Interesting Junction Layouts/Phases

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Re: Interesting Junction Layouts/Phases

Post by Fenlander » Wed Oct 09, 2019 12:48

Johnathan404 wrote:
Tue Oct 08, 2019 17:16
You will struggle to find a road improvement made in the last 40 years that local drivers will approve of. That's the way people are. It's not a very good measure of anything.

Given that pretty much the only way you can improve a roundabout is by adding more traffic lights, and people will complain about that, I'd suggest what was built is the lesser of two evils, especially in terms of safety and NMU accessibility.
I live in Spalding, I work in and around it. The A16 Spalding bypass, the A16 Spalding to Boston and more recently the A16 Spalding to Peterborough are massive improvements on the former routes. Same with the various A17 realignments and the A17 Long Sutton bypass along with A17 from Sutton Bridge to Kings Lynn. Yes a lot of those roads could have been better still with a bit of thought and a lot of money, some of them could be improved with a tin of white paint and some lane markings for example.
On a more local scale the traffic lights nearest my house & the next set along move traffic through quicker than the olds ones, the final linking up of 2 housing estates has created another route around town.

These are just local ones, the A1 roundabout improvements, A1/M62 junction, M62 improvements, M62/M57 junction & Switch Island changes have made our long distance family visits much easier and occasionally quicker.

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Re: Interesting Junction Layouts/Phases

Post by Johnathan404 » Wed Oct 09, 2019 13:16

Fenlander wrote:
Wed Oct 09, 2019 12:48
I live in Spalding, I work in and around it. The A16 Spalding bypass, the A16 Spalding to Boston and more recently the A16 Spalding to Peterborough are massive improvements on the former routes. Same with the various A17 realignments and the A17 Long Sutton bypass along with A17 from Sutton Bridge to Kings Lynn. Yes a lot of those roads could have been better still with a bit of thought and a lot of money, some of them could be improved with a tin of white paint and some lane markings for example.
On a more local scale the traffic lights nearest my house & the next set along move traffic through quicker than the olds ones, the final linking up of 2 housing estates has created another route around town.

These are just local ones, the A1 roundabout improvements, A1/M62 junction, M62 improvements, M62/M57 junction & Switch Island changes have made our long distance family visits much easier and occasionally quicker.
As is often the case with generalised criticisms of society, SABRE members are excluded because they tend to be rational and take an interest in how things work.

This is very different from the everyday people you will hear from in community groups, local newspapers and Facebook pages, where people will resent anything that has changed in their lifetime. Check in to any Facebook heritage group if you want to see people sincerely complaining about road layouts that were changed in the 1970s; "it worked fine until then", they say.
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Re: Interesting Junction Layouts/Phases

Post by jackal » Wed Oct 09, 2019 13:31

Johnathan404 wrote:
Tue Oct 08, 2019 17:16
Richardf wrote:
Tue Oct 08, 2019 17:01
Yes. Many local drivers see these junctions as unnecessary, the changes wasteful, and would rather have the roundabouts back. The original roundabout junctions could have been improved in other ways for less money.
You will struggle to find a road improvement made in the last 40 years that local drivers will approve of. That's the way people are. It's not a very good measure of anything.

Given that pretty much the only way you can improve a roundabout is by adding more traffic lights, and people will complain about that, I'd suggest what was built is the lesser of two evils, especially in terms of safety and NMU accessibility.
If there's space you can add filter lanes, and there's usually space. Something like this takes barely more than a lane's width and adds more capacity than signals:
https://www.google.co.uk/maps/@51.62428 ... 384!8i8192

And drivers are not wrong that signalised 'improvements' tend to increase overall journey times:
https://assets.publishing.service.gov.u ... _draft.pdf

The safety/NMU case for signalised crossroads is also dubious due to the lack of deflection.

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Re: Interesting Junction Layouts/Phases

Post by Bryn666 » Wed Oct 09, 2019 13:45

jackal wrote:
Wed Oct 09, 2019 13:31
Johnathan404 wrote:
Tue Oct 08, 2019 17:16
Richardf wrote:
Tue Oct 08, 2019 17:01
Yes. Many local drivers see these junctions as unnecessary, the changes wasteful, and would rather have the roundabouts back. The original roundabout junctions could have been improved in other ways for less money.
You will struggle to find a road improvement made in the last 40 years that local drivers will approve of. That's the way people are. It's not a very good measure of anything.

Given that pretty much the only way you can improve a roundabout is by adding more traffic lights, and people will complain about that, I'd suggest what was built is the lesser of two evils, especially in terms of safety and NMU accessibility.
If there's space you can add filter lanes, and there's usually space. Something like this takes barely more than a lane's width and adds more capacity than signals:
https://www.google.co.uk/maps/@51.62428 ... 384!8i8192

And drivers are not wrong that signalised 'improvements' tend to increase overall journey times:
https://assets.publishing.service.gov.u ... _draft.pdf

The safety/NMU case for signalised crossroads is also dubious due to the lack of deflection.
A medium speed free-flow lane applied in an urban context does nothing for pedestrian or cyclist safety. Signal junctions aren't about journey times (people who say they are miss the point entirely); they're about reallocating time so that dominant flows don't cause a total network breakdown.

Any roundabout with an ICD larger than 40m rapidly becomes dangerous for NMUs due to circulatory speeds rising beyond safe gap creation. But if the only metric of success where a road scheme is concerned is that precious car drivers can go a little bit faster then this won't be relevant.
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Re: Interesting Junction Layouts/Phases

Post by Chris5156 » Wed Oct 09, 2019 14:19

Johnathan404 wrote:
Wed Oct 09, 2019 13:16
This is very different from the everyday people you will hear from in community groups, local newspapers and Facebook pages, where people will resent anything that has changed in their lifetime.
Indeed. I remember seeing one Facebook comment on a council proposal to change this roundabout (now a signalised junction) to something more cycling-friendly. The woman making the comment said she was distraught because the council were "destroying a part of her childhood". How do you even begin to engage with that sort of nonsense?!

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Re: Interesting Junction Layouts/Phases

Post by jackal » Wed Oct 09, 2019 14:24

Bryn666 wrote:
Wed Oct 09, 2019 13:45
jackal wrote:
Wed Oct 09, 2019 13:31
If there's space you can add filter lanes, and there's usually space. Something like this takes barely more than a lane's width and adds more capacity than signals:
https://www.google.co.uk/maps/@51.62428 ... 384!8i8192

And drivers are not wrong that signalised 'improvements' tend to increase overall journey times:
https://assets.publishing.service.gov.u ... _draft.pdf

The safety/NMU case for signalised crossroads is also dubious due to the lack of deflection.
Signal junctions aren't about journey times (people who say they are miss the point entirely)
So why do 88% of the 54 schemes in the linked study have decreased journey times as an objective (one that is generally not realised)? You are simply proving the point that signalised junctions are frequently used on the basis of false assumptions.
they're about reallocating time so that dominant flows don't cause a total network breakdown.
So you're claiming that an unsignalised junction will cause a total network breakdown, yet still have quicker overall journey times? How do you square that circle?
Any roundabout with an ICD larger than 40m rapidly becomes dangerous for NMUs due to circulatory speeds rising beyond safe gap creation. But if the only metric of success where a road scheme is concerned is that precious car drivers can go a little bit faster then this won't be relevant.
The metric is overall transport efficiency and safety of the junction. What that requires will vary with the circumstances. For instance, for larger roundabouts with significant volumes of NMUs, some signalisation may well be appropriate, perhaps alongside other interventions such as filter lanes (which can have signalised crossings if necessary). But where there are few NMUs (as in the first Weymouth example we're discussing here, which doesn't have any pedestrian access) it's far from clear that signals are at all helpful.

That is what I object to - signals as the kneejerk response, regardless of circumstances.

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Re: Interesting Junction Layouts/Phases

Post by Bryn666 » Wed Oct 09, 2019 14:45

Perhaps your objection should be with the DMRB then, as that basically forces signals as a go to once a roundabout starts to break down. Weird and wonderful GSJs such as those used in France are anathema to our standards wonks who want high speed motor dominance at grade.
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Re: Interesting Junction Layouts/Phases

Post by jackal » Wed Oct 09, 2019 14:53

Bryn666 wrote:
Wed Oct 09, 2019 14:45
Perhaps your objection should be with the DMRB then
In part, but there are also systematic inaccuracies with estimating journey time savings, not all of which can be laid at DMRB's door.

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Re: Interesting Junction Layouts/Phases

Post by RichardA35 » Wed Oct 09, 2019 15:01

jackal wrote:
Wed Oct 09, 2019 14:24
But where there are few NMUs (as in the first Weymouth example we're discussing here, which doesn't have any pedestrian access) it's far from clear that signals are at all helpful.

That is what I object to - signals as the kneejerk response, regardless of circumstances.
However there are significant numbers of NMU's as can be seen from the pedestrian waiting in the 2019 GSV and the well-worn desire line along the northern verge that have all been ignored in the design and assessment to make the junction work. The roundabout was IMV far worse and invited inappropriate speed and risk taking to enter a gap.

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Re: Interesting Junction Layouts/Phases

Post by Richardf » Wed Oct 09, 2019 21:50

Peter Freeman wrote:
Tue Oct 08, 2019 23:44
These intersections are not 'interesting' in the sense of being rather strange or unusual: they represent the world-standard way, and in my opinion usually the best way, of allowing two fairly major roads to cross each other at-grade.
No they aren't unusual, but they are the only ones I know of that I thought would be of interest. That's what I thought this thread was about.
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Re: Interesting Junction Layouts/Phases

Post by Peter Freeman » Thu Oct 10, 2019 03:02

jackal wrote:
Wed Oct 09, 2019 13:31

And drivers are not wrong that signalised 'improvements' tend to increase overall journey times:
https://assets.publishing.service.gov.u ... _draft.pdf
This is an interesting and objective report. What it stresses though is that the (shorter journey times and congestion reduction) benefits almost always do result at peak times, but are countered by dis-benefits at off-peak times. The benefits balance in most cases is positive, but that doesn't make off-peak users happy.

As we have often discussed on Sabre, the problem is that roundabout signalisation is virtually always 24x7. I'm pleased to see that this is referred to in the foreword of the report:

" Optimising benefits across the 24 hour period – The evaluation has shown that across a 24 hour period we are not sustaining the journey time benefits we generate during peak periods, and predominantly this has been a consequence of 24 hour signalisation. As a company, we are considering a range of options to optimise journey times across a 24 hour period, such as using adaptive traffic control technologies (like SCOOT and MOVA2) which are responsive to the traffic conditions. "


We know that there are issues (vehicle safety, NMU safety, and geometry) with part-time signalisation, but the mention is, at least, encouraging.

It is important to note that when this report refers to signalisation, most of its examples are the signalisation of roundabout approaches, not replacement with cross-roads or T's. Replacement introduces one (perhaps long-ish) delay at the signal. Full signalisation of a roundabout introduces multiple stops (depending on which turn you are making, and subject to the quality of signal phase tuning). This, amongst other reasons, is why I am so critical of signalisation of existing roundabouts, while acknowledging that it is a cheap and cost-effective quick-fix.

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Re: Interesting Junction Layouts/Phases

Post by Peter Freeman » Thu Oct 10, 2019 03:59

jackal wrote:
Wed Oct 09, 2019 13:31

If there's space you can add filter lanes, and there's usually space. Something like this takes barely more than a lane's width and adds more capacity than signals:
https://www.google.co.uk/maps/@51.62428 ... 384!8i8192
I agree, left-turn filter lanes are, in many cases, the very best quick capacity boost. They are so good that they should almost be regarded as a standard part of initial roundabout design. In Australia (where we do build roundabouts, and where they work perfectly well in the right circumstances, as do yours) many roundabouts have filter lanes from commissioning day. Is the UK also doing this? - most that I know of are retrofits.

The filter lane downside is that it exacerbates what I see as an inherent roundabout problem. Each entry's give-way line used to be (in the idealised world of the 1960's …!) the beginning of a short low-speed weaving section, where vehicles safely juggled each other into the correct positions to either exit or continue to circulate. This mode of operation still occurs in urban gyratories. But nowadays roundabouts (UK) are large, they operate at high speeds, and the entry/exit proximity leaves no weaving possibility. Thus, the give-way line is a 'starting-grid' from which to launch a high acceleration forced entry into an unforgiving traffic platoon. It's actually a glancing-angle cross-roads of two one-way roads. While waiting at that give-way line you're looking for a gap that might be created by someone using the exit that lies just to your right, but often it's impossible to predict the intention of an approaching vehicle until it is too late to exploit the gap. If you plan to turn immediately left you might be tempted to do it in the belief that the approaching vehicle is continuing on the circulatory carriageway, but this is often unclear (again until too late). Also, if you plan to turn immediately left you might be tempted to proceed in the belief that you can perform a safe side-by-side usage of the exit, but will the approaching vehicle actually stay in-lane and allow that to work?

I think I'm saying "Hey, roundabouts don't work so well after all" … ? (At least, big UK ones on major roads). (I drive frequently in the UK).

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Re: Interesting Junction Layouts/Phases

Post by jackal » Thu Oct 10, 2019 11:44

Peter Freeman wrote:
Thu Oct 10, 2019 03:02
jackal wrote:
Wed Oct 09, 2019 13:31

And drivers are not wrong that signalised 'improvements' tend to increase overall journey times:
https://assets.publishing.service.gov.u ... _draft.pdf
This is an interesting and objective report. What it stresses though is that the (shorter journey times and congestion reduction) benefits almost always do result at peak times, but are countered by dis-benefits at off-peak times. The benefits balance in most cases is positive, but that doesn't make off-peak users happy.
Journey time 'benefits' are on the whole slightly negative across the 54 schemes (monetised as £5.1m gain in peaks, £5.4m loss offpeak).
It is important to note that when this report refers to signalisation, most of its examples are the signalisation of roundabout approaches, not replacement with cross-roads or T's. Replacement introduces one (perhaps long-ish) delay at the signal. Full signalisation of a roundabout introduces multiple stops (depending on which turn you are making, and subject to the quality of signal phase tuning). This, amongst other reasons, is why I am so critical of signalisation of existing roundabouts, while acknowledging that it is a cheap and cost-effective quick-fix.
Yes, I agree signalised intersections are much more efficient at moving traffic than signalised rbts. The problem is that they don't have the safety benefits (lack of deflection etc). So I don't think that's a one size fits all answer - it will be site specific.
Peter Freeman wrote:
Thu Oct 10, 2019 03:59
I agree, left-turn filter lanes are, in many cases, the very best quick capacity boost. They are so good that they should almost be regarded as a standard part of initial roundabout design. In Australia (where we do build roundabouts, and where they work perfectly well in the right circumstances, as do yours) many roundabouts have filter lanes from commissioning day. Is the UK also doing this? - most that I know of are retrofits.
Absolutely on board with you. Whereas the choice between rbt, signalised rbt and intersection is quite subtle and context specific IMO, the choice between filter lane and no filter lane is not a hard one for a busy movement. They should be the 'default' improvement, not signals.

As for building them from the start, I'd like to see that much more as well, though it does happen. Here's a nice example in Watford:

https://www.google.co.uk/maps/@51.65521 ... a=!3m1!1e3
The filter lane downside is that it exacerbates what I see as an inherent roundabout problem. Each entry's give-way line used to be (in the idealised world of the 1960's …!) the beginning of a short low-speed weaving section, where vehicles safely juggled each other into the correct positions to either exit or continue to circulate. This mode of operation still occurs in urban gyratories. But nowadays roundabouts (UK) are large, they operate at high speeds, and the entry/exit proximity leaves no weaving possibility. Thus, the give-way line is a 'starting-grid' from which to launch a high acceleration forced entry into an unforgiving traffic platoon. It's actually a glancing-angle cross-roads of two one-way roads. While waiting at that give-way line you're looking for a gap that might be created by someone using the exit that lies just to your right, but often it's impossible to predict the intention of an approaching vehicle until it is too late to exploit the gap. If you plan to turn immediately left you might be tempted to do it in the belief that the approaching vehicle is continuing on the circulatory carriageway, but this is often unclear (again until too late). Also, if you plan to turn immediately left you might be tempted to proceed in the belief that you can perform a safe side-by-side usage of the exit, but will the approaching vehicle actually stay in-lane and allow that to work?

I think I'm saying "Hey, roundabouts don't work so well after all" … ? (At least, big UK ones on major roads). (I drive frequently in the UK).
I see what you're saying about roundabouts generally (except I'm not sure how filters could make things worse?). To be honest, the roundabouts I know that have been signalised were quite intimidating to enter pre-signalisation. That said, I never actually saw a collision, and if there were any they would have been low speed side-swipe type affairs. More than anything I marvelled at how well they flowed given what they had to deal with.

It is absolutely true that the signalised replacements feel safer, but in the case of the crossroads at least I suspect that when there are collisions they may be more serious. I've seen cars miss red lights at signalised crossroads, and been driven through a red at speed once overseas, and though this is a low frequency event it poses a completely different level of danger to anything at a roundabout.

I saw a study along these lines once upon a time (sadly don't recall where) which basically said that signalised crossroads are the best of both worlds from a safety perspective, which is plausible but unfortunate given their negative journey time effects.

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Re: Interesting Junction Layouts/Phases

Post by Peter Freeman » Thu Oct 10, 2019 13:09

jackal wrote:
Thu Oct 10, 2019 11:44
Yes, I agree signalised intersections are much more efficient at moving traffic than signalised rbts. The problem is that they don't have the safety benefits (lack of deflection etc). So I don't think that's a one size fits all answer - it will be site specific.
Fully agree, of course.
jackal wrote:
Thu Oct 10, 2019 11:44
I see what you're saying about roundabouts generally (except I'm not sure how filters could make things worse?).
Ok, they don't make it worse. It just sort-of looks worse. Immediate left turns, the easiest movement, have gone elsewhere, so they no longer occur at the give-way line. All the vehicles waiting there are now in the same boat, looking for their chance to barge in, or barge across. Hence there is little movement, and queueing drivers perhaps have more of that 'getting nowhere' feeling.

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Re: Interesting Junction Layouts/Phases

Post by WHBM » Fri Oct 11, 2019 06:54

jackal wrote:
Thu Oct 10, 2019 11:44
I saw a study along these lines once upon a time (sadly don't recall where) which basically said that signalised crossroads are the best of both worlds from a safety perspective, which is plausible but unfortunate given their negative journey time effects.
This is fine provided the phasing is efficient and reflects the actual usage.

Squandering capacity on wasted green time where nothing is coming, pedestrian phases when there are no pedestrians, etc, and providing insufficient green time on other flows so it takes multiple phases to get through, is surely something the signal engineering world should have sorted out long before now.

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Re: Interesting Junction Layouts/Phases

Post by Bryn666 » Fri Oct 11, 2019 08:26

In other words "as long as my journey isn't disrupted" screw everyone else?

One of many reasons why main roads get red signals when "there is nothing coming" is to prevent the road becoming a drag strip by people bombing along knowing they won't need to stop for anything. Even the car loving Americans do this and accept the logic.

Also blame the DMRB which is totally unsuitable for urban signal junction design and will eventually... when the DfT hasn't got to deal with sodding Brexit... be replaced by Chapter 6 of the TSM.
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Re: Interesting Junction Layouts/Phases

Post by WHBM » Fri Oct 11, 2019 13:40

Bryn666 wrote:
Fri Oct 11, 2019 08:26
In other words "as long as my journey isn't disrupted" screw everyone else?

One of many reasons why main roads get red signals when "there is nothing coming" is to prevent the road becoming a drag strip by people bombing along knowing they won't need to stop for anything. Even the car loving Americans do this and accept the logic.
If you can point me to a screw everyone attitude, please do. I must have missed it.

Sequencing signals, whether to give green waves, or the opposite, has just gone. I blame engineers recruited with minimal experience who don't know how to do it, or even where to start (and are sufficiently uneducated that they likely couldn't spell "optimise" if they tried). Going round a signalised motorway junction roundabout and getting a change to red in your face at each of four consecutive signals after midnight, when the only car on the road (M25 J23 I'm looking at you) is not done to prevent drag racing.

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Re: Interesting Junction Layouts/Phases

Post by Skipsy » Sat Oct 12, 2019 23:58


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Re: Interesting Junction Layouts/Phases

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Re: Interesting Junction Layouts/Phases

Post by M4 Cardiff » Sun Oct 13, 2019 12:35

WHBM wrote:
Fri Oct 11, 2019 13:40
Bryn666 wrote:
Fri Oct 11, 2019 08:26
In other words "as long as my journey isn't disrupted" screw everyone else?

One of many reasons why main roads get red signals when "there is nothing coming" is to prevent the road becoming a drag strip by people bombing along knowing they won't need to stop for anything. Even the car loving Americans do this and accept the logic.
If you can point me to a screw everyone attitude, please do. I must have missed it.

Sequencing signals, whether to give green waves, or the opposite, has just gone. I blame engineers recruited with minimal experience who don't know how to do it, or even where to start (and are sufficiently uneducated that they likely couldn't spell "optimise" if they tried). Going round a signalised motorway junction roundabout and getting a change to red in your face at each of four consecutive signals after midnight, when the only car on the road (M25 J23 I'm looking at you) is not done to prevent drag racing.
Even more silly as there are some signalised roundabouts where, if you pull off from a green sensibly, you get stopped at the next one, but if you accelerate as hard as possible and hoon it round, you can just beat the red wave. Surely this is dangerous as it only encourages this sort of behaviour.
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