Why doesn't the UK have any diverging diamond intersections?

[Crithylum]

Is it that roundabouts are better*? Or is it that no-one wants to be the first in case it goes wrong?

*better could mean higher capacity, cheaper, safer, more convenient for pedestrians/cyclists, or anything relevant

 

[ABB125]

Is it that roundabouts are better*?

It depends. However, roundabouts are the British way. Thinking outside the box (circle!) simply doesn't happen. National Highways seem to be terrified of anything which isn't a roundabout, and everyone else just copies them.

Or is it that no-one wants to be the first in case it goes wrong?

There are hundreds (maybe thousands?) of diverging diamonds in use around the world in more enlightened countries. We would not be the first.


The UK has a phobia of anything "new" or "different", and seemingly refuses to use best practise from abroad. And on the rare occasions when a project does manage to get past all the political, financial and "not invented here" hurdles, the industry manages to make a complete mess of it. (See also HS2.)

 

[HSTEd]

It depends. However, roundabouts are the British way. Thinking outside the box (circle!) simply doesn't happen. National Highways seem to be terrified of anything which isn't a roundabout, and everyone else just copies them.

Then they stick traffic lights on them, destroying the entire point of the roundabout in the first place!

 

[ABB125]

Then they stick traffic lights on them, destroying the entire point of the roundabout in the first place!

Indeed. In many (most?) cases a properly-designed signalised crossroads (or diverging diamond!) would be far more effective than a signalised roundabout.
Of course, that assumes a 3- or 4-arm junction. Anything more than that and you get collective panic from the client (who has no expertise in road junctions) and consultant/designer ("something complicated! We can't cope with that, it actually requires effort") and thus a roundabout (or, depending on the exact junction layout, a squareabout) gets built.

 

[Ediswan]

There are hundreds (maybe thousands?) of diverging diamonds in use around the world in more enlightened countries. We would not be the first.

Source please.

 

[edwin_m]

Looking at these, they do create a potential for a near head-on collision in the event of a "SPAD" of the signalised junction where the directions of the cross-road swap sides. With a signalised roundabout the worst case is a sidelong collision and with a signalised crossroads it's front to side.

 

[renegademaster]

with a signalised crossroads it's front to side.

And at road speed , so quite faster than someone turning out a roundabout.

The UK has a phobia of anything "new" or "different", and seemingly refuses to use best practise from abroad

Theirs also a section of Brits, who seem to think everything done in Britain is inferior and we should just blindly copy whatever Germany is doing.

 

[John Webb]

Can someone explain what a 'Diverging Diamond Intersection' is, please? I've never heard of them before.

 

[HSTEd]

Can someone explain what a 'Diverging Diamond Intersection' is, please? I've never heard of them before.

A relatively new style of crossing, first developed in France but popularised in the US.

It involves one of the two roads involved temporarily reversing its customary traffic direction (left hand traffic becomes right hand traffic), allowing all relevant crossing moves on the flat with only two phases on its traffic lights.
Here's an image from the Wikipedia page that shows what it looks like.

Looking at these, they do create a potential for a near head-on collision in the event of a "SPAD" of the signalised junction where the directions of the cross-road swap sides. With a signalised roundabout the worst case is a sidelong collision and with a signalised crossroads it's front to side.

The slaloming probably has a traffic calming efect though, I believe in the US they are believed to be safer than what came before.

 

[ABB125]

Source please.

According to Wikipedia*, there were "over 160" in use in late 2022, with another 80 or so under construction (mostly in the USA)

*Yes, I know Wikipedia isn't a primary source

 

[John Webb]

Thanks for the explanation.

 

[Peter Mugridge]

with a signalised crossroads it's front to side.

Not necessarily; at least at this one there is a high potential for a head on...

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[JLH4AC]

Then they stick traffic lights on them, destroying the entire point of the roundabout in the first place!

Installing traffic lights does not destroy the entire point of roundabouts, when done right signalising the roundabout stops the roundabout from being blocked by imbalanced traffic flows or traffic going over its capacity which keeps traffic flowing more freely and aids the entry of traffic from more minor roads, also in more urban settings signalised roundabout can reduce risks for pedestrians and cyclists while still reducing the number of possible collision points and increasing the capacity of the junction compared to more conventional signalised junctions.

 

[mpthomson]

A relatively new style of crossing, first developed in France but popularised in the US.

It involves one of the two roads involved temporarily reversing its customary traffic direction (left hand traffic becomes right hand traffic), allowing all relevant crossing moves on the flat with only two phases on its traffic lights.
Here's an image from the Wikipedia page that shows what it looks like.



The slaloming probably has a traffic calming efect though, I believe in the US they are believed to be safer than what came before.

Although the US is now rapidly adopting roundabouts as the default on new build roads.

 

[simonw]

It depends. However, roundabouts are the British way. Thinking outside the box (circle!) simply doesn't happen. National Highways seem to be terrified of anything which isn't a roundabout, and everyone else just copies them.

There are hundreds (maybe thousands?) of diverging diamonds in use around the world in more enlightened countries. We would not be the first.


The UK has a phobia of anything "new" or "different", and seemingly refuses to use best practise from abroad. And on the rare occasions when a project does manage to get past all the political, financial and "not invented here" hurdles, the industry manages to make a complete mess of it. (See also HS2.)

Wiki says there are 160. A bit removed from thousands.

 

[bangor-toad]

Hi,
I think one of the barriers to adoption is that such a design would fail to pass UK safety standards.

I believe there are there are some quite strict rules about the length of slip roads. I think the deceleration length for a 50mph section is 80m, possibly 110m if it then counts as dual carriageway. For the double diamond you'd need two, one for the initial join and one for the departing. This would mean the slip roads would need to be separated by a minimum of 160m. This is much more than many roundabouts and there would be an increased land requirement.
Then there is the issue of 'weaving' where traffic will have to cross lanes to merge with and leave from the through lane. 160m is way too short for this and would be viewed as a safety hazard (an example of this is the Coventry inner ring road which is certainly 'interesting' for this type of manoeuvre...)

Have a read of some UK standards to build your own interpretation: Link

I can't ever see such a junction being built in the UK as I can't see how you can build them small enough to be safe or to have the capacities required.
Cheers,
Mr Toad

 

[swt_passenger]

There’s a long running discussion about this in the “Sabre” roads forum that people here might find interesting:
https://www.sabre-roads.org.uk/forum/viewtopic.php?t=39327&hilit=Diverging+Diamond

 

[Dent]

Another issue is capacity on the straight-through route. This layout means that traffic can only flow through in one direction at a time so each directions has to be stopped for more than half the cycle, whereas with a conventional signal-controlled junction traffic can flow straight through in both directions for most of the time with only brief stops to allow for joining traffic or right turns.

 

[Tetchytyke]

I'm still not entirely sure what problem they solve. They restrict throughput because only one side of the diamond can move at any one time.

For crossroads the UK model of having filter lanes works well- straight on goes red then the two opposing right-turn filters go green- and for big interchanges a roundabout or freeflow slip roads would make more sense anyway.

 

[Statto]

Then they stick traffic lights on them, destroying the entire point of the roundabout in the first place!

Because some roundabouts have become so busy with traffic (particularly at Motorway junctions) that traffic lights are needed so traffic can enter the roundabout

 

[Crithylum]

I'm still not entirely sure what problem they solve. They restrict throughput because only one side of the diamond can move at any one time.

For crossroads the UK model of having filter lanes works well- straight on goes red then the two opposing right-turn filters go green- and for big interchanges a roundabout or freeflow slip roads would make more sense anyway.

A conventional diamond interchange needs 3 phases to facilitate all moves, a diverging diamond only needs 2. The issue with a conventional diamond is that the 2 right turns both conflict with each other and going straight.
According to Missouri DoT:

For higher traffic volumes, the DDI design has better performance and offers lower delays, fewer stops, lower stop time and shorter queue lengths when compared to the performance of the conventional diamond design. For lower volumes, performance of the DDI and a conventional diamond interchange are similar.

A DDI has a higher capacity for all signalized movements when compared to the conventional diamond interchange. Especially, the capacity of left-turn movements is twice that of the corresponding capacity of left-turn movements of the conventional diamond interchange. Exclusive left-turn lanes are not necessary for the DDI.

To be comparable to a 4-lane DDI, a conventional diamond interchange would require 6 lanes to provide the same capacity. When additional future capacity is needed, it would be advantageous to convert a conventional diamond interchange into a DDI instead of pursuing the more costly option of widening the major and minor roadways in the interchange (includes widening the bridge) and adding additional lanes to the ramps.

While the DDI design does not allow through movements from off- to on-ramps, it permits u-turn movements with fewer conflicts than a conventional diamond interchange.

The potential for wrong way traffic movements in a DDI can be minimized with geometrics, signing, pavement marking, signals and lighting.

Nearby signalized intersections may reduce the effectiveness of a DDI. The simplified signal phasing of the DDI may impact the phasing of nearby traffic signals.

 

[Dent]

A conventional diamond interchange needs 3 phases to facilitate all moves, a diverging diamond only needs 2.

Why is having two phases instead of three, at the expense of greatly reducing the capacity of the junction, a benefit?

The issue with a conventional diamond is that the 2 right turns both conflict with each other and going straight.

I don't really see an issue or a conflict there. Traffic going straight and the two right turns are on different phases of the traffic lights, so there is no conflict.

 

[Royston Vasey]

Indeed. In many (most?) cases a properly-designed signalised crossroads (or diverging diamond!) would be far more effective than a signalised roundabout.

You trying driving through American towns when you're stopped every 200m at a "signalled crossroads". It's a start stop nightmare. Ridiculous statement. Crossroads are infinitely worse than roundabouts. I have no opinion on diverging diamonds

 

[Tetchytyke]

The issue with a conventional diamond is that the 2 right turns both conflict with each other and going straight.

But in a diverging diamond the two straight-on phases conflict with each other.

I can see how they're a specific solution at specific locations, but equally I get the distinct impression they're mostly due to the US aversion to roundabouts. And at really big busy intersections they'd be best off investing the money in a proper free-flow interchange.

 

[Dent]

Indeed. In many (most?) cases a properly-designed signalised crossroads (or diverging diamond!) would be far more effective than a signalised roundabout.

Many locations have signalised crossroads where they are more effective than a roundabout. There already are various different types of junctions in use in the situations where they are appropriate.

Your claim of National Highways being "terrified of anything which isn't a roundabout" is completely untrue, as are your claims of a "phobia" of anything new and "not invented here" hurdles.

 

[Bletchleyite]

Because some roundabouts have become so busy with traffic (particularly at Motorway junctions) that traffic lights are needed so traffic can enter the roundabout

It's when you have uneven flows you need lights on a roundabout. You don't need them on all legs in every case, which makes more efficient use of road space because there's no "dead" phase with everything on red.

 

[Crithylum]

I don't really see an issue or a conflict there. Traffic going straight and the two right turns are on different phases of the traffic lights, so there is no conflict.

If they are on different phases, only one can be green at a time. This means that on average, a phase can be green at most 1/3 of the time (some phases may be longer at the expense of other phases). Whereas this is 1/2 with a DDI, a 50% increase.

Why is having two phases instead of three, at the expense of greatly reducing the capacity of the junction, a benefit?

Because it increases the capacity of the intersection? You have not provided any evidence for your claim that it greatly reduces capacity. Here is a link to my source: http://epg.modot.org/index.php/234.6_diverging_diamond_interchanges

 

[mpthomson]

But in a diverging diamond the two straight-on phases conflict with each other.

I can see how they're a specific solution at specific locations, but equally I get the distinct impression they're mostly due to the US aversion to roundabouts. And at really big busy intersections they'd be best off investing the money in a proper free-flow interchange.

Which, as I said earlier, is now disappearing and they now appear to be installed regularly at new major intersections.

 

[Dent]

Because it increases the capacity of the intersection? You have not provided any evidence for your claim that it greatly reduces capacity. Here is a link to my source: http://epg.modot.org/index.php/234.6_diverging_diamond_interchanges

I already have explained how it greatly reduces the capacity of the straight through route, see Post 18.

 

[scrapy]

Junction 26 of the M60 (near Stockport) has seen it's fair share of incidents with traffic going wrong way onto the motorway and dual carriageway. Maybe this has put National Highways off.