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The Pont Tywysog Cymru signs haven't even gone up yet and the Second Crossing project already has some botched ugly roadsigns. Never noticed that one before.
They've done the sensible thing of putting a 50mph speed limit there, but they've done the stupid thing of having the speed limit start arbitrarily half a mile from where the road works start.
I still don't understand why this junction is with a nothing rather than the A403, and why go the expense of building the M49 as motorway standard when it's chocked by the roundabout at the end, you'd think they'd at least have the brains to put in a left turn filter or they may as well have just built a WS2 road as the weak link in capacity is that roundabout.
ajuk wrote: ↑Mon Apr 01, 2019 00:55
I still don't understand why this junction is with a nothing rather than the A403, and why go the expense of building the M49 as motorway standard when it's chocked by the roundabout at the end, you'd think they'd at least have the brains to put in a left turn filter or they may as well have just built a WS2 road as the weak link in capacity is that roundabout.
Very wrong on both counts. Have a very quick glance at the Avonmouth side of the M49 and you'll very clearly see that there is not "nothing" there - or anything close. And I've never known the roundabout to be any problem whatsoever apart from the occasional overturned lorry - it's the M5 that's the weak link with congestion on the M49 almost always a direct result of congestion on the M5. That said, the weaving section between J18 and 18A is starting to become an issue in itself. Plus it'll be interesting to see what the roundabout becomes like when you start adding regular M4 east traffic to it.
ajuk wrote: ↑Mon Apr 01, 2019 00:55I still don't understand why this junction is with a nothing rather than the A403, and why go the expense of building the M49 as motorway standard when it's chocked by the roundabout at the end, you'd think they'd at least have the brains to put in a left turn filter or they may as well have just built a WS2 road as the weak link in capacity is that roundabout.
A roundabout is easily capable of handling the traffic load of a moderately busy two-lane dual carriageway. It would be nice to see a free-flowing junction where the M4 meets the M49, yes, and too often we build a roundabout where a free-flow junction could have been used. But I don't think you can seriously claim the M49 is in any way choked by the roundabout. It works fine!
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!
Let's put this into context. According to the traffic count figures on SABRE maps, the M49 has an AADT of ~18k. The M50 between the M5 and J1, where the M5 junction was changed from a freeflow trumpet to a roundabout, has an AADT of around 30k, plus the M5 junction also has to deal with southbound Strensham services traffic. Yet it too very rarely suffers from congestion. Traffic levels on the M49 would probably have to at least triple before the roundabout becomes an issue, and as Johnathan says the M4 and M5 will be far bigger problems than the M49 ever will be.
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?
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.
Of course it could have just been budgetary or based on bad forecasts that more traffic would leave/join at the M49, or an assumption that congestion could drive people to use the M48 bridge to cross the Severn. With that in mind, the idea of terminating the M48 on a roundabout at J23 in the future is nuts.
AAndy wrote: ↑Mon Apr 01, 2019 12:48
The M49 should always have been a free-flowing GSJ where it meets the M4 as it is where it meets the M5.... poor foresight/planning....
Cost was not the issue as I understand. Somebody thought it was better to have a roundabout.
Given there must be close to zero traffic that bridge bound traffic needs to give way to, and only an absolutely minuscule amount of traffic turning right into the M4, it effectively is free-flowing. Granted, the new junction on the M49 will lead to a bit more traffic using the east-facing slips, but I can’t see it being a problem for a very long time.
Presumably it’s only a roundabout in order to not be a dead end if the bridge is closed?
Herned wrote: ↑Mon Apr 01, 2019 22:51
Presumably it’s only a roundabout in order to not be a dead end if the bridge is closed?
I should think the main reason it's a roundabout is that we like building roundabouts. Deviating from that norm requires more thought, especially when working with the estuary as a constraint.
A secondary benefit is that you wouldn't need to drive to Avonmouth every time you want to close the bridge, which would allow a lot more traffic to slip through and that could be a problem in an emergency.
It was probably built in anticipation of the new M49 junction too.
Finally, a very minor benefit but still a benefit: it avoids trying to find somewhere to display the toll prices on the Avonmouth Bridge.
Herned wrote: ↑Mon Apr 01, 2019 22:51
Presumably it’s only a roundabout in order to not be a dead end if the bridge is closed?
I should think the main reason it's a roundabout is that we like building roundabouts. Deviating from that norm requires more thought, especially when working with the estuary as a constraint.
A secondary benefit is that you wouldn't need to drive to Avonmouth every time you want to close the bridge, which would allow a lot more traffic to slip through and that could be a problem in an emergency.
It was probably built in anticipation of the new M49 junction too.
Finally, a very minor benefit but still a benefit: it avoids trying to find somewhere to display the toll prices on the Avonmouth Bridge.
A trumpet could have just about fit in the space there of course, which would also have allowed you to escape a closed bridge.
Bryn Terminally cynical, unimpressed, and nearly Middle Age already. She said life was like a motorway; dull, grey, and long.
The M4/M49 roundabout also helps maintenance crews on the bridge to turn around without having to drive almost to Bristol.
The Welsh side is a little easier as you can turn around at the ex toll booths.
Tony Alice (they,them)
~~~~~
Owner of a classic rust heap/money pit, and other unremarkable older vehicles.
Usually found with a head in an old map or road atlas.
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