Grade-separated Junctions on Single Carriageway Roads

[jackal]

The echelon junction (which just has one connecting bidirectional sliproad) makes more sense in that context. OTOH, unlike the CGSJ, I wouldn't really call it grade-separated because it has no freeflowing movements other than one "straight on" movement for each road (you could add filter lanes for a left turn movement from one road and a right turn (= left+left+left) movement from the other, but this is something that's rarely done in practice)).

It doesn't really have any freeflowing movements - even the 'straight on' movements are impeded by right turns unless dedicated filter lanes for the straight movement are provided (which they rarely if ever are). I believe the final version of the A14 Huntingdon upgrade even features an echelon variant at Bar Hill with signals on both mainlines

[wrinkly]

To clarify, the type of echelon junction which I was referring to when I said

Several mentioned earlier in the thread are examples of that. I think it's a currently recommended type for certain traffic levels.

is one in which two roads cross by a bridge, and are connected by one link road in one of the four quadrants, and the link road is two-way, and there is a flat, all-movements T junction at each end of the link road - no LILOs.

The through roads are usually single carriageways but I suppose they could in some cases have single-lane dualling at one or both of the constituent T-junctions.

If such a junction is being designed, and if land is available in all four quadrants as possible sites for the link road, I imagine the designers would choose the quadrant which minimises the expected number of right turns from the link road onto (rather than off) the through routes.

For example, if the roads run N-S and E-W, and the heaviest turning movement is traffic approaching from the east and turning north or vice versa, and the other turning movements are much smaller, the link road should be in the NW quadrant.

Personally I would say we have no choice but to count it as a GSJ, because it's a junction which includes a road-over-road bridge. Whether it's within what the OP intended this thread to be about is a separate question.

[AndyB]

Free-flowing interchanges for single carriageway roads are pretty rare in the UK - only other one I could think of was on the A483 south of Wrexham and even here the major road is dual carriageway north of the junction.

Any others? I'm primarily looking for junctions in which the major road is single carriageway, and flows freely through the junction (rather than, for example, two roads which pass over each other through a bridge, but are connected by a link road with at grade junctions). Cheers!

Just saying

[crowntown100]

Well, I've found two Cornish examples. However, neither are full access.

The first one is one I should have picked up on straight away. The A30 is single Carriageway at Zelah (although, hopefully, not for too much longer) and has a GSJ with a bridge designed to allow dualling which forms the GSJ here.

The second is a hybrid Trumpet on the A390 at the eastern end of the Sticker bypass here.

In both cases, the old road gets used as one of the slip roads in the junction.

I also think that there are a few examples on the A361/A39 NDLR.

[A9NWIL]

A CGSJ (compact grade-separated junction) is where you have, typically, a single-carriageway road crossing a double-carriageway road, via the use of two bidirectional (usually S2) sliproads. For each slip road, one end connects to the single-carriageway road at a T-junction, the other end connects to the dual carriageway as a LILO. With one connection to each side of the dual carriageway, this makes all twelve movements possible at a relatively cheap cost, and through traffic on the dual carriageway is free-flowing.

This junction type is encouraged in the DMRB as an alternative to at-grade junctions between a major road and a minor road in cases where both the through and turning traffic on the minor road is relatively low, allowing the major road to be grade-separated without spending the money for a more expensive junction type like a diamond. OTOH, when joining a fast and busy road, it's fairly dangerous due to the typical lack of proper merges (often joining traffic has to stop to find a gap in traffic, then after joining is doing just-over-0 when everyone else is doing 60 or 70). Here's an example from CBRD's Bad Junctions (and there are several similarly substandard CGSJs on that section).

Doing this on a pair of single carriageway roads is a little weird because the main purpose of the two LILOs is so that you don't need central reservation gaps. The echelon junction (which just has one connecting bidirectional sliproad) makes more sense in that context. OTOH, unlike the CGSJ, I wouldn't really call it grade-separated because it has no freeflowing movements other than one "straight on" movement for each road (you could add filter lanes for a left turn movement from one road and a right turn (= left+left+left) movement from the other, but this is something that's rarely done in practice)).

LILO junctions are quite weird I often find that I have to put my foot right down so I can accelerate as fast as possible, when joining a dual carriageway this way. The other way round means slowing to at least 40MPH so that its not a dangerous turn!

[Gareth Thomas]

The A338/B3078 junction at Fordingbridge in Hampshire.

[Stevie D]

Hmm, so it's basically half a CGSJ? Interesting design. I assume it'd only be an improvement over a regular staggered junction if the straight-on movements were fairly popular on both roads, and that it'd be substandard unless the turning movements were relatively light. That combination can't be all that popular but I assume it happens on occasion.

They are sometimes used where there is a significant height gain needed on one road. For example, where the A35 Axminster bypass crosses A358, it has a long climb up Raymond's Hill, so flying it over A358 means the gradient is reduced and, even though the junction doesn't merit grade separation per se (it would probably have been a flat roundabout otherwise), by having an uninterrupted movement on the mainline it makes it much easier for heavy vehicles to manage the hill.

[Owain]

We missed this one on the A427; it's very crude, but has a slip road each way, so it counts.

[jackal]

I don't think A52/A61 in Derby and A444/A4082 in Coventry have been mentioned.

There's also a couple of 'accidental' ones on South Bridge and George IV Bridge in Edinburgh:
https://www.google.co.uk/maps/@55.94838 ... 338,17.02z

[Duncan macknight]

Has everyone forgotten the A9 Pitlochry North Junction?

[Owain]

Funny this thread should come up now .... I discovered this one this morning, and it doesn't appear to have been mentioned.

[Burns]

Has everyone forgotten the A9 Pitlochry North Junction?

It's not on a single carriageway, I'm afraid. Before they hatched each side down to one lane, I used to take advantage of the tiny piece of dual carriageway to pass tractors.

[Owain]

Has everyone forgotten the A9 Pitlochry North Junction?

It's not on a single carriageway, I'm afraid. Before they hatched each side down to one lane, I used to take advantage of the tiny piece of dual carriageway to pass tractors.

Does the hatching have a solid or broken line?

If it's the latter, I'm sure no one would mind.

[Bryn666]

Having not read the thread in detail I'm going to go on a limb and say Livingston is the capital of the single carriageway GSJ.

[vlad]

I don't think A52/A61 in Derby and A444/A4082 in Coventry have been mentioned.

I wouldn't say either of those are on single-carriageway roads.

Unless you argue that they count as they're only grade separated in one direction.

[Burns]

Has everyone forgotten the A9 Pitlochry North Junction?

It's not on a single carriageway, I'm afraid. Before they hatched each side down to one lane, I used to take advantage of the tiny piece of dual carriageway to pass tractors.

Does the hatching have a solid or broken line?

If it's the latter, I'm sure no one would mind.

It looks like this. If I were desperate, I might still use it but shh...

[Owain]

Does the hatching have a solid or broken line?

If it's the latter, I'm sure no one would mind.

It looks like this. If I were desperate, I might still use it but shh...

Ahh.... What a waste of tarmac. And paint!

[jackal]

I don't think A52/A61 in Derby and A444/A4082 in Coventry have been mentioned.

I wouldn't say either of those are on single-carriageway roads.

Unless you argue that they count as they're only grade separated in one direction.

They're not on SCs, so yes, they don't fall within the scope of the exact wording the OP used.

But the thread has clearly become a more general discussion of SC GSJs, and includes several examples of single carriageway GSJs that are on roads that are otherwise DCs. That's what these are examples of.

[orudge]

Funny this thread should come up now .... I discovered this one this morning, and it doesn't appear to have been mentioned.

Evidently designed with future dualling in mind!

[lefthandedspanner]

This junction for the Ulter Folk and Transport Museum, on the A2 between Belfast and Bangor, allows full access from both sides of the road.