Unbuilt four-way full freeflow interchanges

[jackal]

There was no extension planned south of the M8 but the A82 end was definitely meant to continue. The quality of the map is poor but it appears the junction could allow all movements in all four directions.

On the Google Earth overlay there are movements from the M898 to the new arm, but no new movements to/from the A82.

http://www.cbrd.co.uk/articles/glasgow/maps

[Truvelo]

Here's another full four way junction

[Chris5156]

Here's another full four way junction

One of several in that report, I think!

[jackal]

In the OP I mention a gothic interchange (this sort of thing), which I think was an alternative at the same location. This was from CBRD, but the link is dead now - maybe lost in the redesign.

[Truvelo]

Yes, there were a few of those shown in the report.

[M19]

Yes, there were a few of those shown in the report.

Malfunction Junctions.

[Chris5156]

In the OP I mention a gothic interchange (this sort of thing), which I think was an alternative at the same location. This was from CBRD, but the link is dead now - maybe lost in the redesign.

Gone temporarily but back soon!

[jackal]

I've added a couple more to the list. The first is the rejected option 11 for the M2 Junction 5 scheme, referred to as a '4-tier intersection' in the scheme brochure and as a 'fully free-flowing interchange' in the TAR. Between the two it seems clear it is a stack.

The second is this geometrically unusual cloverstack, which fell victim to the financial crisis:

[jackal]

A full cloverleaf is one of the options for the A1079/A164 Jock's Lodge improvement:

http://www2.eastriding.gov.uk/EasySiteW ... lId=675358

[Truvelo]

A full cloverleaf is one of the options for the A1079/A164 Jock's Lodge improvement:

http://www2.eastriding.gov.uk/EasySiteW ... lId=675358

The purple option would be the easiest to build along with another freeflow left turn in the northwest quadrant. Such 6-ramp parclos are the default junction type in Canada. A pity we only have a couple as 6 of the 8 turns are freeflow.

The blue option is disgusting. Introducing roundabouts with nasty deflection, as clearly shown on the map , will bugger up the A1079.

The green option has elongated slips to presumably increase the weaving length.

So long as the blue option is junked I don't mind which of the others is finally chosen.

[Burwellian]

In fact the result of the missing movements is to cause chaos when the slightest thing goes wrong. For example a northbound incident on the M11 means that anybody wanting to get to the Park and Ride , University west and Madingley Rise area from the M11 southnound has no alternative but to go through the centre of Cambridge simply because the highways designers have come to the conclusion that nobody driving south on the M11 would ever want to go there. Even with 'normal' traffic its common between 8.30 and 9.30 to find traffic queues northbound in lane 1 for the Madingley Road that reach back to the Barton Road junction. Many of those vehicles are only they have been forced to use that route. It is so routine now that 90% of the users know that local custom is to queue for Madingley Road on the hard shoulder.

The NW Cambridge site off Madingley road which is well underway is aimed to add the following
1,500 homes for University and College staff
1,500 private houses for sale
Accommodation for 2,000 postgraduates
100,000 sqm of academic and research and development space of which up to 40% may be private research with University connection or Research Institutes
Community facilities such as a primary school, community centre, health centre, supermarket and local shops.
A hotel
Care home

Improvements to the M11 junction - none. But they are promising cycle paths so that's all right then.

...and you've sort of touched on why. The City Council was the only local authority in the area opposed to the A14 upgrade, mainly because of Huntingdon Road. Madingley Road has the same issues; if you make it easy for traffic to approach Cambridge from the North that way, it drops them onto the city streets, either into the mess at Mount Pleasant (Huntingdon Road) or onto The Backs.

Quite simply, the Uni wouldn't want it, and it's probably them that'd stop any such upgrades going in. Much better to force them further along the A14 and in on Milton Road (which is thus jammed as a result) or further down the M11 to use Trumpington Road, not that the city wants the cars in town in the first place if they can help it. Even Newnham allows traffic to drop onto the Fen Causeway, away from the scenic stuff and the one-way system mess.

This is probably a fair part of why there's talk of a busway towards Cambourne to alleviate the A1303 Madingley Road and make things at least a little more tolerable without encouraging more car use.


Back on topic, for unbuilt 4-ways, do the various past proposals for the Girton rebuild count?

[jackal]

Back on topic, for unbuilt 4-ways, do the various past proposals for the Girton rebuild count?

Indeed there was! This is a Highways Agency design from 2009 redrawn by Truvelo:

https://www.sabre-roads.org.uk/wiki/ima ... _links.png

Smart Cambridge Transport's current (highly unofficial) proposal to add the 'missing' movements wouldn't be, as the new A428<>A14 movements would be at-grade:

http://www.smartertransport.uk/wp-conte ... 40x652.jpg

[jackal]

In 2015 this was the 'emerging preferred' option for the Coolagh junction on the N6 Galway bypass. In the 2017 scheme refresh it was replaced with a more basic GSJ.

[jackal]

I've updated the OP with a couple of rejected options from the current M42 J6 and A428 Black Cat schemes.

I've also added one I missed when I originally looked at the Glasgow plans - the interchange between the Hamilton-Cumbernauld motorway and what I'm calling the Airdie-Coatbridge Radial. If anyone knows the latter road's proper name, let me know.

[Truvelo]

It's shown as Coatbridge-Airdrie Expressway on the 1965 plans.

[Peter Freeman]

In the OP I mention a gothic interchange (this sort of thing), which I think was an alternative at the same location. This was from CBRD, but the link is dead now - maybe lost in the redesign.

Gone temporarily but back soon!

Is 'Gothic' the (unofficial) name for that design? I wasn't sure whether any of those had actually been built. It's a free-flowing system-interchange variant of the DDI. I've usually thought of its type name as 'double crossover interchange'.

An interesting thing about that one is that it's 5-armed: four motorway arms plus two street-connecting one-way half-arms. Sydney Australia is about to (partially) open a 5-armed 4-level stack as a mainly-service interchange on its new M8 motorway. It will not cater for all possible turns, and two of its arms will not be motorways. I'll post about it elsewhere soon.

[Bryn666]

In the OP I mention a gothic interchange (this sort of thing), which I think was an alternative at the same location. This was from CBRD, but the link is dead now - maybe lost in the redesign.

Gone temporarily but back soon!

Is 'Gothic' the (unofficial) name for that design? I wasn't sure whether any of those had actually been built. It's a free-flowing system-interchange variant of the DDI. I've usually thought of its type name as 'double crossover interchange'.

An interesting thing about that one is that it's 5-armed: four motorway arms plus two street-connecting one-way half-arms. Sydney Australia is about to (partially) open a 5-armed 4-level stack as a mainly-service interchange on its new M8 motorway. It will not cater for all possible turns, and two of its arms will not be motorways. I'll post about it elsewhere soon.

The cleanest example - I-695/95 in Baltimore was demolished and replaced with a more conventional stack. The wrong side exits and entries are just undesirable for high speed running.

[jackal]

It's shown as Coatbridge-Airdrie Expressway on the 1965 plans.

Thanks. I also finally got round to adding some of your examples to the OP.

In the OP I mention a gothic interchange (this sort of thing), which I think was an alternative at the same location. This was from CBRD, but the link is dead now - maybe lost in the redesign.

Gone temporarily but back soon!

Is 'Gothic' the (unofficial) name for that design? I wasn't sure whether any of those had actually been built. It's a free-flowing system-interchange variant of the DDI. I've usually thought of its type name as 'double crossover interchange'.

Yes, I think because it's supposed to look like a gothic cross. The only real world examples I know of are the one in Birmingham, Alabama linked above and the removed one Bryn mentioned:



A freeflow DDI, or 'Double crossover merging interchange' (DCMI) as it's apparently called, has a similar basic layout to a DDI but with bridges rather than traffic lights. Accordingly only one of the mainlines has the carriageways swapping over, not both as in a gothic interchange:



I don't think a DCMI has ever been built, but there are a couple of US interchanges that have the carriageways on one mainline swap over in a non-DDI-style configuration:

Charlotte: https://www.google.co.uk/maps/@35.27241 ... 456434,16z
Grand Rapids: https://www.google.co.uk/maps/@42.97232 ... 775898,16z

All the long turns merge and diverge from the offside in a gothic cross, so given modern standards there is basically no situation where you would build one. By contrast 'only' half the long turn merges/diverges in a DCMI are to the offside. So it does a similar job to an offside cloverstack (e.g. M50/M4 and M50/N2), though I have doubts about land take and structural cost.

[Peter Freeman]

A freeflow DDI, or 'Double crossover merging interchange' (DCMI) as it's apparently called, has a similar basic layout to a DDI but with bridges rather than traffic lights. Accordingly only one of the mainlines has the carriageways swapping over, not both as in a gothic interchange:

So the system interchange version ('gothic cross') could be thought of as a 'double double crossover merging interchange', 'two-dimensional DCMI' or 'M2M DCMI'.

I'm not too surprised that the DCMI (diagram above) has not actually been implemented. It sacrifices at least two of the DDI's attributes (cost-effectiveness and urban compactness) in order to achieve full-free-flow and thereby to be just usable as a system interchange.

In addition to replacing traffic signals by grade-separation, the diagram above adds the 'Minor Side Bridge'. This is perhaps not an essential for a DCMI, but is an improvement as it eliminates offside exits and their associated severe weave-points. Weaving problems are not severe, or even present, in a standard DDI, owing to (a) the crossover signals breaking up flow and (b) signals usually being placed at the off-ramp ends.

All the long turns merge and diverge from the offside in a gothic cross, so given modern standards there is basically no situation where you would build one. By contrast 'only' half the long turn merges/diverges in a DCMI are to the offside. So it does a similar job to an offside cloverstack (e.g. M50/M4 and M50/N2), though I have doubts about land take and structural cost.

I realise that modern practise has long been to avoid offside exits (and entries), but I question how important this is at high-turning-volume (and high-joining-volume) locations, where more traffic turns/joins than continues ahead. The criticism in that case is perhaps more that it causes driver confusion (TOTSO) than causes actual merge/diverge difficulty … ? (southbound M6J11A ?)

[jackal]

Well, in the UK the guidance has been to avoid offside merges/diverges. There is, however, an interpretive issue here. E.g. the recent rebuild of Girton seems on its face to have replaced a nearside exit (loop) with an offside one. But I imagine the engineers though of the rebuild as the westbound A14 carriageway turning the corner, with a nearside diverge to A428. This is a geometric absurdity (topography doesn't respect road numbers) but the guidance is not specific enough to exclude this kind of rule bending.

Now, I think the Girton rebuild is very good, but that's because I think offside exits (and merges) are appropriate where most traffic turns, as you say. And this seems to be the rule used in most countries. Given the nonsensical UK rule, it's probably for the best that it's vague and open to abuse, but it would be better to have a more appropriate rule.