Asymmetrical lane drops

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ChrisH
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Asymmetrical lane drops

Post by ChrisH »

The M40 around its junction with the M25 has some rather odd lane arrangements.

Westbound, the three lanes coming out of London on the A40 drop to two through j1, then go back to three but no lanes are dropped at the - much busier - M25 junction. The M25 onslip adds a fourth lane which continues west.

Eastbound, there is a double lane drop from four to two at the M25 junction, with a single lane gain going back to three afterwards and no lane drop in j1.

Is there a reason for this asymmetry? Or other similar examples?
booshank
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Re: Asymmetrical lane drops

Post by booshank »

The M4 westbound has a nasty lack of lane gain after the Almondsbury interchange. The two lanes from the M5 are forced to merge with the two of the mainline. In the opposite direction there's third lane from the M48 that drops at Almondsbury. Really at a minimum it should be mirrored.
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Bryn666
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Re: Asymmetrical lane drops

Post by Bryn666 »

booshank wrote: Mon Aug 10, 2020 20:46 The M4 westbound has a nasty lack of lane gain after the Almondsbury interchange. The two lanes from the M5 are forced to merge with the two of the mainline. In the opposite direction there's third lane from the M48 that drops at Almondsbury. Really at a minimum it should be mirrored.
My belief is they thought the M49 removed the need to widen the M4 but that rather ignored the growth of traffic from London. And now the tolls are gone people won't divert via the A419 to get to Brecon etc. It's going to be a severe bottleneck as a result. I dare say it should be four lanes each way there.
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booshank
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Re: Asymmetrical lane drops

Post by booshank »

Bryn666 wrote: Tue Aug 11, 2020 13:00
booshank wrote: Mon Aug 10, 2020 20:46 The M4 westbound has a nasty lack of lane gain after the Almondsbury interchange. The two lanes from the M5 are forced to merge with the two of the mainline. In the opposite direction there's third lane from the M48 that drops at Almondsbury. Really at a minimum it should be mirrored.
My belief is they thought the M49 removed the need to widen the M4 but that rather ignored the growth of traffic from London. And now the tolls are gone people won't divert via the A419 to get to Brecon etc. It's going to be a severe bottleneck as a result. I dare say it should be four lanes each way there.
Yeah I think the whole thing is looking rather long in the tooth. I hadn't actually noticed it before despite being fairly local (Bath) as it isn't normally on my way to or from anywhere, but on Sat a road closure and presumably some heavy traffic on the ring road took me on a route to south Wales that involved joining the M5 northbound from the A38 and it was definitely a bottleneck.

Could really do with D5 from the M32 to M5 and D4 as far as the M48.
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jackal
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Re: Asymmetrical lane drops

Post by jackal »

Perhaps counterintuitively there is a massive drop off in M4 traffic west of Almondsbury - volumes are half those of the M4 to the east, so there's not really a case for D4. The time has now come to add a third lane westbound, but given this is 25 years after the second crossing and 55 years after the first one, you might say the existing road has earned its crust.
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the cheesecake man
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Re: Asymmetrical lane drops

Post by the cheesecake man »

M18 northbound has always dropped a lane at J2 (A1(M)); it now regains it, then redrops it at J3 (A6182 for Doncaster and Bawtry) until the M180.

M18 southbound remains two lanes between J3 and J2, causing regular delays as the traffic is held up by the large amount of traffic joining at J3 and by exit slip road queues at J2. IMHO it would have been sensible to put the third lane in both ways while you were there.
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Re: Asymmetrical lane drops

Post by WHBM »

booshank wrote: Mon Aug 10, 2020 20:46 The M4 westbound has a nasty lack of lane gain after the Almondsbury interchange. The two lanes from the M5 are forced to merge with the two of the mainline. In the opposite direction there's third lane from the M48 that drops at Almondsbury. Really at a minimum it should be mirrored.
This one arises because the layout is what it's always been since the M4 opened up the hill to Almondsbury in 1966, when it had an extra eastbound climbing lane provided from the beginning on an otherwise D2M road. When the new bridge was built it just tied in to this.

Loss of climbing lanes with subsequent changes is not uncommon; the one on the M25 eastbound from J26 up to J27 at Epping was lost when the whole thing became D4M; it's characterised by still having a hard shoulder when the rest does not ! Likewise the one eastbound from Birmingham to Coventry on the M6 was lost when the M6T tied in. I presume that HE have some justification why climbing lanes are "no longer required" when convenient in a project, just like motorway lighting being "no longer required".
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RichardA35
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Re: Asymmetrical lane drops

Post by RichardA35 »

ChrisH wrote: Mon Aug 10, 2020 17:13 The M40 around its junction with the M25 has some rather odd lane arrangements.

Westbound, the three lanes coming out of London on the A40 drop to two through j1, then go back to three but no lanes are dropped at the - much busier - M25 junction. The M25 onslip adds a fourth lane which continues west.

Eastbound, there is a double lane drop from four to two at the M25 junction, with a single lane gain going back to three afterwards and no lane drop in j1.

Is there a reason for this asymmetry? Or other similar examples?
IMV it was a play on the rules to give the layout with the maximum possible weaving length between the J1 on and J1A off slip. It is still very short (substandard @ <1km?) even with the onslip having been reconstructed to maximise the length.
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Re: Asymmetrical lane drops

Post by PhilC »

WHBM wrote: Tue Aug 11, 2020 14:28 Likewise the one eastbound from Birmingham to Coventry on the M6 was lost when the M6T tied in. I presume that HE have some justification why climbing lanes are "no longer required" when convenient in a project, just like motorway lighting being "no longer required".
The climbing lane on the M6 between Birmingham and Coventry is still there, it's just that it's now laid out as four lanes, rather than three plus a climbing lane marked SLOW VEHS. Back in the 1970s lorries did not have the same power as modern ones, so would often slow right down on some gradients. It must have made sense to keep such slow moving traffic in a lane by itself. Nowadays most lorry traffic can maintain its regulation 90 km/h up most motorway gradients, so the need for segregation is less. An extra overtaking lane must be deemed more useful.
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Re: Asymmetrical lane drops

Post by DB617 »

jackal wrote: Tue Aug 11, 2020 13:49 Perhaps counterintuitively there is a massive drop off in M4 traffic west of Almondsbury - volumes are half those of the M4 to the east, so there's not really a case for D4. The time has now come to add a third lane westbound, but given this is 25 years after the second crossing and 55 years after the first one, you might say the existing road has earned its crust.
I've always seen the opportunity for a quick win between Almondsbury and the M48. For a short extension to the smart motorway westbound, they could have taken over the HS and at least allowed for a much longer weaving/merging length prior to the M48 diverge, plus anyone continuing west could choose to remain on the M48 to avoid messing about in congested M4 lanes. It would make big difference to capacity and it blows my mind that it wasn't part of the original HSR scheme, let alone a D3 expansion all the way to the M49.
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Re: Asymmetrical lane drops

Post by Brenley Corner »

I'm surprised that no-one has mentioned the infamous A31 where it multiplexes with the A338 near Ringwood.

- Eastbound 4 lanes between A338 junctions
- Westbound 2 lanes only widening to 4 lanes after river bridges

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Re: Asymmetrical lane drops

Post by Micro The Maniac »

Brenley Corner wrote: Wed Aug 12, 2020 08:49 I'm surprised that no-one has mentioned the infamous A31 where it multiplexes with the A338 near Ringwood.
TBH, more annoying is the Cadnam junction where there isn't a lane drop at the off-slip, but there's then a lane merge within the junction - which always causes queues. Why not take Lane 1 off to the A31/B3079 roundabout?

Given the lane gain northbound is in the middle of the junction, I guess at least it is symmetrical... so off-topic for this thread (sorry!)
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ChrisH
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Re: Asymmetrical lane drops

Post by ChrisH »

Micro The Maniac wrote: Wed Aug 12, 2020 11:22
Brenley Corner wrote: Wed Aug 12, 2020 08:49 I'm surprised that no-one has mentioned the infamous A31 where it multiplexes with the A338 near Ringwood.
TBH, more annoying is the Cadnam junction where there isn't a lane drop at the off-slip, but there's then a lane merge within the junction - which always causes queues. Why not take Lane 1 off to the A31/B3079 roundabout?

Given the lane gain northbound is in the middle of the junction, I guess at least it is symmetrical... so off-topic for this thread (sorry!)
Begone with your symmetrical lane drop! :D
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Re: Asymmetrical lane drops

Post by skiddaw05 »

Haven't they dropped a lane on the westbound A66 at Bassenthwaite Lake? Or is there now no extra lane to drop, I think GSV predates this
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Re: Asymmetrical lane drops

Post by ManomayLR »

PhilC wrote: Tue Aug 11, 2020 16:14
WHBM wrote: Tue Aug 11, 2020 14:28 Likewise the one eastbound from Birmingham to Coventry on the M6 was lost when the M6T tied in. I presume that HE have some justification why climbing lanes are "no longer required" when convenient in a project, just like motorway lighting being "no longer required".
The climbing lane on the M6 between Birmingham and Coventry is still there, it's just that it's now laid out as four lanes, rather than three plus a climbing lane marked SLOW VEHS. Back in the 1970s lorries did not have the same power as modern ones, so would often slow right down on some gradients. It must have made sense to keep such slow moving traffic in a lane by itself. Nowadays most lorry traffic can maintain its regulation 90 km/h up most motorway gradients, so the need for segregation is less. An extra overtaking lane must be deemed more useful.
When it's a proper 4-lane stretch you'll notice full lane control gantries (except for smart MS4). With climbing lanes mostly you'll just see the standard verge MS3s or central reserve MS1s. So the stretch on the M6 I would term as full 4-lane as it was given full lane control gantries. Obviously now it's been superseded by the smart motorway, where there's 4 lanes throughout, but the stretch is still noticeable by the presence of the hard shoulder and the installation of MS4s on the original corrugated-metal gantries.
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Re: Asymmetrical lane drops

Post by Bryn666 »

The requirement was always if a carriageway had four or more lanes it had overhead signals instead of central reservation mounted. See also the M62 on the Pennines and M1 going into London.

It had no bearing on being a climbing lane or not.
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Re: Asymmetrical lane drops

Post by ManomayLR »

Bryn666 wrote: Thu Aug 13, 2020 17:37 The requirement was always if a carriageway had four or more lanes it had overhead signals instead of central reservation mounted. See also the M62 on the Pennines and M1 going into London.

It had no bearing on being a climbing lane or not.
I've seen plenty of examples e.g M25 clockwise just east of J26 before ALR and M1 southbound between J5 and J4, where a fourth climbing lane did not warrant the installation of lane control gantries, and central reserve MS1s or verge MS3s respectively, were retained.
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Re: Asymmetrical lane drops

Post by Bryn666 »

Length is probably the decision maker there - those weren't very long sections. I thought there were overhead gantries prior to J3 but it's not a length I use often.
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Re: Asymmetrical lane drops

Post by ManomayLR »

Bryn666 wrote: Thu Aug 13, 2020 18:22 Length is probably the decision maker there - those weren't very long sections. I thought there were overhead gantries prior to J3 but it's not a length I use often.
There used to be lane control gantries on the stretch between J4 and J1, aka the Hendon Urban Motorway, but these were replaced with verge-mounted MS4s in 2008.
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Re: Asymmetrical lane drops

Post by BOH »

Brenley Corner wrote: Wed Aug 12, 2020 08:49 I'm surprised that no-one has mentioned the infamous A31 where it multiplexes with the A338 near Ringwood.

- Eastbound 4 lanes between A338 junctions
- Westbound 2 lanes only widening to 4 lanes after river bridges

Tony
On our local FB page there is a warning that lane closures there are happening at off-peak / overnight soon for bore holes to be drilled as preparation for the widening starting soon. Will be chaos when that work finally starts.
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