Fixing a two-bridge roundabout - M4 J33

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DB617
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Fixing a two-bridge roundabout - M4 J33

Post by DB617 »

I've got a bit of a theoretical which I've been pondering and failing to find any solution to, while stuck in traffic approaching M4 J33 on the way to work. Sidenote, stuck in traffic is how I come up with almost all my traffic management ideas and probably how I decided to become a civil engineer, too.

Most of the early Welsh M4 junctions were built as two-bridge roundabouts, which come packaged with the conflict between major flows as we all know well. M4 J33 is the end of the A4232 which is the incomplete Cardiff ring, its incompleteness pushing most eastbound M4 traffic into this junction instead of the other planned end at J30. The tailbacks can be up to a couple of miles from rush hour traffic trying to turn east at the roundabout and grinding to a halt at the traffic lights on the bridge, which themselves are a testament to the increase in AADT because they were originally part-time signals. They certainly aren't anymore.

So my question is, (in theory because SWTRA works are almost always done on the cheap) if we were looking to reduce that shuddering delay at J33, is there any way the movements that require crossing the M4 could be free flowed or somehow improved? Bearing in mind that Cardiff West services sits about 15m from the roundabout and is fed by it on the north side. Are there any examples of similar works elsewhere, or is it something that's considered unprecedented?

Current actual efforts to improve the junction were a couple of free flow links long since completed for the non-motorway-crossing movements, which do make some difference, and a proposed but floundering road link between Barry, the A48 and J34 which would reduce flows through J33. That really is about it.
AtoB
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Re: Fixing a two-bridge roundabout - M4 J33

Post by AtoB »

Not sure if this the answer you wanted to see - or if you've thought of this already -

A new diverge at the M4 eastbound / J33 slip, so that - the existing slip goes up to the r.bout as at present. The new slip diverges to the left of this, then goes underground to emerge on the A4232 s'bound

Additionally - and importantly - building this would require virtually no disruption of existing traffic flows
Last edited by AtoB on Sun Sep 08, 2019 17:46, edited 1 time in total.
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c2R
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Re: Fixing a two-bridge roundabout - M4 J33

Post by c2R »

Yeah, like Fletton Parkway Interchange is probably similar, in that a large amount of traffic leaves the A1 heading northbound to head into Peterborough. It has a direct free flow link using separate bridges.

The services are in a somewhat annoying position, for a cheap freeflow solution - maybe something like Birchanger Green Roundabout could be done - start with some cut throughs, and then realise that some free flowing sliproads are needed....
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From the SABRE Wiki: Fletton Parkway Interchange :


Fletton Parkway Interchange is junction 17 of the A1(M).

Work was completed in 2015 to increase the capacity of the free flowing sliproads between the A1(M) and A1139.

| [[Category:E English Trunk Road

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jackal
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Re: Fixing a two-bridge roundabout - M4 J33

Post by jackal »

Here's my effort, with the major flows freeflowed and only one new bridge required:

M4 A4232 - Copy.jpg

The eagle eyed will notice that one movement (northbound A4232 to the services) is not provided. This traffic is almost certainly close to its origin point so the movement can be sacrificed to freeflow the major movements IMO.
Last edited by jackal on Sun Sep 08, 2019 16:45, edited 1 time in total.
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Johnathan404
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Re: Fixing a two-bridge roundabout - M4 J33

Post by Johnathan404 »

Plenty of space to simply free-flow the eastbound right turn, even if it's of a Meccano construction. I would caution against the temptation to make it too sharp and too steep, as this would reduce its capacity considerably, as per the Irish M50 GSJs.

Consideration would need to be given to whether any additional traffic can be accommodated in Cardiff. If you can get more people in and out then great, if you're just moving the queues around then what's the point?

It would be good to sort out that ludicrous right turn on the westbound offslip while you're there.
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DB617
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Re: Fixing a two-bridge roundabout - M4 J33

Post by DB617 »

Thanks for the ideas; I think a lot of them are feasible. A Fletton Parkway style solution would probably completely relieve the northbound jam, and I had thought (from my own limited regional experience of motorway interchanges) that a tunnelled slip like the one at Lofthouse Interchange would be a possibility. The hard bit is the proximity of the MSA to any civils, but I think it's trivial to move, especially considering the building and its petrol station are a bit like Aust services - tired and in need of renewal. A Meccano style flyover would probably be ruled out in terms of height, simply on the grounds that it would stick out from the landscape like a sore thumb where the M4 here runs just slightly below prominence from the surroundings. Exactly how painful are the costs for fly-under tunnels?

There are issues with traffic further down the road with free-flowing movements into the city, which would have to be handled by Cardiff and the Vale of Glamorgan councils as the road and its junctions are not trunked as of 3 miles south of J33, encompassing Culverhouse Cross, where the southbound slip spills onto the mainline so badly at rush hours that the congestion tends to eventually reach back to the J33 complex. The area where the queue develops is also highly dangerous and sightlines are very poor - it should be permanently reduced to 50mph under the junction. As the existing interrupted flow arrangement congests back from Cardiff to J33 anyway, the only gains to be made from a trunk/motorway perspective are to the northbound carriageway, which would then flow more easily onto the M4 which currently copes well enough. I think the motorway could benefit from the removal of the dense waves of traffic which come down the existing slip, panic braking as the centrelin suddenly disappears into a scary free-for-all.

I like jackal's full rebuild/freeflow loops. I have no concerns over reduction in capacity from the tight loops, as if you look at the current marking arrangement where the A4232 ends, one lane drops to the M4 westbound slip and one lane continues for the eastbound and splits back into two for stacking at the lights, then collapses back into one as mentioned right before joining the M4 in a shambles.

I also think if we're improving the capacity of this junction, why not go the whole hog? At risk of jumping into fantasy roads territory, the M4 here should probably be smart ALR from J32 to J34 at minimum, replacing the weird narrow lane already between J34-J33 eastbound with lane drops at each junction, massively increasing the weaving distance in the process. Currently the weaving eastbound between J33 and J32 is very hairy indeed and when the A4232 backs up onto the westbound M4, a very undesirable queue develops down the hard shoulder where people think they're being terribly clever.
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