Collector/Distributor Roads and Express Lanes

[jnty]

The other factor, at least in my experience, is that once you get above three lanes, a lot of drivers haven't the faintest idea which lane they should be in. Which, outside of peaks where every lane is full, serves to reduce the real-life capacity of the road. I drove along the M4 into London recently and for large distances the inner 2 lanes were virtually empty. I find that is common on 4 lane sections. That's an education problem, but no one wants to tackle it.

Poor driving behaviour - annoying, but is the issue overstated? Was it really so bad? Did it actually cause a problem for you (ie. affect your journey-time)? Was the general flow speed much below the limit? When I'm in the UK, it seems to me that most motorway right lane occupiers are over 70mph.

That frustrating behaviour is not reducing 'the real life capacity of the road'. Extra capacity is there after widening from 3 to 4 or 5 lanes, but it's not being used efficiently off-peak. It does get used efficiently on-peak, and that period is the reason for lanes being added.

'Right-laning' happens in Australia too, despite occasional signs advising "keep left except when overtaking". It's not specifically illegal, but I think the police could charge you under some convenient statute if they wished. Of course, they don't. However, we legally overtake on the left, if necessary (though the police could charge you for that too, if done unsafely). I'm not suggesting the UK should change. You just need to get used to the left lanes being used inefficiently off-peak. Consider them to be on hot standby, and conveniently available for joiners and leavers!

Yes, I think we'd like to believe this is all simply caused by 'bad behaviour' which can be eliminated with enforcement and training, but that's surely a myth. Enforcement against 'middle-lane hogging' on quiet roads may give drivers some satisfaction but, even if eliminated, would do almost nothing for capacity, and conclusively proving 'lane-hogging' in any remotely congested situation must be nearly impossible - advice in serious congestion is usually to 'stay in lane' anyway and, as you say, lane utilisation quickly becomes very good in these situations.

The situation on busy-ish roads where lanes 3-4 become congested with 1-2 quite sparse are frustrating, but it's hard to pin the blame on any particular driver. Lane 4 drivers will feel they are overtaking lane 3; lane 3 drivers probably see a slower vehicle ahead in lane 2, and there will probably be at least one lorry visible in lane 1. To be honest, anyone sitting behind or adjacent to a vehicle travelling slower than they would like to be will feel like they are justified in staying in their lane, and it's hard to argue with that, especially given that "undertaking is illegal." What's the alternative - pull into lane 1, sit behind a truck, and surrender your own rate of progress as a sacrifice to the capacity gods?

I find this can just as easily happen away from junctions as near them, so it may be an irrelevant consideration where C/D lanes are concerned. But it would probably be beneficial to prevent the most "ambitious" local drivers from being tempted to weave straight from lane 1 to lane 4 (or 5, or 6...) then back again when this situation inevitably manifests itself in junction-heavy sections.

[Chris5156]

I'll see that and raise you the truly remarkable Sopraelevata Aldo Moro through the heart of Genoa.

I don't think I realised how unbelievably abrupt some of the junctions are on that road. Ah, Italy!

[Bryn666]

The newly widened A10 in France between J25 and the A85 features more of the D2 converted to 2+2 with a new D3 alongside - this must now be a French standard having been done on the A1 and A13 as well. It is interesting that the outside two lanes always get a weight limit.

[Peter Freeman]

The newly widened A10 in France between J25 and the A85 features more of the D2 converted to 2+2 with a new D3 alongside - this must now be a French standard having been done on the A1 and A13 as well. It is interesting that the outside two lanes always get a weight limit.

Link please.

[Chris5156]

The newly widened A10 in France between J25 and the A85 features more of the D2 converted to 2+2 with a new D3 alongside - this must now be a French standard having been done on the A1 and A13 as well. It is interesting that the outside two lanes always get a weight limit.

Link please.

Seems to only happen around significant underbridges, e.g. here.

[jackal]

The newly widened A10 in France between J25 and the A85 features more of the D2 converted to 2+2 with a new D3 alongside - this must now be a French standard having been done on the A1 and A13 as well. It is interesting that the outside two lanes always get a weight limit.

Link please.

Seems to only happen around significant underbridges, e.g. here.

The modified cloverleaf to the north is pretty mad. Yes, it removes two weaving spaces, but does so by taking the slip road to the loop over three bridges - more bridges than the equivalent slip in a conventional 3 loop interchange (e.g.: https://www.google.co.uk/maps/@51.8553756,5.2294237,15z), which also has a much better slip road alignment. I assume it's some kind of ad hoc upgrade and/or toll-related complexity.

[Chris5156]

The modified cloverleaf to the north is pretty mad. Yes, it removes two weaving spaces, but does so by taking the slip road to the loop over three bridges - more bridges than the equivalent slip in a conventional 3 loop interchange (e.g.: https://www.google.co.uk/maps/@51.8553756,5.2294237,15z), which also has a much better slip road alignment. I assume it's some kind of ad hoc upgrade and/or toll-related complexity.

Yes, it's nuts. It seems to have been done to put a toll plaza on the sliproads linking the north and west arms of the junction - which presumably was not there originally and had to be retrofitted economically. The footprint of the junction suggests it used to be a traditional, symmetrical cloverleaf. But even without that provocation, French highway engineers are not averse to a long, meandering sliproad.

[Owain]

The newly widened A10 in France between J25 and the A85 features more of the D2 converted to 2+2 with a new D3 alongside - this must now be a French standard having been done on the A1 and A13 as well. It is interesting that the outside two lanes always get a weight limit.

Link please.

Seems to only happen around significant underbridges, e.g. here.

Right then, I'm going to nominate the A1 between Bologna and Florence as a cheat!

I've driven the 'Panoramica' because I was on holiday and wanted to see the views. I haven't been on the 'Direttissima' because you don't get to see much when you're under the ground!

Obviously, I know that the latter is a bypass of the former, but the 'Panoramica' could technically count as a 'collector/distributor' road because it has full access to two junctions not accessible from the 'Direttissima'; it also retains its own service areas.

It was very quiet when I drove it!

[Peter Freeman]

Let's formulate a C/D definition, in order to agree on what constitutes a C/D and what doesn't. My original version was -

Definition 1.
A C/D Carriageway diverges left, and then runs left of and parallel to the original carriageway, which is then known as the Express Carriageway, Inner Carriageway or Mainline. From the C/D Carriageway only, one or more grade-separated junctions are accessible, whose merges and diverges therefore do not contact the mainline. A C/D Carriageway must eventually merge back into the Mainline. A C/D Carriageway may exist in only one direction of the motorway, but more usually exists in both directions, forming a pair of Outer Carriageways. 'Merge' and 'diverge' here may be taken to include 'lane-add' and 'lane-drop'.

In disputing certain candidate C/Ds, Jackal stated "My thought is basically that a true collector has traffic going to/from more than one exit/entry". Therefore, I think he would agree with my definition above if "one or more" was changed to "two or more" -

Definition 2.
A C/D Carriageway diverges left, and then runs left of and parallel to the original carriageway, which is then known as the Express Carriageway, Inner Carriageway or Mainline. From the C/D Carriageway only, two or more grade-separated junctions are accessible, whose merges and diverges therefore do not contact the mainline. A C/D Carriageway must eventually merge back into the Mainline. A C/D Carriageway may exist in only one direction of the motorway, but more usually exists in both directions, forming a pair of Outer Carriageways. 'Merge' and 'diverge' here may be taken to include 'lane-add' and 'lane-drop'.

I'm almost amenable to that change, because a C/D that simply serves only one junction still leaves the mainline with one diverge and one merge (per direction) - no benefit. A C/D that serves two junctions reduces the mainline's junction burden to two ramps instead of four - that's the benefit. Let's test this against our real-life disputed examples.

1. The French one (for whatever reason it was created), and any other single-exit bypass, provides no benefit, as the mainline still has two ramps. It meets Def1 but not Def2.

2. M20 J5/6: Without extra carriageways, 2 successive junctions would (in each direction) burden the mainline with 2 diverges, 2 merges, and a short-weave problem. With the extra carriageways, 1 junction (J6) is fully bypassed by the mainline, leaving the mainline with reduced weaving, only 1 diverge, but still 2 merges. Diverge reduction by 1 is a minor flow improvement. It meets Def1 but not Def2.

3. M60 J6-7-8: Without extra carriageways, 3 successive junctions would burden the mainline with 3 diverges, 3 merges, and a short-weave problem. With the extra carriageways, 1 junction (J7) is fully bypassed by the mainline, leaving the mainline with reduced weaving, 2 diverges and 2 merges. Diverge reduction by 1 and merge reduction by 1 is a medium flow improvement (elimination of a too-short weave). It meets Def1 but not Def2.

I'm sure you can see the pattern emerging here - do the arithmetic for 2 J's bypassed, 3 J's bypassed, and so on. Obviously, bypassing more junctions multiplies the flow benefits. More subtle though, is that the entanglement of half a junction at a C/D start or end provides a further half-benefit. This is why, although I could accept the more restrictive Def2, both the M20 and M60 examples show their worth is greater than simply bypassing one junction. C/Ds are intended to reduce mainline weaving, and both of these examples do so. If you won't call them C/Ds, what will you call them?

M20 and M60 are in the same boat - either both qualify, or both don't (in which case we have discovered no C/Ds in the UK).

Here's a definition-based solution to this quandary -

Definition 3.
A C/D Carriageway diverges left, and then runs left of and parallel to the original carriageway, which is then known as the Express Carriageway, Inner Carriageway or Mainline. From the C/D Carriageway only, two or more grade-separated junctions are accessible, whose merges and diverges therefore do not contact the mainline. Certain variations, where a junction is not fully isolated from the mainline but where a net reduction of mainline weaving results and the C/D Carriageway remains continuous, will homologate a C/D. A C/D Carriageway must eventually merge back into the Mainline. A C/D Carriageway may exist in only one direction of the motorway, but more usually exists in both directions, forming a pair of Outer Carriageways. 'Merge' and 'diverge' here may be taken to include 'lane-add' and 'lane-drop'.

The above definition lets M20 and J60 in, but none of the pseudo examples. This is my preference.

An alternative solution would be to stick with Def2, and allow certain partially compliant examples that do reduce weaving to be labelled 'quasi-C/D'. This category would include M20 and M60, but none that is continuous but obstructed, eg. the A299 example that goes through a roundabout before re-joining. It, and some other near-misses, could be labelled 'pseudo-C/D'. (quasi = partial, pseudo = fake).

[Peter Freeman]

Yes, it's nuts. It seems to have been done to put a toll plaza on the sliproads linking the north and west arms of the junction - which presumably was not there originally and had to be retrofitted economically.

Yes, definitely to collect a toll on a new turn (N/W turn previously missing in both directions, ie. no 4th loop). The new eb-to-nb toll plaza would, just, have fitted in the SE quadrant on a conventional link such as in Jackal's counter-example. It would have saved one bridge, but it would have (a) been on a curve, and (b) separated the twin toll plazas, which would be an operational nuisance.

We discussed an unusual major motorway crossing in Italy some time ago that had been done in a sub-optimal ('nuts'!) way so that a toll plaza could be fitted (or, IIRC, so that an existing one would not be bypassed).

It's reminiscent of a USA motorway crossing constructed as a double-ended trumpet, apparently as a nice place to locate a toll plaza. Presumably a better design would have been used if not for the tolling requirement. The one I know well is in Orlando, but I think there are others.

[Peter Freeman]

... the 'Panoramica' could technically count as a 'collector/distributor' road because it has full access to two junctions not accessible from the 'Direttissima'; it also retains its own service areas.

M6Toll too ?!

[Chris5156]

It's reminiscent of a USA motorway crossing constructed as a double-ended trumpet, apparently as a nice place to locate a toll plaza. Presumably a better design would have been used if not for the tolling requirement. The one I know well is in Orlando, but I think there are others.

France is bristling with double-trumpets that incorporate toll plazas too. I see the advantages but they’re very inelegant.

[Bryn666]

The modified cloverleaf to the north is pretty mad. Yes, it removes two weaving spaces, but does so by taking the slip road to the loop over three bridges - more bridges than the equivalent slip in a conventional 3 loop interchange (e.g.: https://www.google.co.uk/maps/@51.8553756,5.2294237,15z), which also has a much better slip road alignment. I assume it's some kind of ad hoc upgrade and/or toll-related complexity.

Yes, it's nuts. It seems to have been done to put a toll plaza on the sliproads linking the north and west arms of the junction - which presumably was not there originally and had to be retrofitted economically. The footprint of the junction suggests it used to be a traditional, symmetrical cloverleaf. But even without that provocation, French highway engineers are not averse to a long, meandering sliproad.

Yes, when the A85 was completed to connect with the A11 and A71 it saw changes to the toll operations. The A10 is an open toll system through Tours and the A85 is a closed toll system either side of a free section here (between Borgueil and east of the A10), hence the difficulty of having a standard junction when the route was completed.

The A85 used to end on what is now the M37, that was a very short lived stretch of motorway, it's now the D751.

[jackal]

Let's formulate a C/D definition, in order to agree on what constitutes a C/D and what doesn't.

I see Wikipedia thinks there's a distinction between C/D lanes and local-express systems, the former including short sections such as those at cloverleafs, the latter being longer. I do think there's something to that. Maybe the M60 example is the only true L-E system in the UK but there are C/D lanes elsewhere.

[ManomayLR]

Let's formulate a C/D definition, in order to agree on what constitutes a C/D and what doesn't.

I see Wikipedia thinks there's a distinction between C/D lanes and local-express systems, the former including short sections such as those at cloverleafs, the latter being longer. I do think there's something to that. Maybe the M60 example is the only true L-E system in the UK but there are C/D lanes elsewhere.

Local-express in terms of parallel routes does exist as a concept - Ax(M) roads and their parallel local access roads, for example.

[jackal]

Let's formulate a C/D definition, in order to agree on what constitutes a C/D and what doesn't.

I see Wikipedia thinks there's a distinction between C/D lanes and local-express systems, the former including short sections such as those at cloverleafs, the latter being longer. I do think there's something to that. Maybe the M60 example is the only true L-E system in the UK but there are C/D lanes elsewhere.

Local-express in terms of parallel routes does exist as a concept - Ax(M) roads and their parallel local access roads, for example.

A local-express system is a different thing from simply having a motorway and LAR: https://en.wikipedia.org/wiki/Local%E2% ... ress_lanes

[Chris5156]

A local-express system is a different thing from simply having a motorway and LAR: https://en.wikipedia.org/wiki/Local%E2% ... ress_lanes

That page lists five of them in the UK, some of which I found a bit surprising:
- A38 Plymouth - I can only assume it means this, which I wouldn't consider a local-express system at all, and which is barely classifiable as C/D lanes
- M5 Bristol - meaning the links between J18 and 19
- M20 Maidstone - which has been hotly debated above
- M55 Preston - a short third carriageway between the M6 and J1, which wouldn't meet any of the definitions above for C/D lanes, let alone local-express
- M60 Manchester - which is the classic example we've been discussing

[owen b]

Let's formulate a C/D definition, in order to agree on what constitutes a C/D and what doesn't. My original version was -

Definition 1.
A C/D Carriageway diverges left, and then runs left of and parallel to the original carriageway, which is then known as the Express Carriageway, Inner Carriageway or Mainline. From the C/D Carriageway only, one or more grade-separated junctions are accessible, whose merges and diverges therefore do not contact the mainline. A C/D Carriageway must eventually merge back into the Mainline. A C/D Carriageway may exist in only one direction of the motorway, but more usually exists in both directions, forming a pair of Outer Carriageways. 'Merge' and 'diverge' here may be taken to include 'lane-add' and 'lane-drop'.

In disputing certain candidate C/Ds, Jackal stated "My thought is basically that a true collector has traffic going to/from more than one exit/entry". Therefore, I think he would agree with my definition above if "one or more" was changed to "two or more" -

Definition 2.
A C/D Carriageway diverges left, and then runs left of and parallel to the original carriageway, which is then known as the Express Carriageway, Inner Carriageway or Mainline. From the C/D Carriageway only, two or more grade-separated junctions are accessible, whose merges and diverges therefore do not contact the mainline. A C/D Carriageway must eventually merge back into the Mainline. A C/D Carriageway may exist in only one direction of the motorway, but more usually exists in both directions, forming a pair of Outer Carriageways. 'Merge' and 'diverge' here may be taken to include 'lane-add' and 'lane-drop'.

I haven't double checked the whole thread, but I don't recall M1 J7/8 coming up so far, rather surprisingly. If I understand correctly, southbound it meets both C/D Definition 1 and Definition 2 (J8 and J7 both only accessible from the C/D lane). Northbound it only meets Definition 1, as J7 has no access from M1 northbound, so only J8 is accessed from the C/D lane.

[wrinkly]

I prefer to talk of C/D roads or carriageways rather than lanes, because calling them lanes suggests that you can cross between the C/D lane(s) and the rest of the carriageway at any point.

[ManomayLR]

I prefer to talk of C/D roads or carriageways rather than lanes, because calling them lanes suggests that you can cross between the C/D lane(s) and the rest of the carriageway at any point.

Well, at times express lanes or HOV lanes can also be physically separated from the main lanes; in those cases would you call them express / HOV carriageways?