SW M25 Relief

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Herned
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Re: SW M25 Relief

Post by Herned » Mon Sep 13, 2021 08:57

Phil wrote:
Mon Sep 13, 2021 02:03
For starters anyone coming from Kent (which includes huge volumes of HGVs to / from mainland Europe) heading for the Midlands / North west can either go via the South West quadrant to the M40 / M1 for free or via Dartford with its tolls.

Similarly those in Surrey / Sussex and Hampshire (A3) Corridor) heading for Cambridge, Norwich (or Freight heading to Felixstowe) has a choice of going via Heathrow for free or via Dartford with a toll.
For the Channel freight, if you look at e.g. Dover to Luton, it is 14 miles further avoiding Dartford, and right now takes 16 mins longer. Does operating an HGV for 14 miles cost more or less than the cost of the Dart charge? I can't imagine an HGV costs less than £21 per hour to operate, so avoiding the tunnel doesn't make sense for that route. For the M40 going via the south side is the only sensible route.

For Guildford-Felixstowe it's similar: 13 miles, 16 minutes further via the north side, for the M11 the north side is the only sensible choice.

There are no doubt a number of edge cases, but as Chris has already said, the proportion of the total traffic on M25 SW those represent is miniscule.

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Re: SW M25 Relief

Post by Peter Freeman » Wed Sep 15, 2021 15:33

jackal wrote:
Wed Sep 08, 2021 08:56
Once upon a time I used to say long slip roads weren't true C/D lanes, as C/D lanes are supposed to split local and express traffic and to do that they have the feature of connections to/from the mainline at both ends. Here the split is between different types of strategic traffic (M4/M25) so there is no need for outercarriageways to link to the M25 mainline at both ends. Other examples of 'long slip roads' are M8/M74/M77 and the forthcoming M2/A2/A289/LTC section; examples of 'true C/D lanes' are M60 J6 to J8 and M20 J5 to J6. I've long accepted that you have to call both types C/D lanes as everyone else does, and they're clearly close relatives.
Yes, close relatives, in the extended family of 'multiple carriageways', and there are some hybrids. However, I think the distinction is important.

Long slips most often occur when a suitable (eg. more spacious) area must be reached before a braid can fit in, or to avoid construction impediments, or sometimes to avoid having to build an extra structure ... etc.

Genuine C/Ds mainly occur in order to insulate express lanes from minor junctions, but sometimes to provide a lower-speed, and safer, zone where weaving within a short space between junctions is tolerated. It's a lower cost option than the preferable braid. For example, J15/16 on the M5 at Almondsbury. The C/Ds within some clover-leaf GSJs are a special and trivial case, but the same logic.

Our Melbourne Australia freeway network has only one pure (ie. non-braided) C/D implementation (M1 Westgate Tunnel Approach, under upgrade to Q3), plus three that are actually to facilitate braiding but almost incidentally do re-join (on M1 and M2). However, we have pure braids, some reached by long, non-rejoining slips.

[ I believe the main reason for our pure C/D is so that Transurban can toll (HGV's only) the outer carriageways that lead primarily to its tunnel: vehicles on the inner carriageways that lead to the bridge will not be tolled. Otherwise it could simply be D5, or even D6 as there is space. The equivalent M1 on the other side of the city is D5, without any weaving problems. ]

I'm not a fan of C/Ds. Often they waste space and capacity. If there's sufficient width to build a Q3 with 6 lanes total each way, then generally there's width to build a D7. This creates an 'auxiliary lane' solution to weaving. Right lanes are insulated from left-lane weaving disruption simply by their distance from it, rather than by a barrier. There's nothing quite like extra space.

(OT digression over)

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jackal
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Re: SW M25 Relief

Post by jackal » Thu Sep 16, 2021 10:29

^ It's a fact of life that merges and diverges can disrupt mainline flow, especially where tightly spaced. If you can put them on a separate carriageway you're making the mainline/innercarriageway much smoother. For instance, in the M20 and M60 examples the innercarriageways flow through with no merges or diverges between the start and end of the C/D lane sections.

Clearly this will improve flow compared to a D5M or D6M that gets disrupted at every merge or diverge. A wide carriageway is not equivalent to split carriageways because congestion spreads laterally - drivers are understandably unwilling to speed past very slow or queuing traffic in adjacent lanes that may pull out, so a delay in lane 1 becomes a delay in lane 2, which becomes a delay in lane 3, etc. A little extra space (often less than a lane's width) is a small price to pay to keep the mainline flowing.

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Gav
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Re: SW M25 Relief

Post by Gav » Thu Sep 16, 2021 12:26

jackal wrote:
Thu Sep 16, 2021 10:29
^ It's a fact of life that merges and diverges can disrupt mainline flow, especially where tightly spaced. If you can put them on a separate carriageway you're making the mainline/innercarriageway much smoother. For instance, in the M20 and M60 examples the innercarriageways flow through with no merges or diverges between the start and end of the C/D lane sections.

Clearly this will improve flow compared to a D5M or D6M that gets disrupted at every merge or diverge. A wide carriageway is not equivalent to split carriageways because congestion spreads laterally - drivers are understandably unwilling to speed past very slow or queuing traffic in adjacent lanes that may pull out, so a delay in lane 1 becomes a delay in lane 2, which becomes a delay in lane 3, etc. A little extra space (often less than a lane's width) is a small price to pay to keep the mainline flowing.
whole reason the M8 was built with four carriageways as it was thought the weaving on that approach would have been horrendous. so split the traffic flows and let the through traffic run centre with the local on the outer carriageways.

A newer example in scotland is the M8 with the outer A8 carriageways around euro central. however the outer carriageways do have to engage with the roundabouts. but it does work - those center carriageways move relatively good.

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jackal
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Re: SW M25 Relief

Post by jackal » Thu Sep 16, 2021 14:22

^ Yes, the M8 has some good examples. I would also include the C/D lanes/extended slips on the Kingston Bridge northbound, and westbound between J22 and J23, a recent retrofit that effectively extends the split between M8 and M74 carriageways. Both these examples should really be C/Ded in the opposite direction as well; the exit for West St queues back across Kingston Bridge disrupting the main M8 flow.

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Re: SW M25 Relief

Post by Peter Freeman » Thu Sep 16, 2021 16:30

Gav wrote:
Thu Sep 16, 2021 12:26
whole reason the M8 was built with four carriageways as it was thought the weaving on that approach would have been horrendous. so split the traffic flows and let the through traffic run centre with the local on the outer carriageways.
If we're still talking about the M74/M8/M77 interchange, that's an example of long slips, not genuine C/D. It was used by Jackal as an example of such. It's built pretty much as I would hope, and is a nice example of proper urban motorway braided design. The good old days in Glasgow! The simplistic design of merging M74 with M8 only one kilometre away from a merge of M77 with M8, would be a totally unworkable mad merging scramble, to put it mildly. The original design fortunately had a lot of foresight built into it, ready for the M74 addition.
A newer example in scotland is the M8 with the outer A8 carriageways around euro central. however the outer carriageways do have to engage with the roundabouts. but it does work - those center carriageways move relatively good.
That has some apparent C/D character, but the outer carriageways are not free-flow roads running parallel to the through route, and which have slip roads leading to a junction. That's what defines C/D. Those outer lanes ARE the junction. Nevertheless, also a nice setup, as long as the roundabouts can continue to cope into the future.
jackal wrote:
Thu Sep 16, 2021 14:22
^ Yes, the M8 has some good examples. I would also include the C/D lanes/extended slips on the Kingston Bridge northbound, and westbound between J22 and J23, a recent retrofit that effectively extends the split between M8 and M74 carriageways. Both these examples should really be C/Ded in the opposite direction as well; the exit for West St queues back across Kingston Bridge disrupting the main M8 flow.
Yes, all nice design and good examples of long slip roads. I've always liked the Glasgow urban motorway system. Pity it wasn't finished.

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ROAD ROVER
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Re: SW M25 Relief

Post by ROAD ROVER » Thu Sep 16, 2021 19:35

My relief route for that part of the M25 is normally sticking to the inside lane & coming off at each junction then immediately rejoining the motorway.

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Vierwielen
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Re: SW M25 Relief

Post by Vierwielen » Thu Sep 16, 2021 21:37

Phil wrote:
Mon Sep 13, 2021 02:03
Herned wrote:
Sun Sep 05, 2021 21:40
Scratchwood wrote:
Sun Sep 05, 2021 21:30
My issue with the toll is that it penalises people going one way around London on the M25/A282 route, but not the other

Imagine if the M60 had a toll section on one small section of it, traffic volumes around the M60 would be distorted to avoid that section.
Does it really though? There can't be many journeys which result in people choosing to go one way rather than the other - M23 to A1 or A10 perhaps it might swing it for one way or the other, but for most journeys the toll is annoying but far better than going around the longer way or through London. The numbers of actual journeys made every day which have a genuine choice must be a minute proportion of the total
More than you might think!

For starters anyone coming from Kent (which includes huge volumes of HGVs to / from mainland Europe) heading for the Midlands / North west can either go via the South West quadrant to the M40 / M1 for free or via Dartford with its tolls.

Similarly those in Surrey / Sussex and Hampshire (A3) Corridor) heading for Cambridge, Norwich (or Freight heading to Felixstowe) has a choice of going via Heathrow for free or via Dartford with a toll.
In certain cases, it might well be appropriate to stop at the last services before getting to the M25 and checking road conditions - is it worthwhile paying the toll or sitting in a traffic jam?

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Re: SW M25 Relief

Post by Peter Freeman » Fri Sep 17, 2021 03:59

Vierwielen wrote:
Thu Sep 16, 2021 21:37
In certain cases, it might well be appropriate to stop at the last services before getting to the M25 and checking road conditions - is it worthwhile paying the toll or sitting in a traffic jam?
Or, more time-efficiently, turn on your live-traffic satnav.

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Re: SW M25 Relief

Post by WHBM » Fri Sep 17, 2021 10:51

Peter Freeman wrote:
Thu Sep 16, 2021 16:30

If we're still talking about the M74/M8/M77 interchange, that's an example of long slips, not genuine C/D. It was used by Jackal as an example of such. It's built pretty much as I would hope, and is a nice example of proper urban motorway braided design. The good old days in Glasgow! The simplistic design of merging M74 with M8 only one kilometre away from a merge of M77 with M8, would be a totally unworkable mad merging scramble, to put it mildly. The original design fortunately had a lot of foresight built into it, ready for the M74 addition.
As I've written here before, I worked (at an extremely junior level) in the design office of the consultants on the Glasgow motorways in the 1970s.

The reason the M8 west of the Kingston Bridge has quadruple carriageways is that the predicted traffic flow exceeded the capacity of a 5-lane carriageway, which at the time was the maximum width allowed (still is, in theory, but seems subject to exceptions being allowed). So the next capacity level was quadruple 3-lane carriageways. And that is what was done. This particular section had the highest predicted flow on the whole Glasgow network. Having determined that, a braiding solution of the junctions was incorporated.

There had been a famous (because they still talked about it years afterwards) visit by the design team some years before to Chicago and Houston in the USA to look at their urban freeways, and there are aspects of those that can be seen in the Glasgow designs.

Bear in mind the current M74 alignment across south Glasgow was not the original plan, which was to continue the alignment along the West Street slips axis, including the famous "ski jumps" ramps that were never used. The plan was to run through Gorbals and make a fairly square box arouns the central area. I actually did some outline drawings for the route for a public enquiry.

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Re: SW M25 Relief

Post by Peter Freeman » Fri Sep 17, 2021 11:26

WHBM wrote:
Fri Sep 17, 2021 10:51
The reason the M8 west of the Kingston Bridge has quadruple carriageways is that the predicted traffic flow exceeded the capacity of a 5-lane carriageway, which at the time was the maximum width allowed (still is, in theory, but seems subject to exceptions being allowed). So the next capacity level was quadruple 3-lane carriageways. And that is what was done. This particular section had the highest predicted flow on the whole Glasgow network. Having determined that, a braiding solution of the junctions was incorporated.
Then well done! But since a connection not far to the east was intended, even then, a braided design would have been necessary (eventually) even if D6 had been allowed.
Bear in mind the current M74 alignment across south Glasgow was not the original plan, which was to continue the alignment along the West Street slips axis, including the famous "ski jumps" ramps that were never used.
Yes, I wondered if that was the case when I saw the West Street ramps after they were mentioned by Jackal. Still, the current M74 alignment seems OK anyway.

I know there's a web site somewhere about the Glasgow motorways. I must read-up.

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jackal
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Re: SW M25 Relief

Post by jackal » Fri Sep 17, 2021 11:32

^ Yes, it's called Glasgow Motorway Archive. Actually I just used it to look up when the northbound carriageway over Kingston Bridge was segregated. 1995 apparently.

PS - going to grass myself up for being off topic, so don't be surprised if this discussion moves

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Bryn666
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Re: SW M25 Relief

Post by Bryn666 » Fri Sep 17, 2021 11:59

I'd argue the design of C/D roads is actually very relevant to the OP so no need to split it up.

The lack of any strategic design on the SW M25 has left us with a huge problem today - be very interesting to see how it pans out. If we ever get sensible politicians (don't hold your breath), we will eventually go down the Dutch route and start rebuilding motorways anyway.
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