Diverging Diamonds

[Bryn666]

Is the WSP report available to read?

See Figure 1-4 of this report. https://www.sthelens.gov.uk/media/33176 ... ptions.pdf

The link has changed to this: https://www.sthelens.gov.uk/media/2091/ ... 7387470000

And here's the DDI in all its glory:


M6 J23 - Copy.JPG

The A49 poses an issue but I'd argue you could easily sort this with some clever realignment either side. Possibly via the A599 for the western side and building a new link road to a new junction further east along the A580 for the eastern side. Not insurmountable.

Likewise, the petrol station would simply need its access reviewing so that you enter via the M6 northbound entry slip and exit onto the eastbound A580, thus retaining full access.

[Peter Freeman]

Bump!

I like Jackal's GSJ hierarchy (subject always of course to local considerations) ...
... 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 (simplification).

... 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 It's basically superseded by the DDI and SPUI, much like the two-bridge roundabout - which doesn't stop them getting built, of course.

My current version of the motorway interchange competence table is as follows -

1. Diamond (unsignalised)
2. Dumbbell
3. Diamond (signalised)
4. Parclo five-ramp (one loop)
5. DDI
6. SPUI
7. Parclo six-ramp (two loops)
8. Full freeflow

That's Jackal's list, but with two additions by me: signalised diamond in position 3 and five-ramp parclo in position 4. It's a USA or international list - the UK version is rather different.

Reviewing the discussion from a couple of years ago, my recent thoughts, with an AU bias, are -

1. Unsignalised diamonds are only suitable for motorway interchanges with minor rural roads. Perhaps dumbbells also, soon.

2. The signalised diamond appears to still be the default non-freeflow GSJ in Australia, and, I believe, in Canada. Possibly most new builds in USA are DDI - I'm not sure. Therefore it remains in the modern hierarchy. It can have very high capacity. AU has many. Here are examples that are topical at present -

a) A new signalised diamond recently replaced an incomplete half dumbbell here that was originally intended to be completed as a full dumbbell. Clearly, the tide in AU has turned against urban dumbbells -
https://www.google.co.uk/maps/place/Pak ... ?entry=ttu

b) The dumbbell here is also currently being replaced. A DDI would fit, but actually it will get a large signalised diamond. Why? Because (my guess) although it will initially have a high west-to-south flow, that movement will in future be carried by a planned motorway-grade corner-cut, leaving this interchange as more straight-through -
https://www.google.co.uk/maps/place/Pak ... ?entry=ttu

c) This struggling signalised diamond will soon be widened, but with no change to its form. Why? Probably not enough space on its south side for a DDI -
https://www.google.co.uk/maps/place/Pak ... ?entry=ttu

3. The above three examples are all on one stretch of Melbourne's M1, built fifteen years ago with five consecutive dumbbells. It's obvious now that they will all eventually be replaced. This echoes what is happening in other AU cities.

4. The DDI marches forward, especially in the USA where about 160 are in operation. AU appears to be in distant second place with 3 built and 4 more committed, and Canada third with 3 built. Its strengths: handling high turning volumes, low cost, and it's an easy upgrade for a struggling diamond.

5. The SPUI continues to reign for heavily-trafficked urban new-builds. Its strength: dynamic adaptability to unbalanced high flows when signalised in four stages. USA has many, AU has about 40.

6. In AU, both DDI and SPUI are usually fully signalised, even the four left turns.

7. The six-ramp parclo is the about the closest you can get to full freeflow, without actually being full freeflow. It's very useful where a busy arterial road crosses a motorway at a skew angle. That generates high volume right turns in opposite quadrants, which is exactly where this type's 270 degree loops are situated. Thus, two right turns become freeflow. The other two rights, and the straight-throughs, remain signalised but only lightly. Here are two of the three that exist in Perth AU -

a) This one is classic layout and provides two freeflow right turns -
https://www.google.com.au/maps/place/Pe ... ?entry=ttu

b) This one is six-ramp but not classic layout because too much other stuff is attached, and so it doesn't quite provide freeflow rights -
https://www.google.com.au/maps/place/Pe ... ?entry=ttu

8. In a five-ramp parclo only one of the four right turns is a loop. This does not produce a freeflow right, but it means that the looped right turn needs to pass through only one signal, instead of the two in a diamond. This suits some situations where one turn dominates. Here are two of the several that exist in Perth -

a) Here's a classic example -
https://www.google.com.au/maps/place/Pe ... ?entry=ttu

b) And here's an impure form, since one loop is a genuine right turn only but the other functions also as a folded left turn. It still does allow two right turns to encounter only one signal -
https://www.google.com.au/maps/place/Pe ... ?entry=ttu

9. Parclo's can be drawn in many different forms. The loops may be for on-ramps or off-ramps, depending which dominant turns need to be favoured. Some, with subsidiary roads thrown into the mix, become complicated, while some are so simple (four ramps) that they are actually just folded diamonds. The 5-ramp and 6-ramp types well deserve their places in our list.

10. The main point of this rather long post is that in AU the most common motorway service interchange type is diamond, usually signalised. It's our equivalent of UK's GSR. I think this will continue, though I'd love to see the DDI take over.

(Edit: trivial word-smithing)

[jackal]

My current version of the motorway interchange competence table is as follows -

1. Diamond (unsignalised)
2. Dumbbell
3. Diamond (signalised)
4. Parclo five-ramp (one loop)
5. DDI
6. SPUI
7. Parclo six-ramp (two loops)
8. Full freeflow

That's Jackal's list, with two additions by me: signalised diamond in position 3 and five-ramp parclo in position 4. It's a USA or international list - the UK version is rather different.

Reviewing the discussion from a couple of years ago, my recent thoughts, with an AU bias, are -

1. Unsignalised diamonds are only suitable for motorway interchanges with minor rural roads. Perhaps dumbbells also, soon.

2. The signalised diamond appears to still be the default non-freeflow GSJ in Australia, and, I believe, in Canada. Possibly most new builds in USA are DDI - I'm not sure. Therefore it remains in the modern hierarchy. It can have very high capacity. AU examples -

a) A new signalised diamond recently replaced an incomplete half dumbbell here that was originally intended to be completed as a full dumbbell. Clearly, the tide in AU has turned against urban dumbbells -
https://www.google.co.uk/maps/place/Pak ... ?entry=ttu

b) The dumbbell here is also currently being replaced. A DDI would fit, but actually it will get a large signalised diamond. Why? Because (my guess) although it will initially have a high west-to-south flow, that movement will in future be carried by a planned motorway-grade corner-cut, leaving this interchange as more straight-through -
https://www.google.co.uk/maps/place/Pak ... ?entry=ttu

c) This struggling signalised diamond will soon be widened, but with no change to its form. Why? Probably not enough space on its south side for a DDI -
https://www.google.co.uk/maps/place/Pak ... ?entry=ttu
...
8. In a five-ramp parclo only one of the four right turns is a loop. This does not produce a freeflow right, but it means that the looped right turn need pass through only one signal instead of two. This suits some situations where one turn dominates. There are two of these in Perth too -

a) Here's a classic example -
https://www.google.com.au/maps/place/Pe ... ?entry=ttu

b) And here's an impure form, since one loop is a genuine right turn only but the other functions also as a folded left turn. It still does allow two right turns to encounter only one signal -
https://www.google.com.au/maps/place/Pe ... ?entry=ttu

Thanks for this, Peter.

For me, there are two use cases for signalised diamonds: 1) upgrade of an unsignalised diamond or dumbbell; 2) insufficient space for a dumbbell or DDI. I guess it's half in my hierarchy. With a blank piece of paper and a reasonable amount of space I can't ever really see the case for one. That's not true of any of the designs 'fully' in my hierarchy.

Urban dumbbells certainly are still appropriate in many places, including urban interchanges. WSDoT give a good comparison with DDIs:

Experience shows that if roundabout ramp terminals work from a capacity and design standpoint, they are a better option due to being more efficient and sustainable than the a [sic] more expensive DDI. DDIs are reserved for those circumstances where either the volume of left turns are too high or where the dominant flows through the intersection create inequities in flow for a significant portion of the day through a roundabout.

It's also at least easier to plug additional roads into a dumbbell than it is the other main designs. There's an image of a slightly crazy dumbbell/DDI hybrid in Missouri from the above link:

I can't claim to know how well this works!

Regarding five ramp parclos, if they have two loops they can have both long turns (right in UK) freeflow. Here's an example Mapper recently posted (with roundabouts but the principle applies the same):

So while I agree the 5 ramp parclo should be in the hierarchy, it's quite hard to place, as it might handle a junction with one dominant pair of turning movements better than a DDI or SPUI, though as you say its general utility is lower.

As an aside, even a four ramp parclo or "folded diamond" can have both long turns freeflow, though that's somewhat eccentric as the short turns are then convoluted compared to the long turn.

[Peter Freeman]

There's an image of a slightly crazy dumbbell/DDI hybrid in Missouri from the above link:

I can't claim to know how well this works!

That is interesting. I actually posted that same picture in this topic on 11 Apr 2022.

It's very clever in retaining the dumbbell's best feature: accommodation of extra attached roads. However, I'm skeptical of how it would stand up under capacity stress. There's a tricky double conflict point where the teardrop brushes the roundabout. And even more simply damning, it brings back exactly the component whose absence makes the DDI so superior: a roundabout!

[jackal]

There's an image of a slightly crazy dumbbell/DDI hybrid in Missouri from the above link:

I can't claim to know how well this works!

That is interesting. I actually posted that same picture in this topic on 11 Apr 2022.

It's very clever in retaining the dumbbell's best feature: accommodation of extra attached roads. However, I'm skeptical of how it would stand up under capacity stress. There's a tricky double conflict point where the teardrop brushes the roundabout. And even more simply damning, it brings back exactly the component whose absence makes the DDI so superior: a roundabout!

So you did!

I agree about the conflicts at the roundabout. But it may make sense at this particular location. The road off the top of the image (Route 291) is a relatively well aligned D3 with large signalised junctions. All the movements between it and the freeway steer clear of the roundabout. The roundabout rather serves the three S2s off the bottom of the image, and may be appropriate for their presumably much lower volumes. https://www.google.co.uk/maps/@38.90293 ... ?entry=ttu

[Peter Freeman]

For me, there are two use cases for signalised diamonds: 1) upgrade of an unsignalised diamond or dumbbell; 2) insufficient space for a dumbbell or DDI. I guess it's half in my hierarchy. With a blank piece of paper and a reasonable amount of space I can't ever really see the case for one. That's not true of any of the designs 'fully' in my hierarchy.

Another case that could arise is replacement of a dumbbell (over which it has vastly higher capacity) at a location where turning volumes onto and off the motorway are low, but straight-through demand is high. In this scenario, it has, for similar carriageway width, up to 100% higher (theoretical) capacity than a DDI (owing to its >50% duty cycle).

In more practical terms though, and at most real-life sites, I too would prefer a DDI over a standard diamond. (And probably a SPUI over either!)

[Peter Freeman]

But it may make sense at this particular location.

Given that road-hierarchy perspective, you're right, it is appropriate. And clever!

[Bryn666]

But it may make sense at this particular location.

Given that road-hierarchy perspective, you're right, it is appropriate. And clever!

Yeah I'm not seeing any immediate "oh no, you didn't do that" about it. It's unorthodox so would never be allowed here because our overlords have no imagination.

[jackal]

There's an image of a slightly crazy dumbbell/DDI hybrid in Missouri from the above link:

I can't claim to know how well this works!

That is interesting. I actually posted that same picture in this topic on 11 Apr 2022.

It's very clever in retaining the dumbbell's best feature: accommodation of extra attached roads. However, I'm skeptical of how it would stand up under capacity stress. There's a tricky double conflict point where the teardrop brushes the roundabout. And even more simply damning, it brings back exactly the component whose absence makes the DDI so superior: a roundabout!

So you did!

I agree about the conflicts at the roundabout. But it may make sense at this particular location. The road off the top of the image (Route 291) is a relatively well aligned D3 with large signalised junctions. All the movements between it and the freeway steer clear of the roundabout. The roundabout rather serves the three S2s off the bottom of the image, and may be appropriate for their presumably much lower volumes. https://www.google.co.uk/maps/@38.90293 ... ?entry=ttu

On closer inspection this is not US 50/Route 291 in Lee Summit, MO as WSDoT and my link suggested.

It's rather I-49/Arnold Avenue in Grandview, MO: https://www.google.co.uk/maps/@38.84392 ... ?entry=ttu

MDoT are quite proud of the design, claiming a world first for their "divergabout": https://americastransportationawards.or ... t-project/

[Peter Freeman]

^ Ah-ha! Still in Missouri though, the most progressive DOT. And still on a low-traffic road, so it's OK. And we now know there are at least two of them.

I've driven down I49 through there.

[Peter Freeman]

Regarding five ramp parclos, if they have two loops they can have both long turns (right in UK) freeflow. Here's an example Mapper recently posted (with roundabouts but the principle applies the same):


... As an aside, even a four ramp parclo or "folded diamond" can have both long turns freeflow, though that's somewhat eccentric as the short turns are then convoluted compared to the long turn.

1. Mapper's design is a 4-ramp (not 5) parclo = a 'diametrically folded dumbbell' (with an extra bridge over the motorway). As drawn, it has only one freeflow right (using the northern bridge), or two if you stretch to a left turn at a roundabout being counted as freeflow. Given more space, the roundabout lefts could be bypassed, making two out of the four right turns genuine freeflow. The cost is that only two of the left turns are then freeflow. You win some, you lose some.

The usual 5-ramp layout, with one loop and with all its freeflow left turns around the outside (not folded), provides no freeflow right turns, but makes the looped one much nicer (only one light to pass through).

So while I agree the 5 ramp parclo should be in the hierarchy, it's quite hard to place, as it might handle a junction with one dominant pair of turning movements better than a DDI or SPUI, though as you say its general utility is lower.

I place it just below DDI and SPUI, as I'm not obsessed by nonstoppedness. But yes, it's hard to place. At the end of the day, more than half of all GSJ's end up as hybrids.

[jackal]

Regarding five ramp parclos, if they have two loops they can have both long turns (right in UK) freeflow. Here's an example Mapper recently posted (with roundabouts but the principle applies the same):


... As an aside, even a four ramp parclo or "folded diamond" can have both long turns freeflow, though that's somewhat eccentric as the short turns are then convoluted compared to the long turn.

1. Mapper's design is a 4-ramp (not 5) parclo = a 'diametrically folded dumbbell' (with an extra bridge over the motorway).

I was assuming that the northbound offslip remained, but just wasn't drawn on (no need given it stays where it is). Not that much rides on 4 versus 5 ramp in this case.