A487 Caernarfon Bypass

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jackal
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Re: A487 Caernarfon Bypass

Post by jackal »

Peter Freeman wrote: Thu Mar 11, 2021 08:06
jackal wrote: Wed Dec 02, 2020 20:51 ... and on a road with several major junctions like the Carnavon bypass there usually will be significant changes in volume along the way that can be accommodated better by a mixture of S2 and D2 than by blanket S2+1.
I agreed with this statement when I first read it. However, on deeper thought, I think the opposite is true. To the first order, this alternative strategy of S2/D2 can provide exactly the same number of overtaking opportunities with an equal area of road surface. However, it is arguable that S2+1 might allow more judicious positioning of its 2-lane sections. This because D2 might put a 2-lane carriageway on the side where it is needed, but wastefully impose 2 lanes on the opposite side even though it is not required there. That wasted area of road surface could be better deployed elsewhere.

Regardless, I favour the S2/D2 mix!
My unstated assumption is that traffic volumes are typically similar in both directions on a bypass such as this. Hence in terms of volumes alone I don't agree that "S2+1 might allow more judicious positioning of its 2-lane sections" - S2+1 by definition has much more capacity in one direction than the other so will be mismatched to volumes.

Where S2+1 is useful is for climbing lanes. So in the case of high gradients I would agree that "S2+1 might allow more judicious positioning of its 2-lane sections".

So in short I think S2+1 is fine up a hill but typically not otherwise. (Some exceptions are where volumes in each direction are significantly different, or short sections adjoining junctions to allow stacking etc.)

Just going off the video it seems some of this bypass is quite steep, so I can understand the S2+1 on those sections at least.
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Re: A487 Caernarfon Bypass

Post by Peter Freeman »

jackal wrote: Thu Mar 11, 2021 08:27
Peter Freeman wrote: Thu Mar 11, 2021 08:06
jackal wrote: Wed Dec 02, 2020 20:51 ... and on a road with several major junctions like the Carnavon bypass there usually will be significant changes in volume along the way that can be accommodated better by a mixture of S2 and D2 than by blanket S2+1.
I agreed with this statement when I first read it. However, on deeper thought, I think the opposite is true. To the first order, this alternative strategy of S2/D2 can provide exactly the same number of overtaking opportunities with an equal area of road surface. However, it is arguable that S2+1 might allow more judicious positioning of its 2-lane sections. This because D2 might put a 2-lane carriageway on the side where it is needed, but wastefully impose 2 lanes on the opposite side even though it is not required there. That wasted area of road surface could be better deployed elsewhere.

Regardless, I favour the S2/D2 mix!
My unstated assumption is that traffic volumes are typically similar in both directions on a bypass such as this. Hence in terms of volumes alone I don't agree that "S2+1 might allow more judicious positioning of its 2-lane sections"
Yes, on volumes alone (and on a flat, straight road) the two scenarios would be equal in performance. In either case, you'd spend half your distance on a one-lane section (travelling in queues at a lower level of service and lower speed) and half on a two-lane section (with the luxury of being able to overtake and achieve a higher average speed).
jackal wrote: Thu Mar 11, 2021 08:27
- S2+1 by definition has much more capacity in one direction than the other so will be mismatched to volumes.
Agreed. Therefore, since with either design we would have a situation where the same volume of traffic will sometimes be in single file and sometimes on an overtaking section, the volumes on 1-lane and 2-lane sections will be the same but the level of service and hence average speed will be different.
jackal wrote: Thu Mar 11, 2021 08:27 Where S2+1 is useful is for climbing lanes. So in the case of high gradients I would agree that "S2+1 might allow more judicious positioning of its 2-lane sections".
And the hypothetical, though unrealistic, illustration of this, would be a 1km road between two roundabouts that rises to a crest half way along and then descends to the final roundabout. With an asphalt budget of 3 lane-kilometres, the optimal design would be S2+1, where the uphill pieces are 2-lane, swapping over at the crest to 1-lane going down the hill. Hardly any delays. An S2/D2 mix, depending on where the break points are placed, would leave you, and/or the other direction, struggling in your car behind a slow truck for up to 500m of that short road.
jackal wrote: Thu Mar 11, 2021 08:27 So in short I think S2+1 is fine up a hill but typically not otherwise. (Some exceptions are where volumes in each direction are significantly different, or short sections adjoining junctions to allow stacking etc.)
I tend to agree. It is, of course, the reason why simple overtaking lanes are normally placed on hills. But if you can't afford D2 then S2+1 is better than just S2.

By the way, in my opinion the 2-lane sections of S2+1 should not occur where the road is curving to the left, for visibility reasons. This design only partly complies with that rule.

In real life, the asphalt budget and my hypothetical up-down road don't exist, and so every design is a compromise and a one-off special. This design is OK - but an S2/D2 mix as you first suggested would be better and, especially, safer.
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JammyDodge
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Re: A487 Caernarfon Bypass

Post by JammyDodge »

Peter Freeman wrote: Thu Mar 11, 2021 12:29 In real life, the asphalt budget and my hypothetical up-down road don't exist, and so every design is a compromise and a one-off special. This design is OK - but an S2/D2 mix as you first suggested would be better and, especially, safer.
If the video on their site is accurate, then they are doing S4 at the approaches to most of the roundabouts
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nowster
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Re: A487 Caernarfon Bypass

Post by nowster »

I went past recently.

The northern end is looking good. The local road is now in place and open. (It wasn't in June.)

The southern end roundabout is being remodelled and the new route is visible from it: the boundary wall has been removed.

The middle roundabout between Caernarfon and Bontnewydd is being built offline and is visible from the existing road. No visibility of the works to the west of there as the hedges and walls are still in place on either side. I estimate another month will bring a tie-in of the existing road to that roundabout, then the existing road will be torn up and the new road laid across it.

Several of the roads it crosses are currently closed, which makes driving parallel to the route difficult. I'll be down there again in a few weeks, and will try to have a good sken.

The recent dry spell will have allowed the works to progress a little more speedily, I suspect.

There's a good possibility that the bypassed road will become the A499 (as it used to be).

In other news from the area, the junction of the A4086 and the A4244 North of Llanberis is being upgraded from T-junction to roundabout.
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nowster
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Re: A487 Caernarfon Bypass

Post by nowster »

News from the area. I was there a couple of weeks ago. It's pretty close to opening.

Roundabouts are complete. Diversion of the local roads is complete. The temporary bridge for works traffic half way between Caernarfon and Bontnewydd is gone.

Some signage is already up. Some evidence of road markings in places.

The old A487 is being renumbered A4871. Pity! I hoped they'd restore the A499 to its original route.

I wouldn't be surprised to hear it of it opening in the next few weeks.

Drone footage from August.
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Re: A487 Caernarfon Bypass

Post by rowan826 »

The A487 Caernarfon Bypass will open on Friday 18th Feburary

https://www.dailypost.co.uk/news/north- ... luk8z2_kuQ
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Re: A487 Caernarfon Bypass

Post by avtur »

I've spent most of my adult life (50 years) going backwards and forwards to various parts of the Lynn Peninsula from south Manchester. In particular a solid spell of nearly 20 years going to and from Stockport to Pontllyfni, just south of Caernarfon, which was our second home every other weekend in the summer months, I know this route so well !!

Unfortunately we had to relinquish our Caernarfon roots in 2018 when we moved down to Sussex. The works to develop the "by-pass" started during our last season of residence in Pontllyfni. The benefit to us would have been outstanding, so many were the times when we made good progress on our journey all the way to Bangor, but then that last leg through and beyond Caernarfon could be just sooo slow.

I'm really glad to hear that the new road will soon be open and I will have to make a visit to the area to drive the new road. It will bring back many happy memories, but things change in life and unfortunately weekend visits to Pontllyfni are no longer a part of our life. However, living in an entirely new part of the country means that almost everyday is a holiday, out and about exploring new places.

OK, we've been here in mid Sussex three and a half years, so we have gained a lot of local knowledge, but it is still very easy to go out and drive new roads and find new places
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wrinkly
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Re: A487 Caernarfon Bypass

Post by wrinkly »

wrinkly wrote: Tue Jan 26, 2021 22:57
nowster wrote: Tue Jan 26, 2021 22:26 Has anyone noticed that the OS has got the route choice wrong?
I hadn't notice that. The route they show seems to be he one that was first chosen and then fairly quickly changed - called the purple route IIRC.
I see it's right now. I hadn't checked it since the above was posted.
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AAndy
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Re: A487 Caernarfon Bypass

Post by AAndy »

rowan826 wrote: Tue Feb 15, 2022 16:23 The A487 Caernarfon Bypass will open on Friday 18th Feburary

https://www.dailypost.co.uk/news/north- ... luk8z2_kuQ
Might be cancelled due to the storm I wonder?
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Big L
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Re: A487 Caernarfon Bypass

Post by Big L »

AAndy wrote: Wed Feb 16, 2022 15:28
rowan826 wrote: Tue Feb 15, 2022 16:23 The A487 Caernarfon Bypass will open on Friday 18th Feburary

https://www.dailypost.co.uk/news/north- ... luk8z2_kuQ
Might be cancelled due to the storm I wonder?
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AAndy
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Re: A487 Caernarfon Bypass

Post by AAndy »

It was cancelled yesterday, the opening , not the road :wink:

https://www.dailypost.co.uk/news/north- ... s-23133180

Open now though: https://www.dailypost.co.uk/news/north- ... n-23152054
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AAndy
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Re: A487 Caernarfon Bypass

Post by AAndy »

Not good https://www.bbc.co.uk/news/uk-wales-60446867

Crash looks to have happened on a s2+1 section.
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Re: A487 Caernarfon Bypass

Post by Nathan_A_RF »

Anyone know what number has been assigned to the old road?
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Chris Bertram
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Re: A487 Caernarfon Bypass

Post by Chris Bertram »

I was going to suggest B487 as a joke, but I find that this exists in Hertfordshire.
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From the SABRE Wiki: B487 :


The B487 is a short B-road in Hertfordshire.

It begins near Marlowes Shopping Centre in Hemel Hempstead, then passes through the suburb of Adeyfield and under a former railway bridge (now Cycle Route 57). At the Industrial Estate the route is split by the A4147 Link Road, but picks up again near Grovehill.

The route of the B487

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