Diverging Diamonds

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Herned
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Re: Diverging Diamonds

Post by Herned »

Does anyone have a suggestion of where would be a good place to try one of these? The Harrow Road junction on the North Circular is the best location I can think of that would only need road alterations and no bridge works to fit it in
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Robunos
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Re: Diverging Diamonds

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M6 Junction 10? Due to be rebuilt soon anyway . . .
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wrinkly
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Re: Diverging Diamonds

Post by wrinkly »

Robunos wrote: Tue Aug 14, 2018 23:04 M6 Junction 10? Due to be rebuilt soon anyway . . .
Isn't a DD a bad idea for any junction with more than four arms?

It's not instantly obvious that there's even a way of doing it.
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Re: Diverging Diamonds

Post by Herned »

That's the problem, they only work with 4 arms, and presumably need fairly balanced traffic flows across the different routes. I can't think of many 'pure' junctions which would be suitable
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Re: Diverging Diamonds

Post by Bryn666 »

Herned wrote: Wed Aug 15, 2018 08:26 That's the problem, they only work with 4 arms, and presumably need fairly balanced traffic flows across the different routes. I can't think of many 'pure' junctions which would be suitable
M60 Junction 7 is the perfect test site for one.
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Re: Diverging Diamonds

Post by Bryn666 »

Robunos wrote: Tue Aug 14, 2018 23:04 M6 Junction 10? Due to be rebuilt soon anyway . . .
Unfortunately not. The DDI would not work here and to use it would undermine it completely as a concept.
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Re: Diverging Diamonds

Post by Bryn666 »

Herned wrote: Tue Aug 14, 2018 22:06 Does anyone have a suggestion of where would be a good place to try one of these? The Harrow Road junction on the North Circular is the best location I can think of that would only need road alterations and no bridge works to fit it in
Alas, no. The arrangement of the slip roads require straight through movements from local roads so you would need u-turn provision on the A404 itself.
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Re: Diverging Diamonds

Post by Herned »

Bryn666 wrote: Wed Aug 15, 2018 10:17 M60 Junction 7 is the perfect test site for one.
(checks map). Yes, certainly looks ideal
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Re: Diverging Diamonds

Post by ais523 »

I'd guess that a diverging diamond interchange would work better than a signalised roundabout but would not be appropriate in places where an unsignalised roundabout would work. I think it might be a "local optimum" for junctions with a single bridge (or two parallel bridges across the main flow of traffic) and exactly two signal stages, in that it avoids any conflicts and the "fill bridges, then empty bridges" timings mean that vehicles will typically have to wait at at most one red light.

I suspect the major disadvantage is that it can't easily be changed to a trigger-based (rather than timing-based) light sequence without throwing all the timings off and causing vehicles to get stuck inside the junction, and thus they might cause extra delays at quiet periods. (I guess that what you'd have to do would be to rest the lights at "fill" position, then when a vehicle reached them, change the lights to "empty" at about the time the vehicle in question would reach the second set of lights. A following vehicle could sneak onto the bridge "behind" it before the lights changed, so this would scale as capacity increased and eventually degenerate into fully timing-based as traffic got heavy. However, this is still considerably worse than a normal signalised junction in cases where you have a small amount of traffic which is all going in the same direction. I guess you could add two more, optional, phases to handle unbalanced flows, but that seems like it may be missing the point behind the junction.)
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Re: Diverging Diamonds

Post by nowster »

ais523 wrote: Wed Aug 15, 2018 14:00 I suspect the major disadvantage is that it can't easily be changed to a trigger-based (rather than timing-based) light sequence without throwing all the timings off and causing vehicles to get stuck inside the junction...
The two phases aren't "fill" and "release" as the default but "left-to-right" and "right-to-left". Traffic getting stuck in the middle will only ever encounter one red light.
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Re: Diverging Diamonds

Post by Bryn666 »

See attached for a demo of who goes and when.
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ddi-stages.png
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Re: Diverging Diamonds

Post by ChrisH »

Bryn666 wrote: Wed Aug 15, 2018 15:45 See attached for a demo of who goes and when.
That suggests limited stacking space for vehicles going straight on E-W - they all have to stop at least once. Does that present a problem at sites where this has been implemented?
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Re: Diverging Diamonds

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ChrisH wrote: Wed Aug 15, 2018 15:47
Bryn666 wrote: Wed Aug 15, 2018 15:45 See attached for a demo of who goes and when.
That suggests limited stacking space for vehicles going straight on E-W - they all have to stop at least once. Does that present a problem at sites where this has been implemented?
I suspect, but you'd have to run a model on it, that you offset that limitation by running shorter stage times rather than having long wait times and stop-start conditions. The American ones of course utilise three and four lane stacking areas because they have acres of room to play with.

I suspect you could still run the streams separately so you don't stop in the middle but you'd have longer wait times on the approach to the junction that way. Either way, it still retains a two stage configuration whichever method you use.
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Re: Diverging Diamonds

Post by Chris5156 »

Bryn666 wrote: Wed Aug 15, 2018 16:23
ChrisH wrote: Wed Aug 15, 2018 15:47
Bryn666 wrote: Wed Aug 15, 2018 15:45 See attached for a demo of who goes and when.
That suggests limited stacking space for vehicles going straight on E-W - they all have to stop at least once. Does that present a problem at sites where this has been implemented?
I suspect, but you'd have to run a model on it, that you offset that limitation by running shorter stage times rather than having long wait times and stop-start conditions. The American ones of course utilise three and four lane stacking areas because they have acres of room to play with.

I suspect you could still run the streams separately so you don't stop in the middle but you'd have longer wait times on the approach to the junction that way. Either way, it still retains a two stage configuration whichever method you use.
My gut feeling is that it would be better to run the streams separately, if only because - on the type you've drawn with a weaving section and uncontrolled entry from sliproads to turn right - you'd have traffic not under signal control trying to weave into a queue and stacking back onto the sliproad. Better to either have them signal controlled, or have traffic on the main flow stopped when entering traffic is weaving over to face a red light.
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Re: Diverging Diamonds

Post by Bryn666 »

Chris5156 wrote: Wed Aug 15, 2018 16:35
Bryn666 wrote: Wed Aug 15, 2018 16:23
ChrisH wrote: Wed Aug 15, 2018 15:47

That suggests limited stacking space for vehicles going straight on E-W - they all have to stop at least once. Does that present a problem at sites where this has been implemented?
I suspect, but you'd have to run a model on it, that you offset that limitation by running shorter stage times rather than having long wait times and stop-start conditions. The American ones of course utilise three and four lane stacking areas because they have acres of room to play with.

I suspect you could still run the streams separately so you don't stop in the middle but you'd have longer wait times on the approach to the junction that way. Either way, it still retains a two stage configuration whichever method you use.
My gut feeling is that it would be better to run the streams separately, if only because - on the type you've drawn with a weaving section and uncontrolled entry from sliproads to turn right - you'd have traffic not under signal control trying to weave into a queue and stacking back onto the sliproad. Better to either have them signal controlled, or have traffic on the main flow stopped when entering traffic is weaving over to face a red light.
Here's how your variant would work - assume on both versions that the entry to the weaving area would indeed be under signal control.
Attachments
ddi-stages2.jpg
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Re: Diverging Diamonds

Post by nowster »

Variant 1 is best for turning traffic.
Variant 2 is best for through traffic.
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Re: Diverging Diamonds

Post by ScottB5411 »

THIS is the one I seem to drive through the most, off I-270 in St. Louis at Dorsett.

This junction was awful until they installed the DDI, now it flows smoothly all day round.
How about some more beans Mr. Taggart?
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Stevie D
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Re: Diverging Diamonds

Post by Stevie D »

Mark Hewitt wrote: Tue Aug 14, 2018 08:34
ScottB5411 wrote: Tue Aug 14, 2018 02:02 The USA could and should probably replace a good 50% of it's 4 way stops with roundabouts
I can see the point of them in urban areas where two roads meet at a crossroads and putting a roundabout in, even a mini one would be a bit of a squeeze. I think the reality is if you dropped a US city into the UK without road markings we'd mark up most of the 4 way stops as simple give ways with priority given to one of the roads.
In residential areas where there is no through traffic at all, that's probably true, but I suspect a lot of them would end up as mini-roundabouts ... gives equal priority to all traffic, but (in theory) keeps speeds low without requiring drivers to come to a complete stop unnecessarily.
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Re: Diverging Diamonds

Post by Mark Hewitt »

ScottB5411 wrote: Wed Aug 15, 2018 18:17 THIS is the one I seem to drive through the most, off I-270 in St. Louis at Dorsett.

This junction was awful until they installed the DDI, now it flows smoothly all day round.
I spy a roundabout on the business park there too!
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Stevie D
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Re: Diverging Diamonds

Post by Stevie D »

Mark Hewitt wrote: Sun Aug 12, 2018 09:19Yes doesn’t the GSJ roundabout not provide the same advantages with the disadvantage of requiring two bridges?
The big advantage of the roundabout interchange over a diverging diamond is that it can accommodate more than a 4-way cross. If you need to plug additional roads in then they would need to join up at a separate junction away from the main intersection, which there may not be space for, particularly in built-up or intensively used areas. A lot of struggling interchanges in the UK are more than 4-way junctions, which makes them unsuitable for DDIs.

I'm not knocking DDIs, I think they are great and we really should be trialling them in the UK, but we have to be realistic about how widely they can be deployed.

(On the other hand, if it means we can stop tacking service stations and retail/industrial estates onto motorway junctions because it won't be possible to tie in the access roads, that's got to be a good thing!)
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