rare in the UK, but they're common in AU and USA for rural freeway interchanges with minor local roads. Cheap, simple and adequate. My favourite minor interchange design though is the dumbbell. A bonus is that it's extremely easy to convert to a DDI.

BTW, at-grade hamburgers are common, but are M6 J19 (sort-of) and M6 J23 the only hamburger GSJs? One was proposed long ago for M1J28, but fortunately it never happened.
M56 J6 (well, sort of, as it's a parclo with a hamburger on one side) and the A57(M)/A56 junction also count.
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Re: Diverging Diamonds

Post by jackal »

M56 J12 is similar to J6 now. A34/A4130 has a unidirectional cut through.
Peter Freeman wrote: Thu Apr 14, 2022 05:19 I like Jackal's GSJ hierarchies (subject always of course to local considerations). Comparing slots 3, 4 and 5 in each version is quite startling. I know which camp I'm in!

However, I would reverse the order of slots 1 and 2. A signalised diamond can handle far more traffic than a dumbbell - it's a function of how wide you flare the off ramps at the signal line (over-simplification).

Also, I'd like an un-signalised diamond as an extra entry at the start of that list -

1. Unsignalised diamond
2. Dumbbell
3. Diamond=parclo (four ramp)
4. DDI
5. SPUI
6. Parclo (six ramp)
7. Full freeflow

I know un-signalised diamonds are rare in the UK, but they're common in AU and USA for rural freeway interchanges with minor local roads. Cheap, simple and adequate. My favourite minor interchange design though is the dumbbell. A bonus is that it's extremely easy to convert to a DDI.
I meant an unsignalised diamond. I don't know that they're rare in the UK, at least if you count the rough approximations that are more a matter of fitting four slips in cheaply, e.g. these on the A30:

https://www.google.co.uk/maps/@50.74078 ... a=!3m1!1e3

https://www.google.co.uk/maps/@50.70700 ... a=!3m1!1e3

While a signalised diamond could have more capacity than a dumbbell, I wouldn't really consider such a junction to be in the modern hierarchy. It's basically superseded by the DDI and SPUI, much like the two-bridge roundabout - which doesn't stop them getting built, of course.
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Re: Diverging Diamonds

Post by Bryn666 »

jackal wrote: Thu Apr 14, 2022 16:33 M56 J12 is similar to J6 now. A34/A4130 has a unidirectional cut through.
Peter Freeman wrote: Thu Apr 14, 2022 05:19 I like Jackal's GSJ hierarchies (subject always of course to local considerations). Comparing slots 3, 4 and 5 in each version is quite startling. I know which camp I'm in!

However, I would reverse the order of slots 1 and 2. A signalised diamond can handle far more traffic than a dumbbell - it's a function of how wide you flare the off ramps at the signal line (over-simplification).

Also, I'd like an un-signalised diamond as an extra entry at the start of that list -

1. Unsignalised diamond
2. Dumbbell
3. Diamond=parclo (four ramp)
4. DDI
5. SPUI
6. Parclo (six ramp)
7. Full freeflow

I know un-signalised diamonds are rare in the UK, but they're common in AU and USA for rural freeway interchanges with minor local roads. Cheap, simple and adequate. My favourite minor interchange design though is the dumbbell. A bonus is that it's extremely easy to convert to a DDI.
I meant an unsignalised diamond. I don't know that they're rare in the UK, at least if you count the rough approximations that are more a matter of fitting four slips in cheaply, e.g. these on the A30:

https://www.google.co.uk/maps/@50.74078 ... a=!3m1!1e3

https://www.google.co.uk/maps/@50.70700 ... a=!3m1!1e3

While a signalised diamond could have more capacity than a dumbbell, I wouldn't really consider such a junction to be in the modern hierarchy. It's basically superseded by the DDI and SPUI, much like the two-bridge roundabout - which doesn't stop them getting built, of course.
Incidentally, the traditional diamond where all four slips meet at two crossroads has been deprecated by DMRB for decades which is why you see the staggered slip roads in your examples. This has a better safety record due to conflict points being spread out but you're into the territory of the landtake making more sense to just build a dumbbell. Both have similar capacities, but the latter offers a bit more safety.

I don't think there are all that many locations in the UK today where an unsignalised diamond would be a good junction choice - dumbbells are probably the better pick for the safety factor.
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Re: Diverging Diamonds

Post by jackal »

As we've previously discussed a traditional diamond is supposed to be signalised nowadays. They do still have the advantage of a potentially smaller footprint than any other full GSJ. There was supposed to be a new one with its slips wedged between the A55 and railway (now binned). In that case the signals would have been pretty silly given it only had three arms and served a small village.
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Re: Diverging Diamonds

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jackal wrote: Thu Apr 14, 2022 19:40 As we've previously discussed a traditional diamond is supposed to be signalised nowadays. They do still have the advantage of a potentially smaller footprint than any other full GSJ. There was supposed to be a new one with its slips wedged between the A55 and railway (now binned). In that case the signals would have been pretty silly given it only had three arms and served a small village.
Yes, that's one of those edge cases where rigid application of standards results in an absurdity. You'd think, in such matters, a bit of flexibility and logic would be applied... academic though of course as said scheme has been binned.
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Re: Diverging Diamonds

Post by Peter Freeman »

jackal wrote: Thu Apr 14, 2022 16:33
Peter Freeman wrote: Thu Apr 14, 2022 05:19 I know un-signalised diamonds are rare in the UK, but they're common in AU and USA for rural freeway interchanges with minor local roads. Cheap, simple and adequate.
I meant an unsignalised diamond. I don't know that they're rare in the UK, at least if you count the rough approximations that are more a matter of fitting four slips in cheaply, e.g. these on the A30:
https://www.google.co.uk/maps/@50.74078 ... a=!3m1!1e3
https://www.google.co.uk/maps/@50.70700 ... a=!3m1!1e3
Yes, the UK does have those pragmatic irregular junctions with minor roads, and, although it's not always obvious, most are topologically diamonds even though they're not diamond-shaped. They're ok.

I'd never thought until now about how rare larger-scale signalised diamonds - the type that is ubiquitous here in AU - are on the UK's motorway network. The only ones that come to mind are on the M60, at A62 and A56. Perhaps because (some of) the M60 is the most recent major motorway?
While a signalised diamond could have more capacity than a dumbbell, I wouldn't really consider such a junction to be in the modern hierarchy. It's basically superseded by the DDI and SPUI, much like the two-bridge roundabout - which doesn't stop them getting built, of course.
Agreed, in virtually all situations, either a DDI or SPUI would be preferable to a signalised standard diamond. Superseded? I've remarked before that, in USA MO at least, the DDI seems to almost be the default now, but I'm not sure that's true elsewhere. In AU it's not, yet - unfortunately. We'll see.

The SPUI has definitely, for some time now, been Australia's choice for the most important motorway service interchanges.
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Re: Diverging Diamonds

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Peter Freeman wrote: Sun Apr 17, 2022 02:35I'd never thought until now about how rare larger-scale signalised diamonds - the type that is ubiquitous here in AU - are on the UK's motorway network. The only ones that come to mind are on the M60, at A62 and A56. Perhaps because (some of) the M60 is the most recent major motorway?
The newest part of the M60 does have some unusual large signalised diamonds, but it's more than 20 years old now. More recent motorways can be divided into rural and urban projects.

The only major urban motorway of the last 20 years is the M74 extension in Glasgow and that does have two new signalised diamonds, though I don't know how large you'd consider them - no stop line at either junction appears to have more than two lanes and many have only one.

There are rather more examples of rural motorways built in the same period - the M6 Toll, the A1(M) from Darrington to Barton, the M8 completion, the M77, and the M80. You could also include the A14 Huntingdon-Cambridge which is arguably a motorway with green signs. In those I count the following entirely new junctions:
Two-bridge roundabout interchanges - 3
Dumbbells - 11
Trumpets - 2 (both on the M6 Toll, for the purpose of incorporating a toll plaza)
Diamond - 3, plus a fourth diamond/dumbbell hybrid that has a roundabout on one side only

Those four diamonds are all very low-powered affairs: A1(M), M80, M77, M77 hybrid.

I think the conclusion I'd draw is that the M60 is a real outlier and the UK still regards dumbbells or two-bridge roundabouts as a necessity for any serious volume of traffic.

To find an exception to that rule you have to start looking at A-road expressways rather than motorways, and the most significant exceptions appears to be this rural diamond on the A90 Aberdeen Western Peripheral, which by UK standards is a relatively high capacity diamond in a place with significant volumes of traffic, and this one on the A92 which was built as part of the same project. The first one is a terrible choice for its location - just look at the wider area and what it's there to do - and the second one was, remarkably, built to replace a free-flow interchange when a new high speed expressway arm was added to the junction. But then all the junctions on the AWPR are decidedly weird choices.
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Re: Diverging Diamonds

Post by WHBM »

Bryn666 wrote: Thu Apr 14, 2022 17:39Incidentally, the traditional diamond where all four slips meet at two crossroads has been deprecated by DMRB for decades
The A13 rebuild through East London around year 2000 has diamond interchanges at several points. They have a notably low throughput, in part by the seeming inability to synchronise the signals on either side of the mainline, and also the low percentage of green time for each leg. In fact they give rise to dangerous wrong-lane abuse, right turners taking the through/left turn lanes then cutting across on green. This one is particularly prone to that

https://www.google.com/maps/@51.5171871 ... a=!3m1!1e3

This one in daytime seems to always take two or three cycles to get through

https://www.google.com/maps/@51.5202114 ... a=!3m1!1e3

The exception is when periodically the signals fail, then there are no queues at all.
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Re: Diverging Diamonds

Post by Peter Freeman »

Bryn666 wrote: Thu Apr 14, 2022 17:39 Incidentally, the traditional diamond where all four slips meet at two crossroads has been deprecated by DMRB for decades which is why you see the staggered slip roads in your examples. This has a better safety record due to conflict points being spread out ...
Since space is not a prime consideration, and AADTs are often extremely low, many Australian rural service interchanges are of the unsignalised 'spread diamond' variety, where the distance between the two pairs of ramps can be up to 700m. Also, the on and off ramps often do not face each other, but are staggered by tens of metres.
... but you're into the territory of the landtake making more sense to just build a dumbbell. Both have similar capacities, but the latter offers a bit more safety.
I don't think there are all that many locations in the UK today where an unsignalised diamond would be a good junction choice - dumbbells are probably the better pick for the safety factor.
In AU too, a dumbbell is much more likely to be selected today than an unsignalised diamond. But at very minor roads, on low-volume freeways, we're willing to compromise the motorway standard and drop in an at-grade intersection. There is a favourite here called the 'wide median' intersection. It looks a bit like a roundabout but doesn't have priority, and it fits easily between the carriageways with sufficient storage space. Here's one -
https://www.google.com.au/maps/@-38.213 ... a=!3m1!1e3

It's an example of Aussie pragmatism, which has its good and bad points ...!
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Re: Diverging Diamonds

Post by Bryn666 »

It's an interesting layout - we had a similar style shown in Roads in Rural Areas but you'd be hard pressed to find those built today. In fact where we have built at grade junctions on new dual carriageways (e.g. the A66) they have been the hideous tiny gap type which are appalling safety wise.

At least the AU version has space to wait without blocking anyone else and decent deceleration lanes.
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Re: Diverging Diamonds

Post by Peter Freeman »

^ Here's an extreme one, on the M31! This one's especially elongated so that trucks emerging from the filling station and needing to go north-east, can quickly get across to a sheltered lane in order to U-turn. The speed limit temporarily reduces in the vicinity from 110 km/hr to 80. Many trucks on the M31 Hume Freeway are B-doubles - they're long!

Despite the M31's interstate importance, halfway up the freeway the AADT is still low by UK standards. The good news now is that the site will soon be grade-separated properly, having been roundly criticised since it opened only a couple of years ago.

ps. the intersection was promoted, and perhaps partially paid for, by the truck filling station wanting to open that site there. The road authority VicRoads didn't want to discourage such a welcome development, so they let it through. A bit like the tin-shed erectors inserting a roundabout into a perfectly good UK bypass, Bryn!

https://www.google.com.au/maps/@-36.102 ... a=!3m1!1e3
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Re: Diverging Diamonds

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Peter Freeman wrote: Sun Apr 17, 2022 10:47 ^ Here's an extreme one, on the M31! This one's especially elongated so that trucks emerging from the filling station and needing to go north-east, can quickly get across to a sheltered lane in order to U-turn. The speed limit temporarily reduces in the vicinity from 110 km/hr to 80. Many trucks on the M31 Hume Freeway are B-doubles - they're long!

Despite the M31's interstate importance, halfway up the freeway the AADT is still low by UK standards. The good news now is that the site will soon be grade-separated properly, having been roundly criticised since it opened only a couple of years ago.

ps. the intersection was promoted, and perhaps partially paid for, by the truck filling station wanting to open that site there. The road authority VicRoads didn't want to discourage such a welcome development, so they let it through. A bit like the tin-shed erectors inserting a roundabout into a perfectly good UK bypass, Bryn!

https://www.google.com.au/maps/@-36.102 ... a=!3m1!1e3
I'd have hoped the one thing Australia would never have imported was the developer led cock up but I guess money talks wherever you place it.
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Re: Diverging Diamonds

Post by Peter Freeman »

Peter Freeman wrote: Sat Apr 09, 2022 11:20 Australia's second DDI, on the M3 in a northern suburb of Brisbane, has reached practical completion. The 3rd is under construction, and the 4th and 5th are planned.

Info and public guidance video on website -
https://www.tmr.qld.gov.au/projects/gym ... tersection

Location -
https://www.google.com.au/maps/place/Ho ... 153.016549
AU's third DDI has now switched to its DDI configuration. This one was a conversion from a UK-style 2-bridge elevated roundabout. The action was simultaneous complete demolition and new build, with careful staging and traffic switching causing relatively little disruption. Much cleanup left to do.

It's at M1 Exit 87 in the city of Gold Coast, Queensland. -
https://www.google.com.au/maps/@-28.114 ... 177352,16z
https://www.youtube.com/watch?v=enk5HbCAyDA&t=0s

(edit - video added)
Last edited by Peter Freeman on Fri Aug 19, 2022 11:12, edited 1 time in total.
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Re: Diverging Diamonds

Post by Peter Freeman »

Plans for Australia's sixth DDI have been revealed -
1. https://roads-waterways.transport.nsw.g ... index.html
2. https://caportal.com.au/tfnsw/homebush-bay-drive

Construction will begin in 2025 and take 18 months. The location is near to, but not at, Sydney's M4. The freeflow road involved is the A3, which is an important non-motorway route that has an AADT around 100,000 near this point.
https://www.google.com.au/maps/@-33.854 ... a=!3m1!1e3

The DDI will replace the current 2-bridge roundabout interchange, where the roundabout is at ground level and the freeflow is on the upper level. One of the many stated reasons for the DDI's selection is the ease of fitting it to the existing spaces between flyover supports - therefore it could form a useful template for UK conversions.

The layout is not quite classic, but close; and a u-turn provision is incorporated at one of the on/off-ramp pairs.
Last edited by Peter Freeman on Fri Jun 30, 2023 14:49, edited 2 times in total.
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Re: Diverging Diamonds

Post by jackal »

We've previously discussed (in a less appropriate thread than this) the freeflow version of a diverging diamond, which is called a double crossover merging interchange:

Image

While I think it's still the case that none have been built, a version of one is now proposed for the Henderson interchange, which is SE of Las Vegas. The interchange is currently a 1 loop full freeflow design but suffers from very high turning volumes as well as weaving and limited access with the adjacent Gibson Rd interchange. The proposed design, with the mainline turning the corner and an east-west double crossover, can be seen here:

https://www.dot.nv.gov/home/showpublish ... 4840870000

Various rejected designs and other info can be seen at the scheme page:

https://www.dot.nv.gov/projects-program ... nterchange
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Re: Diverging Diamonds

Post by Peter Freeman »

^ Interesting: it certainly must count as a double crossover merging interchange, but a very 'impure' or hybridised one. The current spacing from Gibson Road's ramps (less than 500m) is definitely a problem, and the proposal fixes that. Other merge/diverge spacings don't seem particularly close. With the new design bearing quite a (superficial) resemblance to the current one, it seems a busy and expensive scheme for the traffic gains. However, the accompanying documentation is quite thorough, so I don't doubt that they've got it right.
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Re: Diverging Diamonds

Post by jackal »

It certainly isn't an elegant solution and I doubt it's efficient. It has mainline weaving on both east-facing Gibson Rd ramps. There are established designs for a freeflowing junction with a mainline turning the corner. Here's one in Ontario: https://www.google.co.uk/maps/@43.52845 ... a=!3m1!1e3 I'd suggest something like that with a bit of braiding might have worked better. Or don't bother with the freeflow for W Lake Mead Parkway, which has frequent signals and priority junctions.
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Re: Diverging Diamonds

Post by Peter Freeman »

jackal wrote: Mon Jul 18, 2022 15:41 Or don't bother with the freeflow for W Lake Mead Parkway, which has frequent signals and priority junctions.
I too wondered whether WLMP needed to be included in the freeflow junction design, for your reason. Also, I first wondered whether perhaps the N-S flow should have got the double-crossover instead of the E-W, but there's much to be considered that I don't have details for, including space.

I'm sure WLMP has, though not a freeway, high traffic. I'm most struck by its unsignalised turns across multi-lane traffic. Here in AU, we'd probably signalise all four of those, not just two.

Your Ontario counter-example is much easier to understand. I favour standardised designs, preferably in their 'pure' forms, and that one is of course simply a cloverstack with two extra bridges to avoid the major flows appearing to exit. I possibly wouldn't have included those bridges: instead, leave it in the standard shape and simply make the dominant-flow connectors wider. 'Wrong-side' merges and diverges don't worry me too much, given adequate signing and with no other exits nearby (route 6 is far enough away, 3 maybe not).
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Re: Diverging Diamonds

Post by jackal »

Yes, you can take almost any standard 4-way design and make the mainline turn the corner like that. Here's a three loop design in Germany: https://www.google.co.uk/maps/@49.1544869,9.295894,15z

We have some quite bad cases in the UK of wrong side merges and diverges, and while more lanes helps in some respects, it doesn't in others. For instance, adding lanes to the M25 anticlockwise through Chevening with a wider approach would mean that trucks heading for the A21 would have to move across several lanes to get to their exit, disrupting the M25 flow. I'm not fanatically anti-offside exits, but a nearside exit for a much lower volume route is inherently better in my view as it keeps slow traffic out of fast lanes.
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Re: Diverging Diamonds

Post by Gav »

I wonder if a diverging diamond could be done at hermistongait and then that would ease the congestion.
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