Diverging Diamonds

[Peter Freeman]

Ideally you’d have a site without a need for cycling or pedestrian provision to truly take advantage of those left turns.

Why so? The DDI lends itself very well to pedestrian (and probably cycling) provision - more so than roundabouts, which can be a cyclist's nightmare. Often the pedestrian way is down the centre of the bridge, between the reversed carriageways; and all crossing points are signalised in simple synchronism with the traffic phases. Use GE to see the excellent pedestrian provision on the larger urban examples.

[Peter Freeman]

Some motorists get very confused and end up driving the wrong way up a motorway or dual carraigeway, I don't think they could be trusted with a junction that involves intentionally driving on the wrong side of the road.

Having moved to the state of Missouri and living in St Louis where there are several and also having driven all the French ones several times, I have not seen 1 driver hesitate, nor have I seen any incidents on the news about accidents or wrong way drivers caused by a DDI.... in fact all they have done is remove extensive queues at every interchange they have been installed at

For many, the immediate first reaction to the DDI concept is a fear that "driving on the wrong side of the road" will cause confusion and danger. In fact, because you are actually within an intersection, not on the open road, it doesn't have that strange feeling at all. I made a special visit to Kansas City and environs a couple of years ago to study this layout (dedicated Sabristo!) and drove most of MO's examples. The expected 'funny feeling' and confusion really isn't a problem.

[Peter Freeman]

The only real benefit of roundabouts I can think of ...

Oh, despite being a DDI promoter and an ex-fan of, but sometimes a strong critic of, roundabouts, I can think of many real benefits of roundabouts. Here are just two.

First is that, at low volumes or off-peak, they can join multiple roads in an easy-to-understand way and cause hardly any delay. Second is that they're safe because collisions occur at low speeds and are rarely T-boners or head-ons. However, the low-delay benefit does not apply to the UK's many full-time-signalised roundabouts. And the safety benefit does not apply to the UK's many un-signalised high-speed monsters found at motorway and expressway interchanges.

Horses for Courses.

[ScottB5411]

The USA could and should probably replace a good 50% of it's 4 way stops with roundabouts

[Bryn666]

Ideally you’d have a site without a need for cycling or pedestrian provision to truly take advantage of those left turns.

Provision for NMUs at DDIs has already been solved by Mikael Colville-Andersen/the Copenhagenize company (see link).
https://media.springernature.com/origin ... 8_HTML.jpg

Oh that's far too ambitious on top of a DDI... the DMRB crowd would soil their undies in fear of such innovation.

[Bryn666]

The only real benefit of roundabouts I can think of ...

Oh, despite being a DDI promoter and an ex-fan of, but sometimes a strong critic of, roundabouts, I can think of many real benefits of roundabouts. Here are just two.

First is that, at low volumes or off-peak, they can join multiple roads in an easy-to-understand way and cause hardly any delay. Second is that they're safe because collisions occur at low speeds and are rarely T-boners or head-ons. However, the low-delay benefit does not apply to the UK's many full-time-signalised roundabouts. And the safety benefit does not apply to the UK's many un-signalised high-speed monsters found at motorway and expressway interchanges.

Horses for Courses.

The two bridge roundabout has its advantages but once you hit the point where signals are required they're all wiped out. There are no efficient signalised roundabouts - the only benefit of signals is you get to enter the circulatory because the high speeds are eliminated by a red light.

However, compact roundabouts are still massively safer than simple crossroads, etc. There is a reason France has at least 30,000 roundabouts today.

The bulk of UK motorway delays are surely caused by rubbish junctions. We keep adding mainline lanes but beyond tinkering we rarely address the real problem.

[Mark Hewitt]

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.

[wallmeerkat]

I could see the "driving on the wrong side" thing being not as much of an issue if:

- roads crossed each other at as close to a 90 degree angle as possible and the layout of the kerbs dissuades anything but "straight on". Then you're just following the road rather than feeling that you're crossing over.

- fencing between the 2 carriageways. This would probably be a requirement for headlight dazzling, but it would hide the fact that you are on the "wrong" side, in case anyone got confused.

[Bryn666]

I could see the "driving on the wrong side" thing being not as much of an issue if:

- roads crossed each other at as close to a 90 degree angle as possible and the layout of the kerbs dissuades anything but "straight on". Then you're just following the road rather than feeling that you're crossing over.

- fencing between the 2 carriageways. This would probably be a requirement for headlight dazzling, but it would hide the fact that you are on the "wrong" side, in case anyone got confused.

A more perpendicular crossing is emminently feasible - my drawing is squashed a little for space reasons more than anything, but with clever signing and marking you could avoid the "Bredbury Scissors" effect. In fact if you converted two bridge roundabouts to DDIs using existing structures that's exactly what you'd get as the carriageways would be so far apart you'd need tighter radii to get through.

[Mark Hewitt]

I could see the "driving on the wrong side" thing being not as much of an issue if:

- roads crossed each other at as close to a 90 degree angle as possible and the layout of the kerbs dissuades anything but "straight on". Then you're just following the road rather than feeling that you're crossing over.

- fencing between the 2 carriageways. This would probably be a requirement for headlight dazzling, but it would hide the fact that you are on the "wrong" side, in case anyone got confused.

A more perpendicular crossing is emminently feasible - my drawing is squashed a little for space reasons more than anything, but with clever signing and marking you could avoid the "Bredbury Scissors" effect. In fact if you converted two bridge roundabouts to DDIs using existing structures that's exactly what you'd get as the carriageways would be so far apart you'd need tighter radii to get through.

Would converting your standard two bridge roundabout to a DDI offer any advantages?

[wallmeerkat]

I could see the "driving on the wrong side" thing being not as much of an issue if:

- roads crossed each other at as close to a 90 degree angle as possible and the layout of the kerbs dissuades anything but "straight on". Then you're just following the road rather than feeling that you're crossing over.

- fencing between the 2 carriageways. This would probably be a requirement for headlight dazzling, but it would hide the fact that you are on the "wrong" side, in case anyone got confused.

A more perpendicular crossing is emminently feasible - my drawing is squashed a little for space reasons more than anything, but with clever signing and marking you could avoid the "Bredbury Scissors" effect. In fact if you converted two bridge roundabouts to DDIs using existing structures that's exactly what you'd get as the carriageways would be so far apart you'd need tighter radii to get through.

Would converting your standard two bridge roundabout to a DDI offer any advantages?

If an under-bridge signalised roundabout junction was converted to DDI it might offer less light transitions to get from source to destination. For example a motorway west-east with a roundabout underneath it for a road north-south, getting from the south road northbound to eastbound motorway onslip would currently require 3 traffic light stops (at roughly 6 o clock, 9 o clock and 12 o clock), with DDI this could be lessened to 2?

[roadtester]

Isn't a lot of this a question of what's already established in a particular country and what people are used to? Surely the very fact that a particular junction type dominates is a reason for using it for new junctions too, regardless of the inherent merits of a particular layout from a technical point of view.

We are very accustomed to roundabouts in the UK, but if other countries underwent overnight mass conversion of their junctions to roundabouts, the result would be confusion. I suspect the same would happen here in reverse with DDIs.

[Bryn666]

Isn't a lot of this a question of what's already established in a particular country and what people are used to? Surely the very fact that a particular junction type dominates is a reason for using it for new junctions too, regardless of the inherent merits of a particular layout from a technical point of view.

We are very accustomed to roundabouts in the UK, but if other countries underwent overnight mass conversion of their junctions to roundabouts, the result would be confusion. I suspect the same would happen here in reverse with DDIs.

Yes, but the worst excuse in human history for keeping things the same is "well that's just what we've always done". Whilst change for the sake of change is equally as silly, there are demonstrable benefits over conventional two bridge roundabouts and I would like to at least see one be trialled in the UK.

[Bryn666]

I could see the "driving on the wrong side" thing being not as much of an issue if:

- roads crossed each other at as close to a 90 degree angle as possible and the layout of the kerbs dissuades anything but "straight on". Then you're just following the road rather than feeling that you're crossing over.

- fencing between the 2 carriageways. This would probably be a requirement for headlight dazzling, but it would hide the fact that you are on the "wrong" side, in case anyone got confused.

A more perpendicular crossing is emminently feasible - my drawing is squashed a little for space reasons more than anything, but with clever signing and marking you could avoid the "Bredbury Scissors" effect. In fact if you converted two bridge roundabouts to DDIs using existing structures that's exactly what you'd get as the carriageways would be so far apart you'd need tighter radii to get through.

Would converting your standard two bridge roundabout to a DDI offer any advantages?

Fewer signal stages and a better route through for NMUs are the two main advantages.

[Peter Freeman]

A more perpendicular crossing is emminently feasible - my drawing is squashed a little for space reasons more than anything, but with clever signing and marking you could avoid the "Bredbury Scissors" effect. In fact if you converted two bridge roundabouts to DDIs using existing structures that's exactly what you'd get as the carriageways would be so far apart you'd need tighter radii to get through.

No DDI's have been built with anything approaching 90 degrees. Your drawing is actually quite representative of normal angles. It's not a problem.

Yes, a conversion from a 2-bridge roundabout would produce a more perpendicular crossing, which would also be ok. And such a conversion would be worthwhile if that roundabout had run out of options for alleviation of its capacity constraint. A previous thread has discussed the ease/difficulty in matching the footprints.

BTW, we are discussing the DDI here as if it's a theoretical proposal and a decision is pending on whether to try it out. In fact we're far down the track. It's been done. It's been sorted. It works.

[someone]

If an under-bridge signalised roundabout junction was converted to DDI it might offer less light transitions to get from source to destination. For example a motorway west-east with a roundabout underneath it for a road north-south, getting from the south road northbound to eastbound motorway onslip would currently require 3 traffic light stops (at roughly 6 o clock, 9 o clock and 12 o clock), with DDI this could be lessened to 2?

Unless the lights are badly phases then surely it should be just one set. Essentially the junction just alternates turns between northbound and southbound traffic. There should be enough time between the two set of lights for anyone passing the first to clear the second before it goes red.

In theory there is no reason you need to cross over, though doing so is just a better use of space as traffic making a right turn from a slip road can then queue on the bridge while leftwards traffic is taking its turn. It also means the slip roads do not necessarily have to be signalized and helps reduce weaving problems.

[Peter Freeman]

In theory there is no reason you need to cross over, though doing so is just a better use of space as traffic making a right turn from a slip road can then queue on the bridge while leftwards traffic is taking its turn. It also means the slip roads do not necessarily have to be signalized and helps reduce weaving problems.

"No need to cross over" ...?! In that case you won't have a DDI. Unless I'm misunderstanding your point, you seem to be inventing a normal diamond ... ?

[nowster]

BTW, we are discussing the DDI here as if it's a theoretical proposal and a decision is pending on whether to try it out. In fact we're far down the track. It's been done. It's been sorted. It works.

Where "here" is Australia? There are none currently in Britain or Ireland.

[Bryn666]

BTW, we are discussing the DDI here as if it's a theoretical proposal and a decision is pending on whether to try it out. In fact we're far down the track. It's been done. It's been sorted. It works.

Where "here" is Australia? There are none currently in Britain or Ireland.

The US and Aus are leading the way on this one. We will be, as usual, left behind stuck working to a design manual that has had little update since the 60s.

[someone]

In theory there is no reason you need to cross over, though doing so is just a better use of space as traffic making a right turn from a slip road can then queue on the bridge while leftwards traffic is taking its turn. It also means the slip roads do not necessarily have to be signalized and helps reduce weaving problems.

"No need to cross over" ...?! In that case you won't have a DDI. Unless I'm misunderstanding your point, you seem to be inventing a normal diamond ... ?

Indeed, I am just describing the essence of how they operate because it is easier to visualize the basic process when you do not have to also consider the unusual occurrence of the two traffic flows switching sides with each other twice. The aspect of removing right turn conflicts itself does not require traffic to cross over.

But normal diamond junctions do not normally work that way because it would inefficiently leave half of the bridge empty and cause queuing traffic to back up farther down the approaching roads and slips. The diverging design is what combines the benefits of both methods. It takes away the conflicts whilst also maintaing the best use of available road space by allowing both sides to be used at the same time.