Collector/Distributor Roads and Express Lanes

[Peter Freeman]

A discussion in another thread prompts me to start this new topic about C/D roads. First I quote a post from that previous thread. It's in the form of an FYI -

Collector Lanes are the opposite of Express Lanes on a North American freeway with four carriageways. The latter, in the centre, can go for miles without a junction. The Collectors on the outside have frequent on/off ramps to major streets.

A Transfer is where there is the ability to cross between one and the other, and a Rollover is where this is provided for all exchanging movements in both directions. I suppose the nearest UK equivalent to the latter is M4 J2. Here's one on the 401 in Toronto :

https://www.google.com/maps/place/Toron ... ?entry=ttu

(my bold above)

The overlapping ramps at M4J2 are an impressive sight, especially for their time. Unfortunately, as you know, they're not a real transfer or rollover.

There is a pseudo-C/D road there though. It begins, on the A4 westwards, at the off-ramps leading down to Chiswick Roundabout, while Chiswick Flyover continues as Express Lanes. West of the roundabout, at ground level, A4 passes, and serves, some local roads (the final one being Lionel Road) before optionally ascending the ramp that takes it up to re-join the M4 express lanes. That's the end of the pseudo-C/D. A similar configuration occurs eastbound.
C/D start -
https://www.google.com.au/maps/place/Gr ... ?entry=ttu
C/D end -
https://www.google.com.au/maps/place/28 ... ?entry=ttu

That short piece of A4 almost qualifies as a C/D road because, westbound, it serves Chiswick Roundabout (A205/A406), followed by minor local roads and the A4 underneath the M4, and finally re-joins M4. Whether it's really a C/D road is doubtful though. Diverting onto a C/D road and using that instead of the express lanes should allow you to traverse its whole length uninterrupted by at-grade intersections in which you have no interest - and then rejoin.

The downwards wb off-ramp that spectacularly accompanies the upwards on-ramp is an exit only, to A4. It's not part of a rollover, but it is a braid. A4 wb then continues at ground level, serving Clayponds Lane, Ealing Road and Windmill Road before departing from the M4 line that it has shadowed up to that point. But it's definitely not a C/D there either, since it doesn't re-join.

Parallel cariageways that superficially resemble C/D roads are sometimes simply long ramps, often to facilitate a braid. They must rejoin express lanes, or offer the option to rejoin, in order to qualify as C/Ds.

C/Ds are common in the USA, but rare in Australia. Can anyone say where there are others in the UK?

[Bryn666]

We've never really gone for C/D roads in the UK - the M60 J6-8 is fairly unique in that segregated carriageways have their own junction midway along. Other examples are just segregated carriageways to split out weaving movements from the mainline - e.g. M20 J5-6, Almondsbury's inadequate weaving lengths, and M25 J30-31.

The M4 at Chiswick is just akin to the Westway where a motorway is stacked on top of an A road and slip roads connect the two, there's no specific desire to split movements out, it's just multiple routes sharing a corridor. Ingenious use of space, great engineering, but not really designed to assist the motorway itself.

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 whilst lying to ourselves that they are for strategic national traffic.

[jackal]

As Bryn says, the M60 J6-8 is the only true C/D lane example in the UK, with (1) a junction in the middle with express and collector lanes passing through it, and (2) a connection to the mainline at both ends.

The nearest miss is M20 J5-6, which doesn't quite meet (1) as J6 is at the end of the multi-carriageway section, so therefore can be considered to have slip roads for J5 passing through rather than collector lanes. My thought is basically that a true collector has traffic going to/from more than one exit/entry.

Also notable is the M8 at Plantation, which doesn't meet (2) as the outercarriageway doesn't go back to the mainline at the eastern end. (See also the plans for the A2/M2/LTC interface and M25 J15-16.) M8 J6 doesn't really meet (1) or (2) though it is still a little C/D like, with the four carriageways through the middle (two are really long slips).

The M4 at Chiswick is just braiding and doesn't belong in a C/D lane discussion IMO.

Likewise, M25 J30-31 doesn't have any additional grade-separated carriageway going through any junction so doesn't really have anything to do with C/D lanes in my view. It just has some additional slip roads connecting into at-grade junctions (there is a DMRB term that I don't recall right now). This is fairly common, even in quite innocuous places like the A24 at Horsham, where there is obviously no requirement for true C/D lanes.

The final thing I'd add is that I don't really see C/D lanes as the gold standard they're sometimes held up as, as the additional connections to the mainline are inimical to traffic flow, creating extra weaving zones, decision points and merges, compared to the "long sliproad" design that only connects at one end like the M8 at Plantation or the M25 J15-16 plan (which is parked for now). In fact the A2/M2/LTC had a particularly inefficient C/D design in its previous iteration, where the connection to the mainline at the eastern end (A2/M2/A289) was completely redundant, as all movements it covered were already provided at the western end (A2/LTC). Someone may have pointed this out, and the eastern end connection with the mainline was removed in the current version, which is a pure "long sliproad" design.

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.

[B1040]

Does Clifton Bridge in Nottingham qualify as a very brief express lane?
Does the use of a parallel road (A1(M) in Cambridgeshire and Yorkshire or A /M74) count as an alternative to C/D lanes?

[wrinkly]

In addition to the M60 J6-8 example, there's a sort of reduced CD road on the M60 southbound between junctions 12 and 11.

What about the M8/A8 between Baillieston and Newhouse? Surely there are some examples there?

[jackal]

In addition to the M60 J6-8 example, there's a sort of reduced CD road on the M60 southbound between junctions 12 and 11.

What about the M8/A8 between Baillieston and Newhouse? Surely there are some examples there?

I mentioned M8 J6 above.

M60 J11-12 is just the M25 J30-31-style link between adjacent junctions (though with the quirk of being southbound only). There must be 20+ examples in the UK. It has nothing much to do with C/D lanes, and is probably only referred to as such on Sabre because the UK has so few true C/D lane sections (I would say just one).

[WHBM]

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

My own experience on the M25 is over time I have joined and exited at every junction, in both directions (I've just checked !). But very rarely a single on-off motorway hop. There are a few where joining at one I do a motorway-to-motorway divergence at the next, probably the most frequent being from the BA Terminal 5 at Heathrow, onto the M25 then off again onto the M4. But that's all part of the design logic of accessing the overall network, and part of a long distance journey for many. Now there are lots of others joining and leaving at the same points, and sometimes congestion. But I can't see why the majority are doing anything different to me.

Incidentally, there is often congestion on the through clockwise M25 lanes over to the right just in that short shared stretch I do, but that is nothing to do with junction weaving, it is the reduction of the main carriageway at that point from 6 lanes to 3, wholly due to the multiple bridge piers of the stack junction J15 preventing any widening - without this a 4-lane carriageway would be a given.

Regarding other Quad carriageways, some may recall from past posts I worked (in a very junior capacity) on the Quad scheme on the M8 in Glasgow, west of the Kingston bridge. Original outline thought was for dual 5-lane carriageways, with weaving, between the South Flank junction (the third side of the triangle with the notorious West Street "ski jumps", never built) and the divergence of the M77 Ayr Motorway. Modelling showed dual 5-lanes was insufficient, although weaving would surely have been the key issue, and the current braided Quad solution was the result. It later helped the more recent M74 south side extension to tie into this.

[RichardA35]

Incidentally, there is often congestion on the through clockwise M25 lanes over to the right just in that short shared stretch I do, but that is nothing to do with junction weaving, it is the reduction of the main carriageway at that point from 6 lanes to 3, wholly due to the multiple bridge piers of the stack junction J15 preventing any widening - without this a 4-lane carriageway would be a given.

Plenty of room to fit traffic 4 lanes through J15 - just use the H/S and possibly narrow lanes - but how you would merge and diverge north and south of the junction would be interesting!

[Gaz909909]

Hard shoulder is heavily curtailed under the bridge. Not sure D4 is possible as is.

[WHBM]

Plenty of room to fit traffic 4 lanes through J15 - just use the H/S and possibly narrow lanes - but how you would merge and diverge north and south of the junction would be interesting!

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

[RichardA35]

Plenty of room to fit traffic 4 lanes through J15 - just use the H/S and possibly narrow lanes - but how you would merge and diverge north and south of the junction would be interesting!

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

Hard shoulder is heavily curtailed under the bridge. Not sure D4 is possible as is.

The space exists between the parapet and the centre piers of the overbridges. Yes there would need to be work in the centre and the merge & diverge layouts would need serious work and compromises. It appears that a choice has been made to limit the traffic on the structure and barrier off some of the hard shoulder. There may be an underlying structural problem that would make the engineering a little more challenging than usual but not greatly more than on other structures around the network.

[Owain]

It's not a UK example, but the SS131 in Sardinia has a long stretch of D4 between Sestu and Monastir. It's essentially a HQDC with a distributor carriageway to either side for several kilometres.

[Peter Freeman]

Bryn and Jackal: yeah, I was just trying to be nice to Chiswick 'cos it looks so good . Absolutely, it's not a C/D road

As Bryn says, the M60 J6-8 is the only true C/D lane example in the UK, with (1) a junction in the middle with express and collector lanes passing through it, and (2) a connection to the mainline at both ends.

Yes, this one is definitely C/D, since J7 is accessible only from the outside carriageways. One or more junctions being accessible only from the outer carriageways is what defines a C/D.

The total J6-J8 config is interesting. The start and end of the C/Ds are entangled with junctions 6 and 8. These each have two ramps that begin on the main, inner cariageways, and two ramps that merge with the outer, C/D carriageways. This arrangement does help though - you can, almost, avoid getting mixed up with not only J7, but also J6 and J8, by staying right, away from all diverges. And, of course, it helps keep more bad cases of weaving away from the mainline.

The nearest miss is M20 J5-6, which doesn't quite meet (1) as J6 is at the end of the multi-carriageway section, so therefore can be considered to have slip roads for J5 passing through rather than collector lanes. My thought is basically that a true collector has traffic going to/from more than one exit/entry.

I believe this one does qualify: J5 is entangled with the C/D's western merge/diverge (G.Maps even placing the number 6 inside J5's roundabout!). However, J6 is accessible only from the outer carriageways, as it is not quite at the eastern end of the C/Ds. The outer carriageways are true C/Ds at that point, and the J6 slips diverge from / join with them. Very clear wb, less clear eb, because it's a closer call.

I think this is the issue Jackal: you are seeing the wb C/D road branching right off the J6 off-ramp, but I see the J6 off-ramp branching left off the just-entered wb C/D road.

The westbound signage agrees -
https://www.google.com.au/maps/@51.2947 ... &entry=ttu

As with M60 J6-8, this M20 one completely skips only one junction. Still, it's a true C/D.

[jackal]

Bryn and Jackal: yeah, I was just trying to be nice to Chiswick 'cos it looks so good . Absolutely, it's not a C/D road

As Bryn says, the M60 J6-8 is the only true C/D lane example in the UK, with (1) a junction in the middle with express and collector lanes passing through it, and (2) a connection to the mainline at both ends.

Yes, this one is definitely C/D, since J7 is accessible only from the outside carriageways. One or more junctions being accessible only from the outer carriageways is what defines a C/D.

The total J6-J8 config is interesting. The start and end of the C/Ds are entangled with junctions 6 and 8. These each have two ramps that begin on the main, inner cariageways, and two ramps that merge with the outer, C/D carriageways. This arrangement does help though - you can, almost, avoid getting mixed up with not only J7, but also J6 and J8, by staying right, away from all diverges. And, of course, it helps keep more bad cases of weaving away from the mainline.

The nearest miss is M20 J5-6, which doesn't quite meet (1) as J6 is at the end of the multi-carriageway section, so therefore can be considered to have slip roads for J5 passing through rather than collector lanes. My thought is basically that a true collector has traffic going to/from more than one exit/entry.

I believe this one does qualify: J5 is entangled with the C/D's western merge/diverge (G.Maps even placing the number 6 inside J5's roundabout!). However, J6 is accessible only from the outer carriageways, as it is not quite at the eastern end of the C/Ds. The outer carriageways are true C/Ds at that point, and the J6 slips diverge from / join with them. Very clear wb, less clear eb, because it's a closer call.

I think this is the issue Jackal: you are seeing the wb C/D road branching right off the J6 off-ramp, but I see the J6 off-ramp branching left off the just-entered wb C/D road.

The westbound signage agrees -
https://www.google.com.au/maps/@51.2947 ... &entry=ttu

As with M60 J6-8, this M20 one completely skips only one junction. Still, it's a true C/D.

I'm not thinking anything about which side of the road a diverge happens (and haven't even looked at it at that level of detail). I will just repeat my main point:

My thought is basically that a true collector has traffic going to/from more than one exit/entry.

There is no potential collector at all at J5 (i.e. 2 carriageways not 4 through the middle of the junction). J6 does have inner and outercarriageways going through the middle, but all traffic on the outercarriageways is to/from J5, so they are really just long slips from J5 rather than collectors.

I acknowledge that the quoted condition (a more explicit statement of part of (1) above) could be relaxed, with the M20 example then included. Certainly it's reasonable to think of M20 J5-J6 as C/D lanes, and I've said the same myself previously. I suppose I am just pointing out there's one respect in which it is more long slip roads, which does not apply to the M60 example, a purer example of C/Ds.

[Peter Freeman]

^ First, let's look at the definition. You say -

My thought is basically that a true collector has traffic going to/from more than one exit/entry.

I don't think that's worded very clearly. But anyway, why 'more than one' ?

I prefer -

One or more junctions being accessible only from the outer carriageways is what defines a C/D.

I'd add -

It must re-join the same carriageway that it left.

In the case of M20, just imagine that J5 is not there. No roundabout, no ramps, but all else still the same. The next junction is J4, quite far away. That leaves J6 in isolation, no other junctions involved.
J6's on and off ramps, in both directions, join to the outer pair of carriageways (C/D), while the inner pair runs through with no contact with J6's ramps (ie. they're express). Take a careful look.

Now imagine J5's back - reality. No change to the above statement:
J6's on and off ramps, in both directions, join to the outer pair of carriageways (C/D), while the inner pair runs through with no contact with J6's ramps (ie. they're express).

I'm going to put it yet another way. You say -

J6 does have inner and outercarriageways going through the middle, but all traffic on the outercarriageways is to/from J5, so they are really just long slips from J5 rather than collectors.

No, not all traffic. The outer carriageways at J6 go westwards towards J5, but wb traffic can then go left down J5's ramps, or go right to rejoin the inner carriageways. C/D ends here. This occurs before reaching J5. Really, J5 has nothing to do with the C/D.

This is where the C/D road ends, wb -
https://www.google.com.au/maps/@51.2957 ... &entry=ttu

The M20 situation is the same as M60, except that the entanglement with another, irrelevant junction occurs at only one end of the C/D, instead of both. In both cases, one complete junction is skipped by the express lanes. And you accept that M60 qualifies.

QED

[jackal]

^ Why more than one junction? Because if it connects to only one it's just a long slip road for that junction.

M20 wb outercarriageway traffic can get back to mainline as well as access J5: sure, but that's just duplication of innercarriageway movement, not access to a second junction.

Case with no J5: this would be similar to a case in France that someone (Bryn?) posted, where the road arbitrarily gained and lost an extra carriageway, I think so an existing bridge could be used as part of a widening project. I think there may have been a junction on that section but can't find it right now. Anyway, doesn't seem like a full C/D lane set up to me, as the outercarriageways in the middle of the junction just duplicate the innercarriageway movement. Distributor lanes distribute to junctions: no distribution, no distributor.

[Bryn666]

^ Why more than one junction? Because if it connects to only one it's just a long slip road for that junction.

M20 wb outercarriageway traffic can get back to mainline as well as access J5: sure, but that's just duplication of innercarriageway movement, not access to a second junction.

Case with no J5: this would be similar to a case in France that someone (Bryn?) posted, where the road arbitrarily gained and lost an extra carriageway, I think so an existing bridge could be used as part of a widening project. I think there may have been a junction on that section but can't find it right now. Anyway, doesn't seem like a full C/D lane set up to me, as the outercarriageways in the middle of the junction just duplicate the innercarriageway movement. Distributor lanes distribute to junctions: no distribution, no distributor.

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).

A variant on the theme is the new A9/A709 set-up in Montpellier: https://maps.app.goo.gl/phu8mQvvst6cLJH79
Here the A709 starts as outer-carriageways alongside the A9 (passing a junction in the process), before it branches off and resumes the original alignment of the 1970s A9 - the new A9 is now continuously tolled bypassing the city - and then at the other end it passes another junction having resumed as outer-carriageways.

I still do not consider the M20 to be pure C/D lanes - if they'd extended to J7 before rejoining the mainline they definitely would be.

Incidentally, the arrangements on the A4 around Rotterdam's stack interchange with the A12 is precisely how we should've done Almondsbury. Some good segregation of flows and braiding there!

[Owain]

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).

A variant on the theme is the new A9/A709 set-up in Montpellier: https://maps.app.goo.gl/phu8mQvvst6cLJH79
Here the A709 starts as outer-carriageways alongside the A9 (passing a junction in the process), before it branches off and resumes the original alignment of the 1970s A9 - the new A9 is now continuously tolled bypassing the city - and then at the other end it passes another junction having resumed as outer-carriageways.

A1/A3 north of Paris?

[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).

A variant on the theme is the new A9/A709 set-up in Montpellier: https://maps.app.goo.gl/phu8mQvvst6cLJH79
Here the A709 starts as outer-carriageways alongside the A9 (passing a junction in the process), before it branches off and resumes the original alignment of the 1970s A9 - the new A9 is now continuously tolled bypassing the city - and then at the other end it passes another junction having resumed as outer-carriageways.

A1/A3 north of Paris?

That's one of those truly Parisian hard to quite nail down examples like the A6a/A6b tangle north of Orly. The French do love slip road spaghetti.

[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.