A47 Corridor improvement programme

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jackal
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A47 Corridor improvement programme

Post by jackal »

Don't think this has it's own page (mods, feel free to merge if it does).

Anyway, this is one of the bunch of schemes due to start construction by Spring 2020. A few details are here, including an overall cost of £300m. I found the cost breakdown here interesting, especially the range of £67m-£103m for Thickhorn. Surely some freeflow, or at least a stackabout, can be provided for that price?

Note also that part of the A12 will be renamed A47 as part of the scheme.
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Re: A47 Corridor improvement programme

Post by wrinkly »

jackal wrote:I found the cost breakdown here interesting, especially the range of £67m-£103m for Thickhorn. Surely some freeflow, or at least a stackabout, can be provided for that price?
I think I've seen a crude plan of a Chieveley-style diversion of the A11 mainline there, possibly in a BBC item. It might even be mentioned on here somewhere but I've no idea which thread.

Correction: it wasn't like Chieveley because it connected to the existing at-grade roundabout.

www.sabre-roads.org.uk/forum/viewtopic.php?f=1&t=32439

http://www.edp24.co.uk/news/politics/no ... _1_3249001
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Re: A47 Corridor improvement programme

Post by roadtester »

Isn't that actually quite similar to what was done at M40/A46 Longbridge Island?
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Re: A47 Corridor improvement programme

Post by jackal »

roadtester wrote:Isn't that actually quite similar to what was done at M40/A46 Longbridge Island?
Yes, very similar. Personally, though I appreciate that it removes some conflicts, I don't like to see expensive bridges like that just dumping traffic into a roundabout. I note as well that the report I linked to gave a poor BCR of 0.9 for the Longbridge-style bypass. IMO it would be better to use the same bridges to freeflow onto the A47E.
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Re: A47 Corridor improvement programme

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jackal wrote:
roadtester wrote:Isn't that actually quite similar to what was done at M40/A46 Longbridge Island?
Yes, very similar. Personally, though I appreciate that it removes some conflicts, I don't like to see expensive bridges like that just dumping traffic into a roundabout.
That was exactly the thing I kept moaning on about in the Longbridge Island thread - A46 traffic exchanging one roundabout for a mother a few hundred yards away!
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Re: A47 Corridor improvement programme

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Somewhat similar at King's Lynn.
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Re: A47 Corridor improvement programme

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Yes, perhaps one of the pros can explain what the thinking is with this sort of layout and how it relieves pressure on the main roundabout - there's obviously a bit of a fashion for it and presumably it must work.
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Re: A47 Corridor improvement programme

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roadtester wrote:Yes, perhaps one of the pros can explain what the thinking is with this sort of layout and how it relieves pressure on the main roundabout - there's obviously a bit of a fashion for it and presumably it must work.
It simply removes conflicts between major flows. For instance, A11sb->A11sb and A11nb->A47eb presently have to cross paths. With the bypass, these two movements do not come into contact with each other. It's the same principle as any grade-separated junction, just with the annoyances of lower speeds and more remaining conflicts (e.g. A11nb->A11nb and A11sb->A47 still conflict).
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Re: A47 Corridor improvement programme

Post by c2R »

roadtester wrote:Yes, perhaps one of the pros can explain what the thinking is with this sort of layout and how it relieves pressure on the main roundabout - there's obviously a bit of a fashion for it and presumably it must work.

It does work... so, one bridge is required (or two, depdendent on location).

At King's Lynn, traffic heading s/b on the A149 from Hunstanton can exit at the first exit from the roundabout junction and head either towards Norwich or towards Wisbech/the North on the A47/A17. It does this without impacting upon the A10 n/b carriageway, which can head into King's Lynn without being interrupted.
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Re: A47 Corridor improvement programme

Post by roadtester »

Thanks for the replies - I can see now why it works but it still seems a bit messy!
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Re: A47 Corridor improvement programme

Post by roadtester »

PS - is there a proper name for this arrangement whereby a big heavily used roundabout is relieved by the construction of a flyover and a smaller second roundabout? Sun and planet roundabouts? Penny farthing roundabouts? Any other suggestions?
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Re: A47 Corridor improvement programme

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wrinkly wrote:Somewhat similar at King's Lynn.
I've visited the roundabout in question several times, and think I can explain how it's an improvement over a roundabout GSJ at that location. (I don't know if the A11 junction would work the same way, as I'm less familiar with the traffic flows there.)

The basic idea is that it gives an entrance to the A47, and an exit from the A47, at each side of the large roundabout. Most drivers are going to be doing one movement on their way through the junction, then the opposite junction on the way back home. This means that instead of having to go >180° around the large roundabout for one of their two movements, they can do <180° for both (so long as one of the movements involves the A47 to/from Wisbech and the other is a different road at that junction); the total movements using the help of the small roundabout add up to 180°, rather than the 360° that would be needed otherwise. For journeys that don't involve the A47, it neither helps nor hurts. So it's basically a substantial improvement in terms of congestion on the large roundabout, given that a large number of journeys there are to/from Wisbech (the junction is basically the point at which traffic that comes from the west gets to split off to the various parts of Norfolk, and given Norfolk's placement within the UK, most traffic is going to come from the west or possibly the south).

Of course, counterbalancing this is possible congestion on the small roundabout, but I've never known that to be a problem (admittedly, I'm rarely there in peak times). This is partly because the signals on the large roundabout effectively act as ramp metering for the A47, and partly because much of the Wisbech-Norwich stretch of the A47 is S2 (meaning that it carries S2 levels of traffic that can easily be handled by a small roundabout; occasional D2 sections don't change that). The key insight is that all traffic on the small roundabout is using the A47, whereas the large roundabout handles numerous different roads.

So I'd say this sort of junction layout is appropriate for a junction where a) many roads meet, and b) there's a large proportion of turning traffic to/from the "main" through road, compared to the amount of traffic that wants to go straight on on the "main" road.

Does the A11 fit these criteria south of Norwich? It doesn't seem to fit quite as well as the A47, but it still seems to fall into the same basic pattern. Most notably, the A47 at King's Lynn is handling the lion's share of the traffic from the west, which may want to go to many different destinations within Norfolk; the A11 at Norwich is in a similar position for traffic from the south. So there's quite a parallel there. The main obvious difference is that the A47 has a freeflow already at Norwich, whereas there are no freeflows at King's Lynn. This means that the large Norwich roundabout is getting congested on relatively few movements, most of which involve the A11 already, and thus there's less of a gain to be made by pushing half the A11 turning traffic onto a new small roundabout; compared with the King's Lynn junction, there are going to be a much smaller proportion of journeys that wouldn't want to use the small roundabout at all, neither on the way out or the way back.

Out of interest, are there any figures on AADTs or peak flows of the various movements of the A47/A11 interchange? It'd be worth knowing which movements it is that need the most attention.
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Re: A47 Corridor improvement programme

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I use the A10/A47 roundabout a lot - usually a few times a week - but the similarity to what's proposed at Norwich didn't immediately occur to me, perhaps because I'm normally just going north/south on the A10, not negotiating A47 the flyover and the recently (well, fairly recently) added second roundabout.

I have to say, I didn't really notice the roundabout being particularly overloaded before so I didn't think of the flyover/second roundabout scheme mainly as a jam-buster (I have no idea what the actual numbers are/were so I'm prepared to be corrected on that). I actually saw it more in the context of A47 dualling/improvement and shared the widely-expressed disappointment that they bottled the dualling aspect before it got built.

I suppose the net effect is that whereas before, all A47 traffic used to hit the big roundabout, regardless of whether it was through traffic or vehicles wanting to turn off/join, now it is just the leave/join traffic that hits the main roundabout, not the through traffic.
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Re: A47 Corridor improvement programme

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ais523 wrote:Out of interest, are there any figures on AADTs or peak flows of the various movements of the A47/A11 interchange? It'd be worth knowing which movements it is that need the most attention.
I don't have numbers for specific movements, but the AADFs for the nearest count points are:

A11S 41,276
A11N 30,318
A47W 36,940
A47E 49,209

I think this supports the suggestion that freeflow for A11S<->A47E should be provided, a bit like this, rather than the Longbridge-style bodge.
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Re: A47 Corridor improvement programme

Post by jackal »

The county council apparently expect the A47 North Tuddenham to Easton upgrade to take possible extension of the NNDR into account. So far as I'm aware, the council haven't even decided on a route for the extension...

http://www.edp24.co.uk/news/politics/ca ... _1_4744247
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Re: A47 Corridor improvement programme

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A47 junction improvements could be scrapped after Highways England resist roadworks

Presumably HE would not be happy to see their road fouled up with a developer-funded bodge, especially given their nearby plans for Blofield to North Burlingham dualling.
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Re: A47 Corridor improvement programme

Post by 85CF380 »

jackal wrote: I don't have numbers for specific movements, but the AADFs for the nearest count points are:

A11S 41,276
A11N 30,318
A47W 36,940
A47E 49,209

I think this supports the suggestion that freeflow for A11S<->A47E should be provided, a bit like this, rather than the Longbridge-style bodge.
I also notice that A11 Hgvs north of the A47 (Thickthorn) number 837,
while A11 Hgvs south of the A47 number 2353, so roughly 1500 Hgvs are leaving the A11 at Thickthorn for the A47, possibly half of them are making a (slow)270* turn (either east-south or north-east) roughly one a minute. If the £50m improvement does nothing to alleviate this problem then its a waste of money in my opinion.
I'm also no fan of Longbridge. £40m. The A47 isn't free-flow. Coming up the M40 from Oxford for the A46 you're still confronted by 3 sets of lights & a plethora of lanes of which 10% of other road users seem incapable of understanding. Likewise A46 ex Evesham to M40 south. The POPE study was a whitewash too.
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Re: A47 Corridor improvement programme

Post by 85CF380 »

Something else that irks me about the A47 Corridor project. Why, west of King's Lynn does it not encompass the A17? At Kings Lynn west and north bound traffic roughly splits itself in half between the A17 & A47. Note HGV figs.
A47 Wisbech bypass 18700 AADF 970 HGV : A17 Gedney bypass 19300 AADF 2470 HGV. Even at A17 Sutterton HGV numbers rise to whopping 2640 on a S2 carriageway with AADF 15100. (who allowed this road to be de-trunked? & why? I note the HE still manage the A49 with 12000 nth of Hereford).
As for the 'A47 corridor'. West of the A1, the A47 itself has been detrunked! & from my experience traffic going west of P'Boro uses the A605 (another de trunked road!) to the A14 & west. Interlinking network?
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Re: A47 Corridor improvement programme

Post by Fenlander »

The bits of the A47 I use are from the A1 to Kings Lynn, the single carriageway from A1 to the existing dual needs dualling, the single the other side of Peterborough could do with dualling, or at least adding some passing places. At Kings Lynn where the A17 joins it there's the missing Bruges for the flyovers for the A17, finishing that junction off would deal with the A17 traffic better which would leave the A47 traffic to get on and do its own thing without all having to go round the roundabout.
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Re: A47 Corridor improvement programme

Post by doebag »

IME, immediately west of the A1, a large percentage of A47 HGV traffic is accessing the gravel pits south of the A47, then it seems to tail off towards Leicester.
The A605 is too convenient a shortcut between the A1 and the A14.
As for the flyover at the Pullover roundabout A47/A17 I wonder if that will ever happen. It must been over thirty years after the by-pass had been built and the earthworks made that the one at the Hardwick roundabout was completed.
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