160ish, almost all in the US. I don't think that I'd call the US a "more enlightened country" on almost any topic, least of all road design.
That's not a high bar.
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Why doesn't the UK have any diverging diamond intersections?
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ABB125
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I quite agree with you here about stop-start through multiple signalised junctions. Especially when the signal phasing appears to be somewhat uncoordinated between adjacent junctions. (The same also applies to signalised roundabouts surrounded by other signalised junctions.)
I was mainly thinking of more "isolated" junctions (further away from other junctions) where two roads meet, rather than right in the middle of an urban area. Having said that, here's a signalised roundabout which I'm familiar with, in an urban area, which would be interesting to consider the effect of replacing with a crossroads: https://maps.app.goo.gl/Yzit4hnXCc5dmTGq6
Indeed. But nowhere near all.
In some locations. But all too often, the default of a roundabout (of whichever variety) is used, despite lacking the capacity for the junction in question*. For example, M62 J29 and M25 J2, both of which have been partially improved since construction.
And there are plenty of junctions which are completely inappropriate for their situation, whether by design, cancellation of later elements of a scheme, or by continued bodging over the years as additional roads get added. M25 J10 and M6 J26 spring to mind.
*Notwithstanding arguments about "the traffic modelling said it would be OK" or "it was built 50 years ago when there was less traffic".
M25 J10 (again), M6 J11, M1 J24 (and surrounding mess) and M42 J6 (all recent/in progress "upgrades") would beg to differ. A quick glance at Dublin shows what can be done with similar junctions in similar environments if the willpower is there.
Occasionally National Highways/their consultants will come up with something which isn't a roundabout, and is actually quite good. M62/A1(M) interchange is one example, although still suffers from gold-plated (no doubt they're "world beating") DMRB* standards. The Lower Thames Crossing, if it goes ahead, is another example (if anything, the two "interesting" junctions have gone too far, and include unnecessary complexity).
*design manual for roads and bridges - see link in next post
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However, it's worth noting that in many of the cases I've mentioned above, a diverging diamond probably wouldn't be suitable, so is somewhat off topic.
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How about the single point urban interchange? Another can of worms!
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DMRB - Design Manual for Roads & Bridges, I think. https://www.standardsforhighways.co.uk/search/5e2ca037-2573-4d3c-9094-f82666ae1db9
ABB125
Established Member
Yes, correct (sorry, I will edit my post accordingly)
Obviously nowhere near all junctions are signalised crossroads, why would the number of signaled crossroads be anywhere near all junctions when they are not the right solution in all cases.
If you researched the history of these particular examples there would be various reasons for those particular junctions ending up the way they did, but attributing them to some imaginary fear of anything other than roundabouts is nonsense.
ABB125
Established Member
Of course - it was not my intention to imply so, apologies if that is what comes across. "... of junctions with traffic volumes warranting something more than a basic roundabout or priority junction" would perhaps be a suitable suffix to my original post.
By far the majority of junctions in this country would come into the latter category.
I'm well aware of the historical reasons (often financial, or change of government/policies) which resulted in these junctions. That doesn't, however, explain why subsequent "improvements" haven't tried something a bit more effective.
(Yes, DMRB makes grade separation etc expensive, but all that's needed to solve this is political will.)
You seem to have jumped to the conclusion that diverging diamond intersections constitute "something a bit more effective" than roundabouts and are automatically what should be tried in these cases. In reality different solutions have been tried in various cases, but you seem to be discounting trials of anything other than diverging diamond intersections.
They have several disadvantages compared to more common types of junctions, including:
- Creating two points of intersection between the through traffic in each direction, meaning that traffic can's flow straight through in both directions at the same time
- Complexity, making it difficult for drivers to navigate leading to mistakes and accidents
ABB125
Established Member
Yes, there are some disadvantages, but that doesn't mean they should be ruled out. If there were to be a trial at a suitable* location, and it turned out that there was no improvement to traffic flow, that's fine, I would accept that. But there isn't yet in this country...
*what counts as a suitable location will, of course, vary depending on who you ask, and what their bias/desired trial outcome is!
Apologies, that was not my intention. Obviously there are many cases where a diverging diamond would not be appropriate, but there will certainly be a few locations where they may well be the right solution
If we aren’t there already, I imagine in the next few years we will be able to design the most efficient junctions using road traffic simulators. However, even if you can the best junction on a computer it may not be the one that is built due to other issues such as cost, environmental impact etc.
Has there actually been any policy decision to rule them out, or have they just not be found to be the best solution for any junction which was being planned?
I don't think there is any obligation on every country in the world to try every type of junction anyone could possible imagine.
== Doublepost prevention - post automatically merged: ==
Surely that is already done, and has been for a number of decades.
renegademaster
Established Member
Surely its accepted roundabouts have flaws especially for land take and capacity but the advantage being they are safer
One was actually proposed by consultants for J23 of the M6, although it never actually happened, obviously
In which case roundabouts are clearly the best cost vs efficiency design in most situations and that’s the answer to the OPs question…
Crithylum
Member
I guess another reason is that DDIs are a ‘recent’ invention, and the UK has not invested in much infrastructure recently. This means that none have been built partially because there is no money for new roads/intersection rebuilds, unlike certain parts of America, which seem to throw endless money at the pit which is more road capacity that still gets clogged up anyways.
edwin_m
Veteran Member
Check out PTV Vissim (other microsimulations are probably available, that's just the one I know of).
HSTEd
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I could see some advantages of this style of crossing over the traditional bridged over roundabout, at least in some situations.
As an example, the reconstruction of the A52/A46 junction has some pretty dodgy aspects despite being quite new, largely (IMO) due to the decision to have two roundabouts crammed so closely together with such radical changes in direction.
As an example, the reconstruction of the A52/A46 junction has some pretty dodgy aspects despite being quite new, largely (IMO) due to the decision to have two roundabouts crammed so closely together with such radical changes in direction.
The roundabouts with cut-offs and signallised "through-about" lanes, and good balance between the phases, can have very high capacity indeed, and run at capacity without losing too much speed. M6 J19 Tabley Interchange seems to flow smoothly under even the heaviest traffic conditions, and it's signallised and not fully grade-separated.
class397tpe
Member
Apologies to resurrect a month-old thread, but I've always thought M4 J3 would be a great candidate for a diverging diamond interchange. It's a pretty awful roundabout as it causes queues in all directions, including standing traffic back onto the 60mph M4 eastbound - pretty dangerous. Doesn't appear the interchange has really been touched since it was built, bar signalisation.
The bridge that takes the M4 over the junction already has space, as it is continuous with multiple arches. You could quite easily fit a generous diverging diamond interchange here, at a relatively low cost as no new bridges would be required.
Of course, as the A312 is managed by TFL, the chances of this junction ever being improved are zero. Major roads in London have effectively been abandoned for 25 years with many unsafe and awful layouts remaining that would have been fixed elsewhere in the country years ago.
The A312 itself could do with upgrading to remove the awful Hayes road roundabout to the north, perhaps with a SPUI like below. Of course, more chance for pigs to fly.
The bridge that takes the M4 over the junction already has space, as it is continuous with multiple arches. You could quite easily fit a generous diverging diamond interchange here, at a relatively low cost as no new bridges would be required.
Of course, as the A312 is managed by TFL, the chances of this junction ever being improved are zero. Major roads in London have effectively been abandoned for 25 years with many unsafe and awful layouts remaining that would have been fixed elsewhere in the country years ago.
The A312 itself could do with upgrading to remove the awful Hayes road roundabout to the north, perhaps with a SPUI like below. Of course, more chance for pigs to fly.
I like that plan. I used to use this junction a lot, and it certainly needs an upgrade.
One nice thing about it is that the large amount of space under the bridge means it wouldn't look like a diverging diamond from a motorist's point of view. You could have large hedges between the wrong direction traffic flows, and effectively it would feel like a signalised whirlpool junction driving through it (i.e. Don't try to think of it in a map view, just follow the signs and drive!) This would therefore be a good gentle introduction to the junction type.
Do you need the merge /unmerge for the M4 right turns under the bridge, or could these cross with signals (sync'd with tge adjacent signals so you don't need to stop an extra time) - could make it a little simpler on the ground to navigate with no possibility of going back up the motorway you've just left.
One nice thing about it is that the large amount of space under the bridge means it wouldn't look like a diverging diamond from a motorist's point of view. You could have large hedges between the wrong direction traffic flows, and effectively it would feel like a signalised whirlpool junction driving through it (i.e. Don't try to think of it in a map view, just follow the signs and drive!) This would therefore be a good gentle introduction to the junction type.
Do you need the merge /unmerge for the M4 right turns under the bridge, or could these cross with signals (sync'd with tge adjacent signals so you don't need to stop an extra time) - could make it a little simpler on the ground to navigate with no possibility of going back up the motorway you've just left.
Looking at the M4 J3 diagrams it occurs to me that a diverging diamond is similar to a magic roundabout with the links under the bridges one-way instead of two. Wikipedia seems to suggest that magic roundabouts are a uniquely British phenomenon. Perhaps we don't have diverging diamonds because we have magic roundabouts. If M4 J3 became a magic roundabout with the two links under the motorway running anti-clockwise, how would that compare with the traffic-light-laden diverging diamond?
DM352
Member
I have used one in North America and would say it works fine for where most traffic turns off the diamond overpass.Going straight through the diamond feels like a gauntlet railway on a previous two way track so is impossible for both sides to cross on green.
The junction before the diamond used to be a 4 way traffic signal at grade with a major road which the developers at the time did the bare minimum for their site access roads.
The biggest issue for me is where there are four way traffic light junctions right after the diamond making it a pain to get in the correct lane if traffic is busy and you join it from a diamond slip road.
I'd suggest there are two different forms really - urban and rural/semi-rural.
Note here that I'm talking specifically about diverging diamond interchanges, as per the post title - not diamond interchanges generally.
I would've thought space/land requirements would be a significant barrier in the UK when it comes to urban. It's difficult enough getting planning permission and compulsory purchasing. Diverging diamond interchanges don't take up a monumental amount of space, but they do take up more space than a roundabout in urban settings. Diamond interchanges would work well however.
The places where they would be easier to fit would be leaving major roads onto 'minor' (less major) roads in semi-rural/rural settings. In these situations we often have signal-controlled dumbbells or a pair of signal-controlled roundabouts. By signal-controlling them you've already made part of the infrastructure change require for diverging diamond interchanges, and frankly you'd be better off for ditching the roundabouts imo.
However until the road design manuals change to encourage diverging diamond interchanges, that's the main reason we don't have more. For what it's worth, the UK has a fantastic record on road safety - top 10 in the world per capita last time I checked. Obviously there's always room for improvement and we should strive for it, but on this basis of international comparisons, and the fact that our killed and seriously injured numbers are decreasing despite an increasing population; there's probably a reluctance in the DfT to go making significant changes to road design recommendations.
At a push, I imagine we'd get them at at the rate we're getting 'Dutch style roundabouts' (which is 3 roundabouts in 5 years).
Note here that I'm talking specifically about diverging diamond interchanges, as per the post title - not diamond interchanges generally.
I would've thought space/land requirements would be a significant barrier in the UK when it comes to urban. It's difficult enough getting planning permission and compulsory purchasing. Diverging diamond interchanges don't take up a monumental amount of space, but they do take up more space than a roundabout in urban settings. Diamond interchanges would work well however.
The places where they would be easier to fit would be leaving major roads onto 'minor' (less major) roads in semi-rural/rural settings. In these situations we often have signal-controlled dumbbells or a pair of signal-controlled roundabouts. By signal-controlling them you've already made part of the infrastructure change require for diverging diamond interchanges, and frankly you'd be better off for ditching the roundabouts imo.
However until the road design manuals change to encourage diverging diamond interchanges, that's the main reason we don't have more. For what it's worth, the UK has a fantastic record on road safety - top 10 in the world per capita last time I checked. Obviously there's always room for improvement and we should strive for it, but on this basis of international comparisons, and the fact that our killed and seriously injured numbers are decreasing despite an increasing population; there's probably a reluctance in the DfT to go making significant changes to road design recommendations.
At a push, I imagine we'd get them at at the rate we're getting 'Dutch style roundabouts' (which is 3 roundabouts in 5 years).
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