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Why do British Highway Engineers Hate Free-Flow Junctions?

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ABB125

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But by all accounts the new A556 works well, and it's certainly something I felt was needed from my experience with the old road.
It'll be interesting to see whether the recently "upgraded" M6 J19 has any effect on traffic congestion. Another example of a junction where anywhere else there would be a proper flyover for northbound A556 traffic, but here we have to make do with a hamburger...
What's more surprising is that given the huge flow from London to Manchester that a spur (at the time probably A556 (M) or M556, I suspect) wasn't built to the M56 towards Manchester from day one. Was it anticipated Manchester traffic would go via the M62? Or was London to Manchester traffic much lower back then?
My understanding is that when the M62 was built, Liverpool was expected to be the dominant traffic generator, hence the layout of the M6/M62 junction favouring that city. But yes, it does seem odd that a spur wasn't built originally, especially as it's the only movement which isn't free-flow at the M6/M56 junction
 
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PTR 444

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It'll be interesting to see whether the recently "upgraded" M6 J19 has any effect on traffic congestion. Another example of a junction where anywhere else there would be a proper flyover for northbound A556 traffic, but here we have to make do with a hamburger...

My understanding is that when the M62 was built, Liverpool was expected to be the dominant traffic generator, hence the layout of the M6/M62 junction favouring that city. But yes, it does seem odd that a spur wasn't built originally, especially as it's the only movement which isn't free-flow at the M6/M56 junction
Maybe it was originally anticipated that the M6-M56 via A556 movement would only cater for traffic to Stockport and Manchester Airport (much less developed back then). Whereas for Central Manchester, traffic from the south was expected to use the M62 and M602?
 

ABB125

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Maybe it was originally anticipated that the M6-M56 via A556 movement would only cater for traffic to Stockport and Manchester Airport (much less developed back then). Whereas for Central Manchester, traffic from the south was expected to use the M62 and M602?
That's probably the reason.
 

Bletchleyite

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That sort of makes sense as the M62/602 route goes closer into the city on motorways and feeds the Mancunian Way. But then why was Manchester signed via the A556?
 

krus_aragon

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Roundabouts are fine. Enormous, many-lane signalised roundabouts are less fine. If you need to signalise a roundabout, you'd be far better off replacing it with a proper signalised junction (ie: crossroads), which would be more efficient.
I know it's not quite a motorway, but would you have any thoughts on J19 on the A55 (where it meets the A470, A547, and a local road, right next door to the railway line)? The large roundabout has recently been partially signalised, but I don't know what I'd do differently given the roads coming from seven different directions in a confined area.
 

tomuk

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The 'original' plan back in 1962 was for a motorway link from M6 J19 to M60 J7 an elongated Altrincham–Sale–Stretford Bypass. This never happened but you ended up with the odd A6144 (M) Carrington spur and the repeatedly cancelled A556(M) which has now come to pass as the current improved A556.
 

ABB125

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I know it's not quite a motorway, but would you have any thoughts on J19 on the A55 (where it meets the A470, A547, and a local road, right next door to the railway line)? The large roundabout has recently been partially signalised, but I don't know what I'd do differently given the roads coming from seven different directions in a confined area.
I'm not sure to be honest, especially as I don't know what the major traffic flows are (other than straight through in the A55!). In some ways this demonstrates the British approach: "aaaargh! It's got more than four arms! We can't cope, just stick a roundabout in and hope it works". A signal-based junction of some sort might be better (as long as the signal timings are sequenced properly, which generally they aren't), but unfortunately I can't say much more without a load more data.
 

krus_aragon

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I'm not sure to be honest, especially as I don't know what the major traffic flows are (other than straight through in the A55!). In some ways this demonstrates the British approach: "aaaargh! It's got more than four arms! We can't cope, just stick a roundabout in and hope it works". A signal-based junction of some sort might be better (as long as the signal timings are sequenced properly, which generally they aren't), but unfortunately I can't say much more without a load more data.
Fair enough. In case you're curious, the major flows are between the A470 and A55. Most commuting traffic leaving the A55 heads north into Llandudno / Jn on the A470 and A547 (and back out), and there's a LOT of daytrippers added to this during the summer, coming to/from the A55 from the East. There is a reasonable amount of traffic coming northbound on the A470 joining the A55, or crossing over toward Llandudno. Again, seasonal flows include tourists coming and going from the England direction. (The A547 east of J19 is pretty quiet.)

Prior to the remodelling, tailbacks on the A470's flyover from Llandudno were standard in the evening from Spring to Autumn, with traffic seeking to turn left onto the A55 toward Chester. The road only becomes two lanes shortly before the roundabout (much of the flyover has a crawler lane for northbound traffic up the steep slope). Only the left lane was for left turns, so as soon as you had half a dozen cars waiting to do so, no other vehicles could filter past to go south/west.

This flyover now allows traffic from either lane to turn left, with the right lane also serving ahead onto the roundabout (for south/west). It's notable that while three of the major roundabout's entrances are signalled (giving a decent route for pedestrians & bikes to cross), the two entrances from the Llandudno / Jn direction are not. The lights elsewhere are timed to periodically hold up other traffic to let vehicles geto out of Llandudno/Jn, and they're free to enter the roundabout at any other time the coast happens to be clear.

There will also be some tailbacks on the A470 northbound toward J19 at peak times, through the adjacent small roundabout. They're not as bad as the queues of tourists leaving Llandudno in the afternoon, though.

All in all, I think it's a pretty good example of improving the existing road layout, and suiting the main traffic flows. The only other thing I could have wished for when the diggers came out was a filter lane from Llandudno/A470 to the A55 eastbound. This would have been tricky to do without rebuilding/modifying the flyover, and would have required additional land, encroaching on the buildings north-east of the roundabout.
 

satisnek

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Worcestershire County Council have been historically notorious for not completing road projects. There's the never-finished Kidderminster Ring Road for starters, and also the A38 Roman Way Droitwich bypass, where the flyover which should have carried it over the A442 junction was never built.

droitwich.jpg
 

Alfie1014

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And here's the A13/A1089 junction, which is hamstrung by sunk cost fallacy (continuing to use the existing trumpet) and the fact that having a junction here means five arms need to be plugged in, always a sign of complexity:
View attachment 118282
For an amusing diversion, here is the original plan for this junction (from a decade ago). Note than up is west not north in this image. Whilst this would be totally inadequate, I'm slightly surprised there's even a trumpet on here. I'd have expected another roundabout, like this:
View attachment 118283
As someone who grew up in house just off to the left of the upper plan of the A13/A1089 junction I would hate to have to live near both the construction and use of that monstrosity of a junction, it would dominate the (mostly flat) landscape for miles and seems to butt up against most of the surrounding settlements. No wonder local opposition to the LTC is so vocal! I’m glad I live no where near it these days!
 

sannox

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Roundabouts are fine. Enormous, many-lane signalised roundabouts are less fine. If you need to signalise a roundabout, you'd be far better off replacing it with a proper signalised junction (ie: crossroads), which would be more efficient.

Gonna need a citation on that one as I think that is perhaps a bit broad-brush. Large signalised crossroads can be very slow, with turns having to be signalised separately and lots more delay. Signalised roundabouts can actually work pretty well, many designed with low cycle times or double cycling so traffic is moving quicker. Of course too much traffic all breaks down.
 

D365

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This is very similar to the traffic problem and debates you get when you play Cities Skylines (which, despite all the add-ons, is basically one big road-traffic simulator). The default/pre-built system interchange you get at the start of a new map is a cloverleaf, which as discussed above is fine for light traffic but as soon as you get a significant amount of traffic it's very easy for the whole thing to become gridlocked, especially if your heaviest flow is trying to turn right. This is because traffic turning right joins its target road before traffic already on the target road gets to its junction to let it turn right, so what you end up with is a single section of road taking traffic for multiple destinations, and you can't separate it into lanes because traffic joins on the left and then different traffic leaves on the left so there's a lot of weaving needs to be done in-between the two junctions.
It doesn't fall under the topic of British motorway interchanges, but I always replace my cloverleafs with a "four leg" variant.
 
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