Diverging Diamonds
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Re: Diverging Diamonds
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
Re: Diverging Diamonds
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.
Re: Diverging Diamonds
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
Re: Diverging Diamonds
M60 Junction 7 is the perfect test site for one.
Bryn
Terminally cynical, unimpressed, and nearly Middle Age already.
She said life was like a motorway; dull, grey, and long.
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Terminally cynical, unimpressed, and nearly Middle Age already.
She said life was like a motorway; dull, grey, and long.
Blog - https://showmeasign.online/
X - https://twitter.com/ShowMeASignBryn
YouTube - https://www.youtube.com/@BrynBuck
Re: Diverging Diamonds
Unfortunately not. The DDI would not work here and to use it would undermine it completely as a concept.
Bryn
Terminally cynical, unimpressed, and nearly Middle Age already.
She said life was like a motorway; dull, grey, and long.
Blog - https://showmeasign.online/
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Terminally cynical, unimpressed, and nearly Middle Age already.
She said life was like a motorway; dull, grey, and long.
Blog - https://showmeasign.online/
X - https://twitter.com/ShowMeASignBryn
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Re: Diverging Diamonds
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.
Bryn
Terminally cynical, unimpressed, and nearly Middle Age already.
She said life was like a motorway; dull, grey, and long.
Blog - https://showmeasign.online/
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YouTube - https://www.youtube.com/@BrynBuck
Terminally cynical, unimpressed, and nearly Middle Age already.
She said life was like a motorway; dull, grey, and long.
Blog - https://showmeasign.online/
X - https://twitter.com/ShowMeASignBryn
YouTube - https://www.youtube.com/@BrynBuck
Re: Diverging Diamonds
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.)
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.)
Re: Diverging Diamonds
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.
Re: Diverging Diamonds
See attached for a demo of who goes and when.
Bryn
Terminally cynical, unimpressed, and nearly Middle Age already.
She said life was like a motorway; dull, grey, and long.
Blog - https://showmeasign.online/
X - https://twitter.com/ShowMeASignBryn
YouTube - https://www.youtube.com/@BrynBuck
Terminally cynical, unimpressed, and nearly Middle Age already.
She said life was like a motorway; dull, grey, and long.
Blog - https://showmeasign.online/
X - https://twitter.com/ShowMeASignBryn
YouTube - https://www.youtube.com/@BrynBuck
Re: Diverging Diamonds
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?
Re: Diverging Diamonds
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.
Bryn
Terminally cynical, unimpressed, and nearly Middle Age already.
She said life was like a motorway; dull, grey, and long.
Blog - https://showmeasign.online/
X - https://twitter.com/ShowMeASignBryn
YouTube - https://www.youtube.com/@BrynBuck
Terminally cynical, unimpressed, and nearly Middle Age already.
She said life was like a motorway; dull, grey, and long.
Blog - https://showmeasign.online/
X - https://twitter.com/ShowMeASignBryn
YouTube - https://www.youtube.com/@BrynBuck
Re: Diverging Diamonds
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.Bryn666 wrote: ↑Wed Aug 15, 2018 16:23I 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.
Chris
Roads.org.uk
Roads.org.uk
Re: Diverging Diamonds
Here's how your variant would work - assume on both versions that the entry to the weaving area would indeed be under signal control.Chris5156 wrote: ↑Wed Aug 15, 2018 16:35My 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.Bryn666 wrote: ↑Wed Aug 15, 2018 16:23I 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.
Bryn
Terminally cynical, unimpressed, and nearly Middle Age already.
She said life was like a motorway; dull, grey, and long.
Blog - https://showmeasign.online/
X - https://twitter.com/ShowMeASignBryn
YouTube - https://www.youtube.com/@BrynBuck
Terminally cynical, unimpressed, and nearly Middle Age already.
She said life was like a motorway; dull, grey, and long.
Blog - https://showmeasign.online/
X - https://twitter.com/ShowMeASignBryn
YouTube - https://www.youtube.com/@BrynBuck
Re: Diverging Diamonds
Variant 1 is best for turning traffic.
Variant 2 is best for through traffic.
Variant 2 is best for through traffic.
- ScottB5411
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- Location: St. Louis, Missouri, USA
Re: Diverging Diamonds
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.
This junction was awful until they installed the DDI, now it flows smoothly all day round.
How about some more beans Mr. Taggart?
Re: Diverging Diamonds
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.Mark Hewitt wrote: ↑Tue Aug 14, 2018 08:34I 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.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
- Mark Hewitt
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- Location: Chester-le-Street
Re: Diverging Diamonds
I spy a roundabout on the business park there too!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.
Re: Diverging Diamonds
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.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?
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!)