Why does the UK not have any D2+1 roads?

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bothar
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Re: Why does the UK not have any D2+1 roads?

Post by bothar »

Al__S wrote: Thu Jan 06, 2022 06:57 how much, realistically, does that save over a D2?
In the ROI, two or three sections of D2+1 were built, based on Swedish principles but with some stupid economies. These were not seen to be entirely successful and now a D2 format is likely to be used.
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Re: Why does the UK not have any D2+1 roads?

Post by jnty »

A320Driver wrote: Thu Jan 06, 2022 15:24 Going by the nomenclature in another recent thread, shouldn’t it be D1+1? I.e. dual, one-lane carriageways, one of which has an extra lane?
I've always read it like S(2+1)=S3, which makes sense as it's sort of a special case of S3, but I suppose the same reading for D(2+1)=D3 makes no sense. Oh no!
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Re: Why does the UK not have any D2+1 roads?

Post by FosseWay »

As mentioned above, these are very common in Sweden. They generally work fine on routes with moderate rather than heavy traffic flows, and allow the speed limit to be higher than it would otherwise be. But if there is a crash or breakdown in a single lane section, it is likely to prevent traffic passing, even at walking speed. I've sat on a bus a few km from home for up to an hour on occasions, when something large enough to prevent the bus passing it has broken down, or on one occasion when a vehicle collided with a herd of pigs.

Conclusion: they are better if there is enough room to reliably get a bus or HGV past a stationary vehicle, but the more extra width you add to deal with such emergency situations, the less of a financial/land take gain you get over building a D2 in the first place.
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Re: Why does the UK not have any D2+1 roads?

Post by c2R »

jnty wrote: Thu Jan 06, 2022 14:45
I suppose rural S4s probably end up in a 'survival of the unfittest' situation - the ones wide enough to provide a reasonable level of safety will be relatively easy to upgrade to D2, and the ones that aren't will stick around and look unsafe (and probably get changed to something else as well.) I don't know of any NSL rural S4s but I'd imagine they're on roads so quiet that safety's not a huge consideration. I'm not sure any cost savings for "WS4" vs. D2 "from scratch" would ever be worth it.
The less famous section of the A556 springs to mind: https://www.google.com/maps/@53.287097, ... 384!8i8192
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Re: Why does the UK not have any D2+1 roads?

Post by A1OZZ »

Fenlander wrote: Thu Jan 06, 2022 11:11 The easiest way to upgrade parts of the A17/A16/A47 and no doubt many other roads round here would be with the odd bit of S2+1. It doesn't need to be Motorway, it doesn't even need to be dual carriageway, just give us the odd bit of clear road to get past the slower stuff safely.
Certainly helps on the A47 at Tugby, East Norton and Wardley. Baffles me why the A47 is still so lame, a nightmare to drive on.
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Re: Why does the UK not have any D2+1 roads?

Post by KeithW »

jnty wrote: Thu Jan 06, 2022 14:45
skiddaw05 wrote: Thu Jan 06, 2022 13:11
jnty wrote: Thu Jan 06, 2022 12:53 Thinking about WS2+1 vs. D2+1 raises a question - what is it about central barriers that makes a (quieter, country) road safer?
I've sometimes wondered this about that even rarer beast, the rural S4
I suppose rural S4s probably end up in a 'survival of the unfittest' situation - the ones wide enough to provide a reasonable level of safety will be relatively easy to upgrade to D2, and the ones that aren't will stick around and look unsafe (and probably get changed to something else as well.) I don't know of any NSL rural S4s but I'd imagine they're on roads so quiet that safety's not a huge consideration. I'm not sure any cost savings for "WS4" vs. D2 "from scratch" would ever be worth it.

There's also the fact that it's two overtaking vehicles likely to be meeting at the centre line which are likely to both be going faster, especially if it's quite a short section, I suppose.
I know of some in the North East - here is one to start with
https://www.google.co.uk/maps/@54.58910 ... 8192?hl=en

There used to be a lot more but some were marked down to provide cycle paths and wider footpaths.
https://www.google.co.uk/maps/@54.56979 ... 6656?hl=en

Here is another stretch.
https://www.google.co.uk/maps/@54.57220 ... 8192?hl=en
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Re: Why does the UK not have any D2+1 roads?

Post by A303Chris »

jnty wrote: Thu Jan 06, 2022 14:45
skiddaw05 wrote: Thu Jan 06, 2022 13:11
jnty wrote: Thu Jan 06, 2022 12:53 Thinking about WS2+1 vs. D2+1 raises a question - what is it about central barriers that makes a (quieter, country) road safer?
I've sometimes wondered this about that even rarer beast, the rural S4
I suppose rural S4s probably end up in a 'survival of the unfittest' situation - the ones wide enough to provide a reasonable level of safety will be relatively easy to upgrade to D2, and the ones that aren't will stick around and look unsafe (and probably get changed to something else as well.) I don't know of any NSL rural S4s but I'd imagine they're on roads so quiet that safety's not a huge consideration. I'm not sure any cost savings for "WS4" vs. D2 "from scratch" would ever be worth it.

There's also the fact that it's two overtaking vehicles likely to be meeting at the centre line which are likely to both be going faster, especially if it's quite a short section, I suppose.
The only NSL S4's I know of are on the A30 in Cornwall, all short sections between S2+1's herem,

Connor Downs

Hayle bypass

Zelah Bypass

Although the last one is being converted to full D2 as part of the Chiverton to Carland Cross scheme
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Re: Why does the UK not have any D2+1 roads?

Post by Bryn666 »

A303Chris wrote: Fri Jan 07, 2022 09:03
jnty wrote: Thu Jan 06, 2022 14:45
skiddaw05 wrote: Thu Jan 06, 2022 13:11
I've sometimes wondered this about that even rarer beast, the rural S4
I suppose rural S4s probably end up in a 'survival of the unfittest' situation - the ones wide enough to provide a reasonable level of safety will be relatively easy to upgrade to D2, and the ones that aren't will stick around and look unsafe (and probably get changed to something else as well.) I don't know of any NSL rural S4s but I'd imagine they're on roads so quiet that safety's not a huge consideration. I'm not sure any cost savings for "WS4" vs. D2 "from scratch" would ever be worth it.

There's also the fact that it's two overtaking vehicles likely to be meeting at the centre line which are likely to both be going faster, especially if it's quite a short section, I suppose.
The only NSL S4's I know of are on the A30 in Cornwall, all short sections between S2+1's herem,

Connor Downs

Hayle bypass

Zelah Bypass

Although the last one is being converted to full D2 as part of the Chiverton to Carland Cross scheme
The A556 being signed as an explicit 60 mph S4 west of the M6 is a weird example of course, I'd like to know if that was approved by the DfT post detrunking or if it's a trunk road hangover.

S4 is not a permitted design for high speed roads these days with the notable exception of transitional lengths between two S2+1s as per the A30 examples.
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Re: Why does the UK not have any D2+1 roads?

Post by WHBM »

The alternating S2+1 on the A303 Ilminster bypass in Somerset has I believe the worst serious accident record for the whole of the A303. I did read it has had at least one fatal accident every year since it was built (and in some years, several).

The A556 S4 was an on-line widening design feature of the onetime Cheshire County Council in the late 1930s, it initially ran from Northwich right into Altrincham. They also did the A41 this way for much of Chester to Birkenhead, and a few others. Lancashire did some at the same time (Preston to Blackpool was one), but I'm not aware of any other former authority that favoured them.
Last edited by WHBM on Fri Jan 07, 2022 10:13, edited 2 times in total.
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Re: Why does the UK not have any D2+1 roads?

Post by A303Chris »

Bryn666 wrote: Fri Jan 07, 2022 09:30
The A556 being signed as an explicit 60 mph S4 west of the M6 is a weird example of course, I'd like to know if that was approved by the DfT post detrunking or if it's a trunk road hangover.

S4 is not a permitted design for high speed roads these days with the notable exception of transitional lengths between two S2+1s as per the A30 examples.
I never realised that length of the A556 was 60mph, I have not driven it that often and just assumed it was 50mph as was the former S4 section between the M6 and M56.

The A583 Kirkham bypass used to be S4 60 mph, but now its 50 mph with average speed cameras
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Re: Why does the UK not have any D2+1 roads?

Post by KeithW »

Herned wrote: Thu Jan 06, 2022 15:08 So is it:
KeithW wrote: Thu Jan 06, 2022 14:24 The main reason for not being D2 seems to have been the cost of digging the cutting.
or
KeithW wrote: Thu Jan 06, 2022 14:24 Good question - take it up with whoever wrote the wiki entry. My guess was that it was here where one carriageway has been marked down to 1 lane.
As your replies seem very contradictory.

I would guess that entire section including the junction was built as D2 and then later, probably after some nasty accidents, it was rebuilt the way it is today
Once again all I did was quite the Sabre wiki - if you think that is contradictory so be it.

As for the why it was reduced to a single lane I have no idea.
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Re: Why does the UK not have any D2+1 roads?

Post by Owain »

There's a rural S4 outside Coleraine. It was S2+1, but got widened to S4 in 2014/15, albeit with a 50 limit on it:

https://maps.app.goo.gl/tNgqBEJPDSoAYeMA9

I thought there was one with NSL on the A6 between Kendal and Shap, but I can't find it.
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Re: Why does the UK not have any D2+1 roads?

Post by nowster »

Owain wrote: Fri Jan 07, 2022 12:49 I thought there was one with NSL on the A6 between Kendal and Shap, but I can't find it.
That was S3 up to about fifteen years ago but has now been marked as S2+1.
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Re: Why does the UK not have any D2+1 roads?

Post by Debaser »

Herned wrote: Thu Jan 06, 2022 07:55 They are a bit of a niche thing, I know Scandinavian countries use them where distances are long and volumes relatively low. In Sweden they are often retrofitted from wide S2 roads, much like the Ilminster bypass, rather than built from new.
I believe this was as a result of the findings of EuroRAP studies, that head-on collisions in rural areas of Sweden was a particular problem. Thus the introduction of a central (IIRC wire rope) barrier and the addition of alternating overtaking lanes to account for speed differentials as a safety measure. Given the motorcycle lobby's hatred of wire rope (aka 'cheese wire') and our current use of concrete as the preferred containment system to prevent head-on crossover collisions on 'true' dual carriageways, I don't think this cross-section would be as cheap a fix as it would have been in Sweden.
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Re: Why does the UK not have any D2+1 roads?

Post by Bomag »

We developed S2+1 on Ilminstser bypass for a better option than WS1. S2+1 is safer than WS1, it may not be as safe as going from WS1 to D2 but that was never an option for Ilminster. New build costs for most roads in England would mean D2 is about the same price (the cost of moving from S2+1 to D2 is not relevant). Scotland developed TD70 as traffic flows were lower on many of their roads and therefore given relative costs, S2+1 came out as affordable whereas D2 wasn't. The last time I checked the A1 upgrade plans for Northumberland had two D2+1 sections between Anwick and Berwick as they were similar in costs and flows to Scottish roads.
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Re: Why does the UK not have any D2+1 roads?

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jnty wrote: Thu Jan 06, 2022 12:53 Thinking about WS2+1 vs. D2+1 raises a question - what is it about central barriers that makes a (quieter, country) road safer? How much of it is that overtaking into the opposite carriageway is prevented and how much of it is cars which 'drift' for whatever reason (crash, inattention, impairment) are physically barred from the other side? The former certainly feels like it would be more common on these roads than the latter. I'm trying to get a sense for how much the UK is 'compromising' by avoiding the barrier given that it feels like almost all wrong-side overtakes can be avoided with frequent WS2+1 plus average speed cameras if necessary.
I'm not sure that any UK national studies go into that much detail. I understand Sweden looked at where their poorly performing roads were (using the EuroRAP risk ratings as a starting point) and determined that it was the potential for high speed head-on collisions on rural roads that was their priority, thereby implementing measures to reduce this potential.
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Re: Why does the UK not have any D2+1 roads?

Post by Bryn666 »

nowster wrote: Fri Jan 07, 2022 12:57
Owain wrote: Fri Jan 07, 2022 12:49 I thought there was one with NSL on the A6 between Kendal and Shap, but I can't find it.
That was S3 up to about fifteen years ago but has now been marked as S2+1.
The S4 is further north around Low Hesket complete with permissive double whites: https://goo.gl/maps/CoqapGFS7Gonda5V8
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Re: Why does the UK not have any D2+1 roads?

Post by nowster »

Bryn666 wrote: Fri Jan 07, 2022 14:28
nowster wrote: Fri Jan 07, 2022 12:57
Owain wrote: Fri Jan 07, 2022 12:49 I thought there was one with NSL on the A6 between Kendal and Shap, but I can't find it.
That was S3 up to about fifteen years ago but has now been marked as S2+1.
The S4 is further north around Low Hesket complete with permissive double whites: https://goo.gl/maps/CoqapGFS7Gonda5V8
Interesting that the side with the danger markings is the side that's permitted to overtake into the opposing traffic.
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Re: Why does the UK not have any D2+1 roads?

Post by KeithW »

nowster wrote: Fri Jan 07, 2022 17:41 Interesting that the side with the danger markings is the side that's permitted to overtake into the opposing traffic.
The markings allowing overtaking if it is safe to do so alternate as you go along see here.
https://www.google.com/maps/@54.8237228 ... 384!8i8192

Its is not considered safe to overtake into a lane containing opposing traffic.
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Re: Why does the UK not have any D2+1 roads?

Post by nowster »

KeithW wrote: Fri Jan 07, 2022 17:59
nowster wrote: Fri Jan 07, 2022 17:41 Interesting that the side with the danger markings is the side that's permitted to overtake into the opposing traffic.
The markings allowing overtaking if it is safe to do so alternate as you go along see here.
https://www.google.com/maps/@54.8237228 ... 384!8i8192

Its is not considered safe to overtake into a lane containing opposing traffic.
But the danger markings don't switch over when the centre markings change...
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