M49 Avonmouth junction

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ajuk
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Re: M49 Avonmouth junction

Post by ajuk »

Johnathan404 wrote: Mon Apr 01, 2019 13:27 My understanding of the point made is that the single-lane slips at the M4 interchange have a lower capacity than the rest of the road, and will therefore one day be a bottleneck.

It doesn't really hold because capacity was never a concern here. A long stretch of D2M is safer than any kind of WS2, and slightly faster, especially a road with a lot of HGVs. Neither of those points have really been compromised by a slight break for a difficult-to-miss roundabout. A simpler way of looking at it is to say traffic can afford to bunch together when navigating a roundabout.

Besides, if and when the day does come that the roundabout needs to go - ignoring the fact that the M4 and M5 would probably be crumbling under the strain by the time this happens - replacing the roundabout will be much easier than upgrading the whole road. Much easier, but probably prohibitively expensive at the time the junction was built.

I think there is a problem on SABRE where so many of the roads we love to talk about are severely overloaded (and therefore do need extra lanes when stopping for a junction), that we don't know how to accept a road that flows freely!
You have a point, I just didn't see why you'd go to the expense of making it a D2M why not just make it a D2 or maybe even an alternating D2+1 with sections of D2 at either end, and spending the money they saved on improving other roads in the area.
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Re: M49 Avonmouth junction

Post by Bryn666 »

Mainly as it is a link between two motorways so little benefit would be gained by building it as a single carriageway all purpose road as you'd have to turn around at the M4 end.

Given that J18 was massively rebuilt it also suggests there was a view a lot of traffic would head to Avonmouth Docks and Bristol avoiding Almondsbury and the M32...
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Re: M49 Avonmouth junction

Post by Truvelo »

I'm sure the tolls had some influence on the M4/M49 junction being a roundabout.

If you think the current layout is bad then count your lucky stars some of the alternative proposals for a second severn crossing weren't built. This one is shocking.
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Re: M49 Avonmouth junction

Post by rhyds »

Truvelo wrote: Tue Apr 02, 2019 22:34 I'm sure the tolls had some influence on the M4/M49 junction being a roundabout.

If you think the current layout is bad then count your lucky stars some of the alternative proposals for a second severn crossing weren't built. This one is shocking.
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jackal
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Re: M49 Avonmouth junction

Post by jackal »

ravenbluemoon wrote: Tue Apr 02, 2019 12:05 The M4/M49 roundabout also helps maintenance crews on the bridge to turn around without having to drive almost to Bristol.
There's a works unit directly off the roundabout, as the northern arm. A similar roundabout interchange immediately to the north of the LTC is proposed with a works unit. Works access may be a significant consideration over the 120 design life of high maintenance structures like these.

There's also a weighbridge.

The roundabout allows traffic to turn around easily, including M49>M49, which even a trumpet wouldn't allow.

And I suspect future proofing for something like J1 may have been envisaged.

I suspect these considerations of general utility will have been the driving factor rather than capital cost. A freeflow fork of reasonable proportions would have cost no more, and perhaps less, than a two-bridge roundabout. But why bother when it's unnecessary in traffic terms and you can't do as much with it.

As for the M49 itself, the most surprising thing in retrospect is that they built it at all. At 18k AADT it's only taking about half a lane's worth of traffic off the M5. That's a pretty poor return on 5 miles of new build motorway, a two-bridge roundabout, and a freeflow interchange complex with braiding and the works. Widening the M5 would have been an obvious alternative.
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c2R
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Re: M49 Avonmouth junction

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DB617 wrote: Mon Apr 01, 2019 21:58
Norfolktolancashire wrote: Mon Apr 01, 2019 21:13 I noticed on my last two trips on a toll free Severn Bridges trip that the D2 M4 from the M5 to the new Severn Bridge was congested. Why wasn't this stretch made D3 when the new Severn Bridge was built?
I wonder if this is something to do with a lane gain for the westbound slip from J22 being considered essential due to the gradient, climbing to meet the M4 which is itself climbing to the apex of the bridge.
I wrote a post about that recently too, as the M4 is reduced to two lanes at the point the M5 in both directions merges into it - the Gloucester Road bridge is the issue - eastbound it is three lanes, due to the gradient - but also perhaps because westbound there were tolls and therefore the AADT was suppressed. There's simply no more room under the bridge, and so it really needs to be demolished and widened to provide D3M all the way to the M4/M48 diverge.

Perhaps for the same reason the M49 was decided upon, rather than widening the M5/M4 - not having to rebuild the Gloucester Road bridge, although short sighted, must have counted in favour of the M49 BCR.

However, the removal of the tolls must have increased traffic flows and probably will continue to, as house prices in south wales are cheaper than in the Bristol and Bath area, so I'd expect increased commuter flows.
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Re: M49 Avonmouth junction

Post by gepree68 »

2019-04-03 M4 M5 M48 M49.png
In the 1990s when they built M49 and the M4 Second Severn Crossing, they should have also built the green dotted line.

The green line would be the new M5 and would be either D3M or D4M.

The existing M5 (most of the red line on the map) would become something like M35.
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Re: M49 Avonmouth junction

Post by betweenmways »

c2R wrote: Wed Apr 03, 2019 00:52
DB617 wrote: Mon Apr 01, 2019 21:58
Norfolktolancashire wrote: Mon Apr 01, 2019 21:13 I noticed on my last two trips on a toll free Severn Bridges trip that the D2 M4 from the M5 to the new Severn Bridge was congested. Why wasn't this stretch made D3 when the new Severn Bridge was built?
I wonder if this is something to do with a lane gain for the westbound slip from J22 being considered essential due to the gradient, climbing to meet the M4 which is itself climbing to the apex of the bridge.
I wrote a post about that recently too, as the M4 is reduced to two lanes at the point the M5 in both directions merges into it - the Gloucester Road bridge is the issue - eastbound it is three lanes, due to the gradient - but also perhaps because westbound there were tolls and therefore the AADT was suppressed. There's simply no more room under the bridge, and so it really needs to be demolished and widened to provide D3M all the way to the M4/M48 diverge.

Perhaps for the same reason the M49 was decided upon, rather than widening the M5/M4 - not having to rebuild the Gloucester Road bridge, although short sighted, must have counted in favour of the M49 BCR.

However, the removal of the tolls must have increased traffic flows and probably will continue to, as house prices in south wales are cheaper than in the Bristol and Bath area, so I'd expect increased commuter flows.

Yes, the M4 from Almondsbury interchange down to the M48 (at the very least) should ideally have a third lane.

These days, there is a jam at the M5 -> M4 W/B on slip every rush hour. This never used to be the case. However, i don't think it is because of the bridge tolls and population. I reckon the main contributor is the variable limits/hard shoulder running on the M4 beforehand - the M4 traffic heading to the M5 flows much better now meaning that the M4 flows better through the M4/M5 interchange. Before that the jams between M32 and M5 were regular, which also restricted throughput towards the Severn. So by freeing up one place it has a knock-on effect a bit further down.

But we are going a bit off topic...
Without the M49 then almondsbury would be buried under traffic by now (no matter how little is taken off it by the M49). There's limited improvement there given how complex it is?
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Re: M49 Avonmouth junction

Post by Norfolktolancashire »

gepree68 wrote: Wed Apr 03, 2019 11:47 2019-04-03 M4 M5 M48 M49.png
In the 1990s when they built M49 and the M4 Second Severn Crossing, they should have also built the green dotted line.

The green line would be the new M5 and would be either D3M or D4M.

The existing M5 (most of the red line on the map) would become something like M35.
The issue would be that the M5 Avonmouth viaduct is congested either at peak hours in the week or in summer weekends, even with it being enlarged to D4. An ideal, albeit expensive, fix would be a parallel D3 from the M4 southwest to Weston (Super Mare), but I could say that for many motorways.
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Re: M49 Avonmouth junction

Post by gepree68 »

According to this South Gloucester Council page, the new M49 junction is going to be called M49 Junction 18A.

Rather than, as I expected, M49 Junction 1.

Should this be reported to South Gloucestershire Council as a problem?
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Re: M49 Avonmouth junction

Post by Bryn666 »

gepree68 wrote: Thu Jun 06, 2019 15:07 According to this South Gloucester Council page, the new M49 junction is going to be called M49 Junction 18A.

Rather than, as I expected, M49 Junction 1.

Should this be reported to South Gloucestershire Council as a problem?
That's absolutely ridiculous, especially given the other junction for Avonmouth is ALSO J18A.

Definite problem. It should be M49 J1.
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Re: M49 Avonmouth junction

Post by SouthWest Philip »

I'd go with M49 jnc 2 with the junction with the M4 being M49 jnc 1 and M5 junction being jnc 3.
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Re: M49 Avonmouth junction

Post by nigelphillips »

I spoke to Highways England - initially they said 18A then a few weeks later after I pointed out that this was a bad idea - they have changed it to Junction 1.
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Re: M49 Avonmouth junction

Post by Truvelo »

Does a tiny little motorway with only one intermediate junction need junction numbers? When the new junction on the M45 was built it never had a number.
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Re: M49 Avonmouth junction

Post by Micro The Maniac »

As a slight tangent, this thread has got me thinking...

According to the map of motorway numberings both the M48 and M49 should be M5x

Or has common sense applied, and like the Thames and Humber, the Severn has been used as the actual zone boundary (at least as far North as Tewkesbury...)?
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Re: M49 Avonmouth junction

Post by IAN »

Does a tiny little motorway with only one intermediate junction need junction numbers? When the new junction on the M45 was built it never had a number.
I suppose it's not essential but until the new junction appears on Sat Nav and due to the fact it's built to serve an industrial area attracting traffic from long distances and it's possible that drivers will not be familiar with it at first, it may be useful to be able to say that drivers should leave the motorway at 'M49 Junction 1'. (Although having said that, I would have thought that most drivers would be sensible enough to realise this!)

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Re: M49 Avonmouth junction

Post by gepree68 »

Truvelo wrote: Thu Jun 06, 2019 20:29 Does a tiny little motorway with only one intermediate junction need junction numbers? When the new junction on the M45 was built it never had a number.
M45 is strange because it already had a junction number (M45 J1) at the point when the new junction was built (with no junction number).

Really they should have renamed J1 to J2, and made the new junction M45 J1.
Last edited by gepree68 on Fri Feb 17, 2023 15:21, edited 1 time in total.
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Re: M49 Avonmouth junction

Post by A9NWIL »

Micro The Maniac wrote: Fri Jun 07, 2019 10:04 As a slight tangent, this thread has got me thinking...

According to the map of motorway numberings both the M48 and M49 should be M5x

Or has common sense applied, and like the Thames and Humber, the Severn has been used as the actual zone boundary (at least as far North as Tewkesbury...)?
Well the M48 could be considered to be in zone 5, but they actually decided to keep the M48 as the zone 4 boundary instead of moving it with the M4! So the M4 crossing the Severn isnt actually the zone 4 boundary!
They then decided that the M4 couldnt be out of zone so the motorway 4 zone squeezes through the big M4-M5 junction and out into the south west corner as well as where you expect it to be.

The real issue is due to naming the M5 the M5 in the first place, this is in a strange place for a zone boundary compared with the way the all purpose road boundaries are placed.

You could say that as 4 is closer to 1 than 5 that it should have priority over the 5th zone. So the M4 would take precedence over the M5 when they cross. That would make all motorways beginning south of the M4 as M3x, meaning that the M49 would need to be the M39. The north side of the M4 west of the M5 would be the 5 zone though meaning the M48 should be an M5x, it couldnt be the M58 though as there already is an M58, so we would need another number M51 is free so it could take that and be logical as well seeing as it would be the southern most M5x motorway.
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Re: M49 Avonmouth junction

Post by Micro The Maniac »

lotrjw wrote: Sat Jun 08, 2019 14:28 Well the M48 could be considered to be in zone 5, but they actually decided to keep the M48 as the zone 4 boundary instead of moving it with the M4!
Good point... I'd overlooked that point.

But the M49 is still wrong...
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Re: M49 Avonmouth junction

Post by A9NWIL »

Micro The Maniac wrote: Sun Jun 09, 2019 15:46
lotrjw wrote: Sat Jun 08, 2019 14:28 Well the M48 could be considered to be in zone 5, but they actually decided to keep the M48 as the zone 4 boundary instead of moving it with the M4!
Good point... I'd overlooked that point.

But the M49 is still wrong...
It certainly feels that way but as I explained they expanded the 4 zone officially, read this: https://www.roads.org.uk/articles/road- ... -anomalies and scroll to 'The Severn mysteries' bit.
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