Collector/Distributor Roads and Express Lanes

[Bryn666]

There are a couple of examples in France - you're probably thinking of when the A1 was widened in the 1990s here: https://maps.app.goo.gl/Uqgc5onncs1yBnmg8 (sortie 17.1 came much later than the widening).

Thanks, that's the one, or at least the kind of thing I had in mind. To me those aren't C/D lanes, though I guess Peter would say they are, given what he says about a hypothetical M20 J6 without J5.

DMRB (sigh) has made this more complicated than it needs to be by defining outer carriageways designed for weaving movements as "link roads*". On any new rural** motorway, junctions spaced less than 2km apart are supposed to have outer carriageways (I'm not calling them "link roads") for weaving but you can handwave this requirement away if there are environmental constraints. There is no compulsion to have them anywhere from what I can gather, and there doesn't seem to be any design guidance for joining up outer carriageways to create a pure C/D network - so this would be why we've barely had any put in because they have to be designed as if they were part of the mainline - so no reduced speed limits or anything that makes them part of an interchange complex. I also imagine there's nebulous arguments about "driver confusion", given very basic globally accepted and workable concepts seem to cause NH to have a meltdown when presented with them. It really is a document written by people who want to feel smarter than they seemingly are.

* "In the context of junctions, a link road is one-way connector road adjacent to but separate from the mainline carriageway carrying traffic in the same direction. It is used to connect the mainline carriageway to the local highway network where successive direct connections cannot be provided to an adequate standard because the junction spacing is too close."

** Another sigh-worthy and stupid distinction, the M60 currently qualifies as a rural motorway despite being surrounded by urban areas purely because it has a 70 limit but the junction spacing across nearly the entire route would require "link roads" if built today! Even more stupidly, so does the M602 (before the air quality limit went in) despite having no offside hard strips and very much being built to reduced characteristics as well...

[Peter Freeman]

1.

Given we know how catastrophic weaving is for the capacity of a motorway it amazes me we haven't had more affinity with splitting out these movements - either through braiding or express lanes for mainline traffic. It rather shows how we are obsessed with encouraging short hop motorway journeys ...

How do C/D lanes encourage short-hop motorway journeys? You could claim 'indirectly, by making close-together junctions more feasible'. But junctions are built where they are needed, either with or without C/Ds. In either case, they will be hoppable.

Braiding is a different matter, and it is true that UK rarely does it (except in Glasgow - congratulations!). Best example of missing braid: M5J15/16, where there is that awful lane-exchange section of shared slips, enforcing a 100% weaving proportion.

2.

It rather shows how we are obsessed with encouraging short hop motorway journeys [euphemism for "other people's journeys"] whilst lying to ourselves that they are for strategic national traffic [euphemism for "my journeys"].

There ... a slight qualification

I agree with WHBM's faux-subtle qualification. Short-hop motorway journeys wouldn't be happening if they weren't effective for the hopper. The frequent obsession with 'separating long-distance traffic from short-distance traffic, strategic from local', etc, is misguided. We need to extract all the value we can from our road assets. The more use we can wring out of them, the better. I would advocate for more junctions on all our motorways (ok, NOT M60!). I would also have routed long-distance motorways to skirt closer to major cities and conurbations, exploiting their availability for local use. ( A view expressed previously - enough!).

[Bryn666]

1.

Given we know how catastrophic weaving is for the capacity of a motorway it amazes me we haven't had more affinity with splitting out these movements - either through braiding or express lanes for mainline traffic. It rather shows how we are obsessed with encouraging short hop motorway journeys ...

How do C/D lanes encourage short-hop motorway journeys? You could claim 'indirectly, by making close-together junctions more feasible'. But junctions are built where they are needed, either with or without C/Ds. In either case, they will be hoppable.

Braiding is a different matter, and it is true that UK rarely does it (except in Glasgow - congratulations!). Best example of missing braid: M5J15/16, where there is that awful lane-exchange section of shared slips, enforcing a 100% weaving proportion.

2.

It rather shows how we are obsessed with encouraging short hop motorway journeys [euphemism for "other people's journeys"] whilst lying to ourselves that they are for strategic national traffic [euphemism for "my journeys"].

There ... a slight qualification

I agree with WHBM's faux-subtle qualification. Short-hop motorway journeys wouldn't be happening if they weren't effective for the hopper. The frequent obsession with 'separating long-distance traffic from short-distance traffic, strategic from local', etc, is misguided. We need to extract all the value we can from our road assets. The more use we can wring out of them, the better. I would advocate for more junctions on all our motorways (ok, NOT M60!). I would also have routed long-distance motorways to skirt closer to major cities and conurbations, exploiting their availability for local use. ( A view expressed previously - enough!).

If you want those local hops, you need to build more C/D lanes. We say motorways aren't for local journeys a lot in the UK, but then build them so they end up that way anyway (look how people complain about the long distances between exits on rural lengths like the M6 J32-33 and A1(M) J50-52). Nobody serious about traffic engineering thinks the M6 through the West Midlands is an example of design excellence do they? J7-8-9 is a classic of the genre of what happens when you mix through traffic with local junction hopping: daily chaos. Had this been built with C/D lanes or even just had a smaller footprint for J8, a lot of today's problems might have been avoided but it still shows how these things are a problem. Any cursory glance at US freeways tells the same story: drivers do not like weaving and closely spaced junctions. The once much lauded stretch in Chicago with an exit and entry every street block is now derided as the worst possible way to design anything.

I've also said numerous times that the car is the worst modal choice for any journey under about 3 miles on every single metric beyond "personal convenience", and any sub-3 mile journey that takes you onto a motorway surely should raise an eyebrow or two as to what are we playing at designing for that - these are trips that walking, cycling, and public transport excel at and in these situations the focus should remain on those.

[jackal]

There are a couple of examples in France - you're probably thinking of when the A1 was widened in the 1990s here: https://maps.app.goo.gl/Uqgc5onncs1yBnmg8 (sortie 17.1 came much later than the widening).

Thanks, that's the one, or at least the kind of thing I had in mind. To me those aren't C/D lanes, though I guess Peter would say they are, given what he says about a hypothetical M20 J6 without J5.

DMRB (sigh) has made this more complicated than it needs to be by defining outer carriageways designed for weaving movements as "link roads*". On any new rural** motorway, junctions spaced less than 2km apart are supposed to have outer carriageways (I'm not calling them "link roads") for weaving but you can handwave this requirement away if there are environmental constraints. There is no compulsion to have them anywhere from what I can gather, and there doesn't seem to be any design guidance for joining up outer carriageways to create a pure C/D network - so this would be why we've barely had any put in because they have to be designed as if they were part of the mainline - so no reduced speed limits or anything that makes them part of an interchange complex. I also imagine there's nebulous arguments about "driver confusion", given very basic globally accepted and workable concepts seem to cause NH to have a meltdown when presented with them. It really is a document written by people who want to feel smarter than they seemingly are.

* "In the context of junctions, a link road is one-way connector road adjacent to but separate from the mainline carriageway carrying traffic in the same direction. It is used to connect the mainline carriageway to the local highway network where successive direct connections cannot be provided to an adequate standard because the junction spacing is too close."

Thanks for that. To be honest we do need a term for them, or they get called C/D lanes, which they really aren't. I guess "link roads" is as good as anything. (From what I recall the DMRB illustration shows the link roads going between but not through junctions, differentiating them from C/D lanes - though going through a junction is a necessary but not sufficient condition for C/D lanes, as the French A1 example shows.)

[A320Driver]

There is a rather curious single-direction C/D lane on the southbound M3 between J13 and J14.

[Peter Freeman]

There is a rather curious single-direction C/D lane on the southbound M3 between J13 and J14.

A rather curious design, but NOT a C/D.

It starts at J13 as a local sb on-ramp, but instead of merging as expected into M3 sb, it becomes absorbed into the large, triangular semi-directional-T (J14) that terminates M3 onto M27. The omission of merging into actual M3 is therefore of no consequence. Its first 2km performs a similar 'lane-exchange' function to Almondsbury J15/16, but here it is perfectly sensible with more than enough length.

What type of carriageway would this one be called? Because it's predominantly part of the s-d-T, it's a 'connector'.

I dislike, in general, complicated interchanges where the routes are bamboozling, even if only at first-sight. However, they often work, and GMaps confirms that this one causes no serious congestion. The only congestion spots in sight are at the M27J5 roundabout and the non-motorway side of Bassett Avenue roundabout (surprised?). We all know how to eliminate those queues don't we (not that it will happen, of course).

Another curiosity is the way M3 continues through the T and terminates at a weird double-roundabout at Bassett Avenue. Weird but not really a problem. I wonder that this point, the third branch of M3's death-split, is not marked as M3J15.

[Bryn666]

There is a rather curious single-direction C/D lane on the southbound M3 between J13 and J14.

A rather curious design, but NOT a C/D.

It starts at J13 as a local sb on-ramp, but instead of merging as expected into M3 sb, it becomes absorbed into the large, triangular semi-directional-T (J14) that terminates M3 onto M27. The omission of merging into actual M3 is therefore of no consequence. Its first 2km performs a similar 'lane-exchange' function to Almondsbury J15/16, but here it is perfectly sensible with more than enough length.

What type of carriageway would this one be called? Because it's predominantly part of the s-d-T, it's a 'connector'.

I dislike, in general, complicated interchanges where the routes are bamboozling, even if only at first-sight. However, they often work, and GMaps confirms that this one causes no serious congestion. The only congestion spots in sight are at the M27J5 roundabout and the non-motorway side of Bassett Avenue roundabout (surprised?). We all know how to eliminate those queues don't we (not that it will happen, of course).

Another curiosity is the way M3 continues through the T and terminates at a weird double-roundabout at Bassett Avenue. Weird but not really a problem. I wonder that this point, the third branch of M3's death-split, is not marked as M3J15.

The M3 used to be the A33 there, it was repurposed as a motorway hence the odd design elements.

[RichardA35]

There is a rather curious single-direction C/D lane on the southbound M3 between J13 and J14.

A rather curious design, but NOT a C/D.

It starts at J13 as a local sb on-ramp, but instead of merging as expected into M3 sb, it becomes absorbed into the large, triangular semi-directional-T (J14) that terminates M3 onto M27. The omission of merging into actual M3 is therefore of no consequence. Its first 2km performs a similar 'lane-exchange' function to Almondsbury J15/16, but here it is perfectly sensible with more than enough length.

What type of carriageway would this one be called? Because it's predominantly part of the s-d-T, it's a 'connector'.

I dislike, in general, complicated interchanges where the routes are bamboozling, even if only at first-sight. However, they often work, and GMaps confirms that this one causes no serious congestion. The only congestion spots in sight are at the M27J5 roundabout and the non-motorway side of Bassett Avenue roundabout (surprised?). We all know how to eliminate those queues don't we (not that it will happen, of course).

Another curiosity is the way M3 continues through the T and terminates at a weird double-roundabout at Bassett Avenue. Weird but not really a problem. I wonder that this point, the third branch of M3's death-split, is not marked as M3J15.

The M3 used to be the A33 there, it was repurposed as a motorway hence the odd design elements.

The particular arrangement also means that J13/Chandler's Ford traffic is unable to use the M3-M27 eastbound for a "3 mile motorway hop" to the airport...

[Bryn666]

A rather curious design, but NOT a C/D.

It starts at J13 as a local sb on-ramp, but instead of merging as expected into M3 sb, it becomes absorbed into the large, triangular semi-directional-T (J14) that terminates M3 onto M27. The omission of merging into actual M3 is therefore of no consequence. Its first 2km performs a similar 'lane-exchange' function to Almondsbury J15/16, but here it is perfectly sensible with more than enough length.

What type of carriageway would this one be called? Because it's predominantly part of the s-d-T, it's a 'connector'.

I dislike, in general, complicated interchanges where the routes are bamboozling, even if only at first-sight. However, they often work, and GMaps confirms that this one causes no serious congestion. The only congestion spots in sight are at the M27J5 roundabout and the non-motorway side of Bassett Avenue roundabout (surprised?). We all know how to eliminate those queues don't we (not that it will happen, of course).

Another curiosity is the way M3 continues through the T and terminates at a weird double-roundabout at Bassett Avenue. Weird but not really a problem. I wonder that this point, the third branch of M3's death-split, is not marked as M3J15.

The M3 used to be the A33 there, it was repurposed as a motorway hence the odd design elements.

The particular arrangement also means that J13/Chandler's Ford traffic is unable to use the M3-M27 eastbound for a "3 mile motorway hop" to the airport...

Almost all of the urban area there is within non-car alternative distance of the airport anyway; the motorway route is longer in terms of distance!

In most cases though I suspect few people are routinely living in Chandlers Ford and then flying from Southampton Airport - certainly not enough to warrant building a dedicated slip road for it.

We've discussed the problems of airport access and net-zero aspirations elsewhere, ironically in a thread about providing C/D roads on the Brussels ring road.

[Peter Freeman]

Another case of C/D lane failure is the 401 in Toronto, which has frequent very short weaving spaces due to all the unnecessary connections between the carriageways. They should have used more braiding and quasi-C/D lane long slip roads. Yes, I know the 401 largely fails due to its massive volumes, but that makes 500m weaving spaces even less acceptable IMO.

OK, just to clarify, I think you mean that 401 'fails' in its traffic performance, whereas most of our preceding comments concerned failure to qualify as true C/D roads. There is of course no doubt that it extensively employs C/D roads! Also worth noting that amongst all those C/Ds, it also has multiple long simple ramps.

Traffic performance - well, 401 certainly shows up as having massive congestion. However, I know some roads, impossible to widen, in my city that consistently have long GMaps red streaks too. We simply have to tolerate some regular stretches of known slow-moving peak traffic. I wouldn't necessarily consider them all to be road failures. 401: maybe.

Regarding the rollovers: the one in the west (linked to by WHBM, and which is only on the north side, and which is more than just a rollover) has serious congestion. Another one, farther east (which is a pure rollover, and occurs on the S and N sides), works better. Yes, there are short weaving lengths on the outer carriageways, but that's somewhat inevitable: that's why the express lanes are required.

Your analysis that transfers between express and C/D are too close together is probably correct, but that's always a fine call. Too far apart means insufficient users are able to exploit the express lanes. Too close together puts you back where you started, with weaving queues forming on the express carriageway - exactly where they are intended to NOT be.

[Peter Freeman]

There are a couple of examples in France - you're probably thinking of when the A1 was widened in the 1990s here: https://maps.app.goo.gl/Uqgc5onncs1yBnmg8 (sortie 17.1 came much later than the widening).

Thanks, that's the one, or at least the kind of thing I had in mind. To me those aren't C/D lanes, though I guess Peter would say they are, given what he says about a hypothetical M20 J6 without J5.

Of course that's a C/D road! By staying on the express lanes (keep left), you don't have to deal with a diverge and a merge associated with sortie 17.1. That's precisely what C/Ds are for - to stretch out the distance between junctions. It removes diverge/merge interactions from the mainline.

There are two unusual points about this French one -
1. It bypasses only one junction (same as M60 and M20).
2. It is only southbound, not both directions (there is a UK example of this too).

Neither of these points has anything to do with its qualification as a C/D.

[Bryn666]

There are a couple of examples in France - you're probably thinking of when the A1 was widened in the 1990s here: https://maps.app.goo.gl/Uqgc5onncs1yBnmg8 (sortie 17.1 came much later than the widening).

Thanks, that's the one, or at least the kind of thing I had in mind. To me those aren't C/D lanes, though I guess Peter would say they are, given what he says about a hypothetical M20 J6 without J5.

Of course that's a C/D road! By staying on the express lanes (keep left), you don't have to deal with a diverge and a merge associated with sortie 17.1. That's precisely what C/Ds are for - to stretch out the distance between junctions. It removes diverge/merge interactions from the mainline.

There are two unusual points about this French one -
1. It bypasses only one junction (same as M60 and M20).
2. It is only southbound, not both directions (there is a UK example of this too).

Neither of these points has anything to do with its qualification as a C/D.

The reality here is they didn't want to replace an existing D2 truss bridge so split the new southbound D3 over it and then built a new junction later. It's an accidental arrangement at best and not really a C/D set up.

[Peter Freeman]

^ It doesn't matter what the history is or the causation was. What is there is there, and it's a textbook C/D. It collects traffic from, distributes traffic to, exit 17.1. There's no "not really" about it.

[Micro The Maniac]

Not much of a hard shoulder to pinch ! The reinforced barrier seems to tell its own story.

https://www.google.com/maps/@51.4942701 ... ?entry=ttu

Pretty sure (but I could be wrong, again) that those barriers are the remnants of the Great Heck panic, when something must be done... the vegetation growth into the hard-shoulder being a sad indictment of maintenance!

[Bryn666]

^ It doesn't matter what the history is or the causation was. What is there is there, and it's a textbook C/D. It collects traffic from, distributes traffic to, exit 17.1. There's no "not really" about it.

By that logic, this is also a C/D lane - https://maps.app.goo.gl/aY5JXY8GMrk6bbAj9

We're not going to agree on this one I don't think

[Bryn666]

Not much of a hard shoulder to pinch ! The reinforced barrier seems to tell its own story.

https://www.google.com/maps/@51.4942701 ... ?entry=ttu

Pretty sure (but I could be wrong, again) that those barriers are the remnants of the Great Heck panic, when something must be done... the vegetation growth into the hard-shoulder being a sad indictment of maintenance!

It does look like railway incursion mitigation rather than protecting the stack itself.

[Peter Freeman]

^ It doesn't matter what the history is or the causation was. What is there is there, and it's a textbook C/D. It collects traffic from, and distributes traffic to, exit 17.1. There's no "not really" about it.

By that logic, this is also a C/D lane - https://maps.app.goo.gl/aY5JXY8GMrk6bbAj9

We're not going to agree on this one I don't think

Of course that's NOT a C/D road/lane!

The stretch that you're talking about, I presume, is this -
https://www.google.com/maps/@51.5155599 ... ?entry=ttu

It begins (if going eastwards) here -
Start (western end) -
https://www.google.com/maps/@51.5149429 ... ?entry=ttu
(Your link shows the same point, in streetview form).

West of this point, A13 Newham Way is plain D3 (3,3), but at this point it splits, becoming 2,1,1,2. This runs eastwards for 500m, passing no entrances or exits. Now the carriageway dividers end, and it once again becomes D3. This is where it ends -
End (eastern end) -
https://www.google.com/maps/@51.5161735 ... ?entry=ttu

Of course that's NOT a C/D road! A C/D road has exit(s), that are bypassed by the express carriageway.

I don't know why the road splits and re-joins like that, but I'm sure some wise Sabristo has a structural or historical explanation.

[WHBM]

Pretty sure (but I could be wrong, again) that those barriers are the remnants of the Great Heck panic, when something must be done... the vegetation growth into the hard-shoulder being a sad indictment of maintenance!

It does look like railway incursion mitigation rather than protecting the stack itself.

Not particularly; there's no such mitigation of the railway over on the opposite carriageway, just the Network Rail notice on the fencing

https://www.google.com/maps/@51.4960159 ... ?entry=ttu

It's a goods train only line where the odd train seems to max out at about 15mph

[Peter Freeman]

Does Clifton Bridge in Nottingham qualify as a very brief express lane? ...

Surprisingly, I think it does, if viewed from the perspective of travelling northbound on the ring road (A52 Clifton Boulevard).

There's a single-lane carriageway that runs northwards on the eastern bridge, contra to four lanes of southbound traffic. This single lane is acting as an express lane for traffic wishing to stay on the ring road. It avoids having to engage with traffic (a) merging in from A453 Clifton Lane, and (b) traffic diverging left to Queens Drive, and (c) traffic diverging left to Thane Road. It re-joins the other northbound traffic (derived from A52 nb and A453 inbound) just before the flyover above Queens Lane roundabout. Use of this express carriageway is signed for A52, but is optional (optional is always the case for express accompanying C/Ds).

It's very unusual - single lane, and length approx 600m. Northbound only: no equivalent southbound. Not part of a motorway.
It begins here -
https://www.google.com.au/maps/place/Cl ... ?entry=ttu
It ends here -
https://www.google.com.au/maps/place/Cl ... ?entry=ttu

Since it is an express lane, free of junctions, the parallel 3-lane northbound carriageway on the western bridge, with junctions, must be, effectively, its matching C/D road. Express and C/D are two sides of the same coin.

[Darren]

The A299 Thanet Way heading westbound is another "close, but not quite" example.

Taking the exit signposted for Herne Bay, you pass not one, not two, but three junctions - all of which can't be reached from the mainline.

The sliproad starts here:

https://www.google.co.uk/maps/@51.36045 ... ?entry=ttu

...and almost immediately there's a "map style" sign for the first junction:

https://maps.app.goo.gl/mLR96WoVNP8imAV9A

The next junction is just signed as a minor one, and after that is a business park junction:

https://maps.app.goo.gl/nKB331fNxMTst1qU6

All the while, the road (2 lanes) runs parallel to the A299 mainline. It then finally reaches the Herne Bay junction, which is in the form of a roundabout. You can then go straight over the roundabout to rejoin the mainline of the A299.

So - although it's close to a C/D lane (and would be, if that roundabout wasn't there), it's instead a long, wide sliproad with several junctions - still pretty unusual, I'd have thought.