Displaced right turn/continuous flow junctions

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jackal
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Displaced right turn/continuous flow junctions

Post by jackal » Tue Sep 12, 2017 09:55

In this junction type, one or more of the right turns cross before the main intersection, reducing the number of conflicts at that point. I know of two examples in the UK:

https://www.google.co.uk/maps/@51.60136 ... z?hl=en-GB

https://www.google.co.uk/maps/@50.48629 ... z?hl=en-GB

The Swindon example is discussed in this paper.

Any more?

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Re: Displaced right turn/continuous flow junctions

Post by wrinkly » Tue Sep 12, 2017 11:34

Are you only looking for signal-controlled junctions?

In the early 1960s at unsignalled rural junctions with minor roads, it was not unusual to have a triangular traffic island with two-way traffic on all three sides. I think it was a recommended layout in the equivalent of the DMRB in certain circumstances. Some that were built then have since been altered. In a sense a minor road junction with some spare space between the fences or hedges can naturally develop into such a junction. Here are two in one view near M6 J19:

https://www.google.co.uk/maps/@53.30789 ... a=!3m1!1e3

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Re: Displaced right turn/continuous flow junctions

Post by Big L » Tue Sep 12, 2017 11:58

Make poetry history.

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Re: Displaced right turn/continuous flow junctions

Post by Chris5156 » Tue Sep 12, 2017 12:44

Surprised the big one on the Reading IDR hasn't been mentioned yet.

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Re: Displaced right turn/continuous flow junctions

Post by Peter Freeman » Tue Sep 12, 2017 13:52

jackal wrote: ... reducing the number of conflicts at that point ...
... and, importantly, enabling 2-phase-only signalisation.

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Re: Displaced right turn/continuous flow junctions

Post by jackal » Tue Sep 12, 2017 14:08

I can see there are going to be some tricky definitional issues here. My intuitive sense is that there isn't really a main intersection with displaced right turns at the Reading IDR example, but rather a series of smaller intersections. On a micro scale this might also apply to wrinkly's unsignalized intersections.

That would at any rate make sense of the claim in the paper I cited that the Swindon example, opened in 2002, is the first in the UK.

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Re: Displaced right turn/continuous flow junctions

Post by Reading » Tue Sep 12, 2017 14:21

Chris5156 wrote:Surprised the big one on the Reading IDR hasn't been mentioned yet.
Only because i was busy

It is also regularly abused - you see cars come down from the slip marked salvation army on your map and make an illegal right turn either straight across or at the yellow box to save having to go down and around the signalised roundabout under the flyover by the oracle.

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Re: Displaced right turn/continuous flow junctions

Post by Peter Freeman » Sun Mar 24, 2019 09:47

Those UK examples are not really offset-right-turn continuous flow intersections (offset-left-turns in the USA), except for (marginally) Swindon. They predominantly enable advanced left turns. Useful, but not proper CFI's.

Melbourne, Australia has just opened its first implementation. It's a re-work of an existing and very congested traditional signalised crossroads. It is a CFI of the 'parallel flow' type, enabling advance execution of a right turn (southbound to westbound). Of the other three right turns, two have to do a P-turn and one (eastbound to southbound) is simply prohibited (alternative routes exist). The efficiency gain results from two-phase signal operation.

It's been operating for one week, so it's too early to say how well it's achieved its aims. I think it will.

https://www.youtube.com/watch?v=86X7XkBX7Vk

https://www.youtube.com/watch?v=kOtq2IW ... e=youtu.be
Last edited by Peter Freeman on Wed Aug 21, 2019 22:34, edited 1 time in total.

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Re: Displaced right turn/continuous flow junctions

Post by Berk » Sun Mar 24, 2019 16:32

I find it notable the Swindon example has an effective ‘contraflow’ with both right-turning lanes parallel to each other giving the impression of ‘driving on the right’.

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Re: Displaced right turn/continuous flow junctions

Post by jackal » Sun Mar 24, 2019 19:42

Peter Freeman wrote:
Sun Mar 24, 2019 09:47
Those UK examples are not really offset-right-turn continuous flow intersections (offset-left-turns in the USA), except for (marginally) Swindon. They predominantly enable advanced left turns. Useful, but not proper CFI's.

Melbourne, Australia has just opened its first implementation. It's a re-work of an existing and very congested traditional signalised crossroads. It is a CFI of the 'parallel flow' type, enabling advance execution of a right turn (southbound to westbound). Of the other three right turns, two have to do a P-turn and one (westbound to southbound) is simply prohibited (alternative routes exist). The efficiency gain results from two-phase signal operation.

It's been operating for one week, so it's too early to say how well it's achieved its aims. I think it will.

https://www.youtube.com/watch?v=86X7XkBX7Vk

https://www.youtube.com/watch?v=kOtq2IW ... e=youtu.be
The Swindon example is textbook CFI. The right turns cross over south of the main intersection, reducing the number of signal phases. I'm not sure how you could think right turns crossing over could be for the benefit of left turns?

The second (Torquay) example is essentially the same but more compact, so the crossover point is very close to the main intersection.

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Re: Displaced right turn/continuous flow junctions

Post by Peter Freeman » Mon Mar 25, 2019 04:36

Your Swindon and Torquay examples are similar, and both ARE technically DRT-CFI's (USA DLT), but hardly classic. The classic DRT intersections originally constructed in Mexico City, and now with more in the USA, are crossroads not T's. Your example in Torquay can only exploit the advantages to a limited extent because (as you observed) the crossed-over section is not long enough: too little storage . This applies less to the Swindon example, and perhaps in both cases it's not a problem if the volumes are low.
Last edited by Peter Freeman on Wed Feb 12, 2020 13:17, edited 1 time in total.

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Re: Displaced right turn/continuous flow junctions

Post by Peter Freeman » Mon Mar 25, 2019 04:44

Berk wrote:
Sun Mar 24, 2019 16:32
I find it notable the Swindon example has an effective ‘contraflow’ with both right-turning lanes parallel to each other giving the impression of ‘driving on the right’.
You're right, and I'm not sure whether this would feel disconcerting. It's even more pronounced in the usual crossroads implementations where a free-left-turn lane runs alongside the crossed-over right-turn lane, making four parallel carriageways. Both internal ones of those four run between opposing flows in their adjacents.

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Re: Displaced right turn/continuous flow junctions

Post by Bryn666 » Mon Mar 25, 2019 05:20

Peter Freeman wrote:
Sun Mar 24, 2019 09:47
Those UK examples are not really offset-right-turn continuous flow intersections (offset-left-turns in the USA), except for (marginally) Swindon. They predominantly enable advanced left turns. Useful, but not proper CFI's.

Melbourne, Australia has just opened its first implementation. It's a re-work of an existing and very congested traditional signalised crossroads. It is a CFI of the 'parallel flow' type, enabling advance execution of a right turn (southbound to westbound). Of the other three right turns, two have to do a P-turn and one (westbound to southbound) is simply prohibited (alternative routes exist). The efficiency gain results from two-phase signal operation.

It's been operating for one week, so it's too early to say how well it's achieved its aims. I think it will.

https://www.youtube.com/watch?v=86X7XkBX7Vk

https://www.youtube.com/watch?v=kOtq2IW ... e=youtu.be
That looks like a hybrid of a CFI and Michigan Left type junction. I like it although I did notice the dreaded Melbourne Hook Turn still remains on the east-west route.
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Re: Displaced right turn/continuous flow junctions

Post by Peter Freeman » Mon Mar 25, 2019 09:10

Bryn666 wrote:
Mon Mar 25, 2019 05:20
That looks like a hybrid of a CFI and Michigan Left type junction. I like it although I did notice the dreaded Melbourne Hook Turn still remains on the east-west route.
Bryn, my mistake in linking to a video that dates from the planning stage. The first link does show a hook turn, but in a design revision it was deleted. That right turn is abolished: not even catered for by a P-turn. And the eastbound route through the intersection is actually single lane, not two as in that video, since the volume is quite low and it leads onto an already-congested shopping/restaurant street.

The northbound approach was also revised: it has 4 lanes ahead, one turn left, and one kerbside bus-lane/bus-stop. No walking across a traffic lane to get to the bus!

The second url shows the final, as-built design.

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Re: Displaced right turn/continuous flow junctions

Post by Peter Freeman » Mon Mar 25, 2019 09:31

^ I don't even know what a Michigan Left Turn is. I know of many of these innovative flat turn types: P-turn, G-turn, Q-turn, Jug-handle, ... I know what some of them are, but not all. I remember when I lived in London in the 70's there were, on Cromwell Road eastbound, several whole-block run-arounds where you turned left-right-right to, in effect, go right at the next crossroads. I don't know whether they're still there.

The Punt Road CFI in Melbourne is rather a 'special'. The main (southbound-to-westbound) right turn only just precedes the real crossroad space, but then it has a significant length of westbound running before crossing over to the correct side. This space is not for storage: it flows simultaneously with E-W and W-E flows (though they get slightly interrupted).

The design takes advantage of un-utilised space on the NW corner. Otherwise it wouldn't fit. It seems to me that most DRT's and other CFI's need quite a lot of space, so whether they can fit depends very much on the exact local circumstances. This means that many will, in practise, be 'specials'.

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Re: Displaced right turn/continuous flow junctions

Post by ais523 » Mon Mar 25, 2019 12:52

The signage at the Swindon example seems rather inadequate to me. You're directing drivers onto what would be, in most junction designs, the wrong side of the road; and all that's there to indicate that is some road markings and a tiny "pass to the right" bollard. I'd expect at least "no entry" signs, rather than just NO ENTRY painted on the road.

The junction design seems like it could be viable, but we'd have to get better at telling drivers what to do when they're there.

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Re: Displaced right turn/continuous flow junctions

Post by Berk » Mon Mar 25, 2019 13:13

Yes, I can’t really see the justification for that either. :confused:

There doesn’t seem to be anything to suggest they couldn’t have done it the right way.

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Re: Displaced right turn/continuous flow junctions

Post by trickstat » Mon Mar 25, 2019 13:45

ais523 wrote:
Mon Mar 25, 2019 12:52
The signage at the Swindon example seems rather inadequate to me. You're directing drivers onto what would be, in most junction designs, the wrong side of the road; and all that's there to indicate that is some road markings and a tiny "pass to the right" bollard. I'd expect at least "no entry" signs, rather than just NO ENTRY painted on the road.

The junction design seems like it could be viable, but we'd have to get better at telling drivers what to do when they're there.
Yes I think more should be done here.

My guess is that, at busy times, people unfamiliar with the junction may be a little confused but won't actually go down the wrong section of road because they would have likely seen cars going in the opposite direction from the area immediately to the left. However, I do think there is quite a high risk of someone getting it wrong before about 7 am or after 7 pm when there may be less traffic about to indicate where you should go.

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Re: Displaced right turn/continuous flow junctions

Post by Berk » Mon Mar 25, 2019 14:34

I think this is only really a fail for at-grade junctions. If you consider some movements at grade-separated junctions (or ones that can be developed as grade-separated), sometimes a wrong-way movement can be easier to build, but give access to a right turn, for example.

I’ve often noted how in Spain, they seem to make right-turns there flow very easily, and left turns have a lot of spaghetti in some cases, but others just flow in the opposite direction. It may help when they have two-way slips (not something I’d want to encourage here).

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Re: Displaced right turn/continuous flow junctions

Post by Bryn666 » Mon Mar 25, 2019 14:42

Berk wrote:
Mon Mar 25, 2019 13:13
Yes, I can’t really see the justification for that either. :confused:

There doesn’t seem to be anything to suggest they couldn’t have done it the right way.
Other than by having a conventional junction you end up with extra stages and more congestion - this layout is fine, I'd have had a larger keep right sign but other than that the driver's eye perspective shows clearly to go ahead not first left.
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