Diverging Diamonds

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ABB125
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Re: Diverging Diamonds

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Is the WSP report available to read?
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Re: Diverging Diamonds

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ABB125 wrote: Sun Oct 24, 2021 12:08 Is the WSP report available to read?
See Figure 1-4 of this report. https://www.sthelens.gov.uk/media/33176 ... ptions.pdf
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Re: Diverging Diamonds

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Chris5156 wrote: Sun Oct 24, 2021 12:07
Bryn666 wrote: Sun Oct 24, 2021 10:29
ravenbluemoon wrote: Sun Oct 24, 2021 00:43 WSP being innovative? :shock: (Sorry, I'm still bitter and holding a grudge after 4 years :lol: )

Denmark are also in the DDI game, having recently rebuilt an interchange on the E20 at Odense. GMaps Link

Didn't get to play on it on my last drive through as it was around midnight, and I had just had fun and games driving into central Copenhagen looking for a Covid test centre (on a Friday night to boot!). Loads of Sabristic stuff along that motorway, including the Great Belt bridge and the Oresund Crossing.
Don't worry it threw me as well! WSP aren't known for innovation :D
No, but then look at what we’re calling innovation - a proven junction design that has been in use in other countries for decades. As innovation goes it’s about on a par with copying someone else’s homework.
Brutal but accurate!
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Re: Diverging Diamonds

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Bryn666 wrote: Sun Oct 24, 2021 13:13
Chris5156 wrote: Sun Oct 24, 2021 12:07
Bryn666 wrote: Sun Oct 24, 2021 10:29

Don't worry it threw me as well! WSP aren't known for innovation :D
No, but then look at what we’re calling innovation - a proven junction design that has been in use in other countries for decades. As innovation goes it’s about on a par with copying someone else’s homework.
Brutal but accurate!
Once again, the perfect candidate for the application of the GG104 risk assessment process (and presumably exactly what it was designed for) - a Type B scheme at worst given the existence of extensive international experience.
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Re: Diverging Diamonds

Post by Peter Freeman »

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
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Re: Diverging Diamonds

Post by Peter Freeman »

Here are two interesting DDI's, both in the USA.
(edit: correction - the first is in France)

The first one has three bridges. It looks weird at first, but a close look shows that one of the long left turns simply branches off early, just before the first crossover. It then runs parallel to the traffic that it separated from, before making its left turn. Why? Perhaps this was a conversion from a standard diamond and the existing two bridges were considered inadequate, so they added another alongside.
DDI 1.jpg

The second one is even weirder. It's a DDI/dumbbell hybrid. One of the DDI's disadvantages is that it can't accommodate extra arms - which is one of the dumbbell's strengths. So this hybrid addresses that issue. The roundabout could of course, if there's sufficient space, follow on after a normal DDI has re-crossed. But since the roundabout's there, it's easy, instead, to use that as the crossover mechanism at one side of the motorway. Is this a good design? I'm thinking about its pro's and con's, and whether it compromises the DDI's inherent superiority. The crossover point on the roundabout's circulation could get rather busy.
DDI 2.jpg
Last edited by Peter Freeman on Mon Apr 11, 2022 15:46, edited 1 time in total.
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Re: Diverging Diamonds

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Peter Freeman wrote: Mon Apr 11, 2022 08:18 Here are two interesting DDI's, both in the USA.

The first one has three bridges. It looks weird at first, but a close look shows that one of the long left turns simply branches off early, just before the first crossover. It then runs parallel to the traffic that it separated from, before making its left turn. Why? Perhaps this was a conversion from a standard diamond and the existing two bridges were considered inadequate, so they added another alongside.

DDI 1.jpg


The second one is even weirder. It's a DDI/dumbbell hybrid. One of the DDI's disadvantages is that it can't accommodate extra arms - which is one of the dumbbell's strengths. So this hybrid addresses that issue. The roundabout could of course, if there's sufficient space, follow on after a normal DDI has re-crossed. But since the roundabout's there, it's easy, instead, to use that as the crossover mechanism at one side of the motorway. Is this a good design? I'm thinking about its pro's and con's, and whether it compromises the DDI's inherent superiority. The crossover point on the roundabout's circulation could get rather busy.

DDI 2.jpg
That first one is French - it's the A1 at Carvin and one of the first DDIs in the world and the extra bridge was added when the A1 was widened to D3 in the early 90s. The one at the A4/A86 junction has been removed in favour of a loop for right turns onto the A4 eastbound removing most of the traffic conflict.
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Re: Diverging Diamonds

Post by Peter Freeman »

Thanks for the correction Bryn. I'd forgotten that France had more than the one that got decommissioned a few years ago. So, did both of France's precede the 2009 Springfield MO starter?

The second one of my pair is in MO near to Kansas City, as so many are -
https://www.google.com.au/maps/place/15 ... 94.5523218

I found a drone view of it -
https://www.youtube.com/watch?v=FV0jycdQOrY

MO's D.O.T. is so innovative.

A few years ago I had a 3-day stopover in Kansas City, just to look at all their DDI's and other interesting stuff. One of the DDI's I visited was on a road named 'Peculiar Way', in a suburb named 'Peculiar', with its DDI colloquially referred to as 'Peculiar interchange'. How apt!
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Re: Diverging Diamonds

Post by Truvelo »

Peter Freeman wrote: Mon Apr 11, 2022 15:42 I found a drone view of it -
https://www.youtube.com/watch?v=FV0jycdQOrY
Traffic seems to be flowing more freely on the roundabout than on the freeway below :roll:
How would you like your grade separations, Sir?
Big and complex.
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Re: Diverging Diamonds

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Peter Freeman wrote: Mon Apr 11, 2022 15:42 Thanks for the correction Bryn. I'd forgotten that France had more than the one that got decommissioned a few years ago. So, did both of France's precede the 2009 Springfield MO starter?

The second one of my pair is in MO near to Kansas City, as so many are -
https://www.google.com.au/maps/place/15 ... 94.5523218

I found a drone view of it -
https://www.youtube.com/watch?v=FV0jycdQOrY

MO's D.O.T. is so innovative.

A few years ago I had a 3-day stopover in Kansas City, just to look at all their DDI's and other interesting stuff. One of the DDI's I visited was on a road named 'Peculiar Way', in a suburb named 'Peculiar', with its DDI colloquially referred to as 'Peculiar interchange'. How apt!
Yes. The French DDIs go back to the early 90s if memory serves - the Americans seem very reluctant to admit DDIs were not their idea.
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Re: Diverging Diamonds

Post by Peter Freeman »

Truvelo wrote: Mon Apr 11, 2022 16:10 Traffic seems to be flowing more freely on the roundabout than on the freeway below :roll:
Ah, yes. But traffic on the freeway is quite heavy (in one direction), while traffic on the roundabout is very low. This is how roundabouts suck us in and then make fools of us: they perform so well at low volumes.
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Re: Diverging Diamonds

Post by Peter Freeman »

Bryn666 wrote: Mon Apr 11, 2022 22:48 Yes. The French DDIs go back to the early 90s if memory serves - the Americans seem very reluctant to admit DDIs were not their idea.
A little unfair -
1. Chlewicki himself quite publicly related his chance encounter with the Paris DDI after he developed his idea, and his dismay that his work had been independently pre-empted.
2. The USA, partly prompted by Chlewicki and his website 'divergingdiamond.com', quickly carried the subject much further.
3. The name he assigned to his invention, 'diverging diamond interchange', still reigns, even though the alternative 'double crossover diamond interchange' (DCD) is, in my opinion, more appropriate.
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Re: Diverging Diamonds

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Bryn666 wrote: Mon Apr 11, 2022 12:11 The one at the A4/A86 junction has been removed in favour of a loop for right turns onto the A4 eastbound removing most of the traffic conflict.
Which goes to show that freeflow is always preferable for a heavy flow, even compared to a relatively high capacity at-grade design like the DDI. I'm not really sure what they were thinking using one to connect the A4 and A86, especially considering the immense expense of the tunnels for these roads.

Indeed, it could be said that the DDI is in a bit of a tight spot, as a design that somewhat prioritises the turning movements at the cost of the straight-on movements, but which will never serve the biggest turns as well as a parclo with freeflow elements. The advantage of the DDI is really just the space and cost saving, making the SPUI its main competitor.

Going back to the A4/A86 junction, a lot of work was needed to fit in the loop, with the eastbound onslip relocated to the south of the junction and travelling along a new section of D2, before crossing the motorway on a new bridge, creating a bizarre retrofitted parclo. But for all the work the A4 east to A86 north right turn is still signalised, and still queues back onto the A4: https://www.google.co.uk/maps/@48.83007 ... a=!3m1!1e3
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Re: Diverging Diamonds

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jackal wrote: Tue Apr 12, 2022 16:39
Bryn666 wrote: Mon Apr 11, 2022 12:11 The one at the A4/A86 junction has been removed in favour of a loop for right turns onto the A4 eastbound removing most of the traffic conflict.
Which goes to show that freeflow is always preferable for a heavy flow, even compared to a relatively high capacity at-grade design like the DDI. I'm not really sure what they were thinking using one to connect the A4 and A86, especially considering the immense expense of the tunnels for these roads.

Indeed, it could be said that the DDI is in a bit of a tight spot, as a design that somewhat prioritises the turning movements at the cost of the straight-on movements, but which will never serve the biggest turns as well as a parclo with freeflow elements. The advantage of the DDI is really just the space and cost saving, making the SPUI its main competitor.

Going back to the A4/A86 junction, a lot of work was needed to fit in the loop, with the eastbound onslip relocated to the south of the junction and travelling along a new section of D2, before crossing the motorway on a new bridge, creating a bizarre retrofitted parclo. But for all the work the A4 east to A86 north right turn is still signalised, and still queues back onto the A4: https://www.google.co.uk/maps/@48.83007 ... a=!3m1!1e3
I'm not actually sure why a direct pair of slip roads has never been provided here - but I suspect the answer lies in the masses of unbuilt autoroutes that were part of the Ile de France masterplan of the 70s. The now demolished A186 and downgraded A199 give some indication of what may have been intended.
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Re: Diverging Diamonds

Post by Peter Freeman »

jackal wrote: Tue Apr 12, 2022 16:39 Which goes to show that freeflow is always preferable for a heavy flow, even compared to a relatively high capacity at-grade design like the DDI. I'm not really sure what they were thinking using one to connect the A4 and A86, especially considering the immense expense of the tunnels for these roads.

Indeed, it could be said that the DDI is in a bit of a tight spot
well yes: horses for courses
as a design that somewhat prioritises the turning movements at the cost of the straight-on movements, but which will never serve the biggest turns as well as a parclo with freeflow elements.
'Parclo' covers a wide range of shapes; and degrees of freeflow, signalisation and priority. A heavily turn-biased location will be best served by a parclo that completely free-flows the heaviest turn, in both directions. But ... you've got to find space for that loop.
The advantage of the DDI is really just the space and cost saving, making the SPUI its main competitor.
Indeed the SPUI is most applicable in most places. Its often-raised structural extravagance is only a problem where the freeflow road is on the lower level and all the turning and signalisation is on the bridge: upside-down versions, where the signalisation is at ground level, are easier. What is certain is that both DDI and SPUI are superior to roundabout-based designs, and either will comfortably fit to replace an elevated roundabout, dumbbell or close diamond.

Looking at the many USA examples, there seem to be two broad categories of DDI implementation. The first category is exemplified by the very first one built, at Springfield MO. It's a simple replacement of a struggling standard close diamond - low cost, same footprint, effective. AU's second DDI, at Bald Hills, is similar. The other category has many members in the USA, and is also exemplified by AU's first one. Its members are mainly new-builds, with much larger footprints. Their motivations are obviously not the saving of space, but presumably structural cost-effectiveness, as for cloverleafs.

I believe that the USA's DDI total, including those in-construction, is now approaching 200. Not a passing fad.
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Re: Diverging Diamonds

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Peter Freeman wrote: Wed Apr 13, 2022 03:07 AU's second DDI, at Bald Hills, is similar. The other category has many members in the USA, and is also exemplified by AU's first one. Its members are mainly new-builds, with much larger footprints. Their motivations are obviously not the saving of space, but presumably structural cost-effectiveness, as for cloverleafs.
Certainly it's a matter of cost-effectiveness, though land take will be a factor there. This six ramp parclo in my preferred configuration, with six of eight turns freeflow, is actually an upgrade of a cloverleaf, and the land take is not much lower. NMU access can also be awkward. It would simply be overkill for the kind of site where a little two-lane DDI is used. The modern hierarchy is something like this I'd suggest:

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

The UK roundabout-centric hierarchy is more like this:

1. Diamond=parclo (four ramp)
2. Dumbbell
3. Roundabout interchange
4. Hamburger interchange
5. Stacked roundabout
6. Full freeflow

PS - Most actual North American six ramp parclos make most of the turns stop at traffic lights, which is not a design I particularly recommend.
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Re: Diverging Diamonds

Post by Bryn666 »

If you have pedestrians then signals probably become a requirement, but these can be set to demand activated.

However I would argue that if you need a six ramp parclo to begin with then you should actually be building Dutch style pedestrian and cycle subways so there are no conflicts at grade between modes. Even the general uselessness that is the DMRB acknowledges this.
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Re: Diverging Diamonds

Post by jackal »

I quite agree about giving pedestrians separate subways or bridges, though the UK has rather turned against them as part of the signalisation craze.

It's quite common to put the NMU route at a DDI along the central reservation (e.g., https://www.google.co.uk/maps/@38.94162 ... a=!3m1!1e3). Logically you could do the same for a freeflow six ramp parclo, with the reservation route extending to the first at-grade junction in each direction. I've never seen that though!

Also notable is that many DDIs in this thread don't have NMU provision anyway (not even the Danish example).
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Re: Diverging Diamonds

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jackal wrote: Wed Apr 13, 2022 19:00 I quite agree about giving pedestrians separate subways or bridges, though the UK has rather turned against them as part of the signalisation craze.

It's quite common to put the NMU route at a DDI along the central reservation (e.g., https://www.google.co.uk/maps/@38.94162 ... a=!3m1!1e3). Logically you could do the same for a freeflow six ramp parclo, with the reservation route extending to the first at-grade junction in each direction. I've never seen that though!

Also notable is that many DDIs in this thread don't have NMU provision anyway (not even the Danish example).
Yes, I came up with a concept for M60 J7 which would have routed cyclists down the middle as this means you have the absolute minimum number of signal stages to make the whole thing operate as opposed to the current offset diamond which has to have five lanes to cope with the conflicting turns. It's embarrassing that we won't even consider DDIs in this country beyond a few attempts that always end up going in the rejected pile.
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Re: Diverging Diamonds

Post by Peter Freeman »

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.
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