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Motorway variable speed limits
Tsst
post Thu, 12 Jul 2018 - 09:10
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Hi,



Last night I was traveling on M25 and the gantry I was heading to was displaying a restricted speed limit of 50 mph. The next gantry displayed 40 mph. However the following 2 gantries were blank ( no speed restriction limit displayed ) and the 3rd gantry following that displayed the "National Speed Limit" sign.



If there is no speed restriction displayed on a particular gantry following a gantry which displays a speed restriction does it mean that there is no restriction anymore ( which is 70 MPH )? As I understand there should be a derestriction sign displayed specifically to inform the motorists that the speed limit restriction has ended but this does not always seem to be the case.



I have drawn a diagram to better explain the query that I have. When passing under gantries 3 and 4 what should the speed limit be?





GANTRY 1: 50 MPH displayed -----> GANTRY 2: 40 MPH displayed ------> GANTRY 3: BLANK ------> GANTRY 4: BLANK -----> GANTRY 5: NATIONAL SPEED LIMIT displayed



Regards
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post Thu, 12 Jul 2018 - 09:10
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Jump in jim
post Mon, 16 Jul 2018 - 06:00
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QUOTE (The Rookie @ Sat, 14 Jul 2018 - 21:10) *
QUOTE (Jump in jim @ Sat, 14 Jul 2018 - 09:25) *
QUOTE (The Rookie @ Sat, 14 Jul 2018 - 08:09) *
I’ll cut you a little slack as a first time poster, but that was never under debate, it was the effect of that blank gantry that was under discussion.

I will cut you some slack too. How about that.

It makes no difference where the illuminated sign is, upon what it is mounted and in what scheme of traffic management. When the sign is off it forms no traffic sign.

Apologies if I have upset your sense of omniscience that you obviously have, hence the slack being cut. It’s not necessary or justified perhaps.

Agreed it forms no sign, the discussion was over the limit that applies, and you’ve singularly failed to add to that discussion by stating someonething which everyone was already agreed on!

It forms no sign so doesn't change the limit in a gantry scheme from the previous displayed sign.

Roads with NSL and special roads NSL have no signs to indicate the maximum speeds of vehicles. NSL applies except where signs indicate a lower speed.

Gantry sign key:
40 = 40 limit lit
X = Unlit
NSL = NSL lit.


Gantries at X...X...X...X = NSL prevails throughout

Gantries at X...40...40...NSL = 40 between gantries 2 to 4 NSL prevails after gantry 4

Gantries at 40...X...X...NSL = 40 between gantries 1 to 4 NSL prevails after gantry 4

Gantries at 40...X...X...X = 40 between gantries 1 to end of variable speed limit scheme shown by permanent signs

It is very simple. If there is an AMI and it is not lit it forms no sign so cannot and does not alter the speed limit from that previously indicated.
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The Rookie
post Mon, 16 Jul 2018 - 06:09
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Genius! All this has already been discussed, perhaps read the thread and realise you just added two parts of nothing, then perhaps starts answering the obvious gaps like how many unilluminated gantries render the lower limit unenforceable.


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Jump in jim
post Mon, 16 Jul 2018 - 06:45
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QUOTE (The Rookie @ Mon, 16 Jul 2018 - 07:09) *
Genius! All this has already been discussed, perhaps read the thread and realise you just added two parts of nothing, then perhaps starts answering the obvious gaps like how many unilluminated gantries render the lower limit unenforceable.

Read my post, unilluminated gantries don’t make the limit unenforceable. Neither does a black cat on the carriageway or special buys at ALDI. Your discussions are nice but making rules up seems like you are just wishfully creating exclusions and conditions when it makes no sense.
It is a simple matter made complex by your delusion.
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notmeatloaf
post Mon, 16 Jul 2018 - 08:11
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QUOTE (Jump in jim @ Mon, 16 Jul 2018 - 07:45) *
Read my post, unilluminated gantries don’t make the limit unenforceable. Neither does a black cat on the carriageway or special buys at ALDI. Your discussions are nice but making rules up seems like you are just wishfully creating exclusions and conditions when it makes no sense.
It is a simple matter made complex by your delusion.

Unfortunately you're not citing in any way how the legislation supports your made up idea. And as it doesn't in any way, because the legislation specifically says that signs displaying no speed limit do have a meaning, then your idea is worthless.

This post has been edited by notmeatloaf: Mon, 16 Jul 2018 - 08:11
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Redivi
post Mon, 16 Jul 2018 - 08:41
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It forms no sign so doesn't change the limit in a gantry scheme from the previous displayed sign

The problem with the situation of 40 X X X is that the driver doesn't see all the gantries at the same time

When he sees Gantry 2 as unlit, he doesn't know if it was lit when he passed Gantry 1
Neither does he know if Gantry 1 is still lit

If the gantries are all switched off immediately after he passes Gantry 1 and X has no meaning, his personal 40 mph speed limit creates a hazard for the following traffic at normal speed
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Jump in jim
post Mon, 16 Jul 2018 - 09:53
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QUOTE (notmeatloaf @ Mon, 16 Jul 2018 - 09:11) *
QUOTE (Jump in jim @ Mon, 16 Jul 2018 - 07:45) *
Read my post, unilluminated gantries don’t make the limit unenforceable. Neither does a black cat on the carriageway or special buys at ALDI. Your discussions are nice but making rules up seems like you are just wishfully creating exclusions and conditions when it makes no sense.
It is a simple matter made complex by your delusion.

Unfortunately you're not citing in any way how the legislation supports your made up idea. And as it doesn't in any way, because the legislation specifically says that signs displaying no speed limit do have a meaning, then your idea is worthless.

No it doesn’t. The part you are quoting explains the 10-second delay in the application of the sign. You can’t just use some of the words in the paragraph, you have to use them all.

There is no legislation anywhere that says a black rectangle conveys a meaning as or indeed is a road traffic sign.

So if you want legislation quoted, you go first. Don’t repeat (4)... you have erred in that. Find the road traffic sign that is a black rectangle and show us all where that is defined. We can go from there.

This post has been edited by Jump in jim: Mon, 16 Jul 2018 - 09:54
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DancingDad
post Mon, 16 Jul 2018 - 09:54
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QUOTE (Redivi @ Mon, 16 Jul 2018 - 09:41) *
It forms no sign so doesn't change the limit in a gantry scheme from the previous displayed sign

The problem with the situation of 40 X X X is that the driver doesn't see all the gantries at the same time

When he sees Gantry 2 as unlit, he doesn't know if it was lit when he passed Gantry 1
Neither does he know if Gantry 1 is still lit

If the gantries are all switched off immediately after he passes Gantry 1 and X has no meaning, his personal 40 mph speed limit creates a hazard for the following traffic at normal speed


And that is the dilemma that a driver faces, do you speed up and risk a ticket with no solid defence or keep obeying the last posted limit and risk a twonker driving into your rear end ?

You posted part of the legislation re the M25 VSL sections but for some reason ignored this bit. (post 10)
QUOTE
(2) A section of a road is subject to a variable speed limit in relation to a vehicle being driven along it if—
(a)the road is specified in the Schedule;
(b)the vehicle has passed a speed limit sign; and
©the vehicle has not subsequently passed—
(i)another speed limit sign indicating a different speed limit; or
(ii)a traffic sign which indicates that the national speed limit is in force.


That is what Jim is detailing and does seem to accord with normal practice on normal roads, that signed speed limits apply until another sign says they don't, not just the absence of a sign.

My interpretation of Para 4 (for what it is worth) re unlit signs is that it only has any relevance if the 10 second "grace period" is in play.
Simple if-then logic, if A applies then B else no effect.
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Jump in jim
post Mon, 16 Jul 2018 - 09:59
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QUOTE (DancingDad @ Mon, 16 Jul 2018 - 10:54) *
QUOTE (Redivi @ Mon, 16 Jul 2018 - 09:41) *
It forms no sign so doesn't change the limit in a gantry scheme from the previous displayed sign

The problem with the situation of 40 X X X is that the driver doesn't see all the gantries at the same time

When he sees Gantry 2 as unlit, he doesn't know if it was lit when he passed Gantry 1
Neither does he know if Gantry 1 is still lit

If the gantries are all switched off immediately after he passes Gantry 1 and X has no meaning, his personal 40 mph speed limit creates a hazard for the following traffic at normal speed


And that is the dilemma that a driver faces, do you speed up and risk a ticket with no solid defence or keep obeying the last posted limit and risk a twonker driving into your rear end ?

You posted part of the legislation re the M25 VSL sections but for some reason ignored this bit. (post 10)
QUOTE
(2) A section of a road is subject to a variable speed limit in relation to a vehicle being driven along it if—
(a)the road is specified in the Schedule;
(b)the vehicle has passed a speed limit sign; and
©the vehicle has not subsequently passed—
(i)another speed limit sign indicating a different speed limit; or
(ii)a traffic sign which indicates that the national speed limit is in force.


That is what Jim is detailing and does seem to accord with normal practice on normal roads, that signed speed limits apply until another sign says they don't, not just the absence of a sign.

My interpretation of Para 4 (for what it is worth) re unlit signs is that it only has any relevance if the 10 second "grace period" is in play.
Simple if-then logic, if A applies then B else no effect.

That is how the control room will set the limits if they do it manually.
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Albert Ross
post Mon, 16 Jul 2018 - 10:31
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Could the problem be in not knowing where a variable speed section starts and finishes?

A new section of the M1 is to become Variable in September http://www.legislation.gov.uk/uksi/2018/819/contents/made

But on the M25 as a circulatory you may finish one scheme and then pass unlit gantries and then happen upon another under a different regulation. How would you know?



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DancingDad
post Mon, 16 Jul 2018 - 10:46
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To me the issue of why it happens comes down to control.
Whether set manually or automatically, to me the system ought to be set up in a way that never (bar equipment failure) leaves a driver guessing.
For instance, if a reduced speed is set on a gantry or set of gantries, the next gantry should automatically display NSL.
If the reduced limits are removed, VSL should be displayed on all gantries affected (including the one already showing NSL) for a period allowing vehicles to go past a gantry showing NSL, never just switched off.
TBH, on the sections of VSL motorways I regularly use (Midlands) I cannot recall seeing a reduced speed section without NSL at the end, just lucky I guess.
But I have seen anomalies in the setting.... 60-50-40-60-40 for instance on sections of M6 from J6 (Spaghetti) northwards.
What's that all about ? The gantries are only a few hundred yards apart, why suddenly stick a 60 between 40s ??

This post has been edited by DancingDad: Mon, 16 Jul 2018 - 10:48
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Jump in jim
post Mon, 16 Jul 2018 - 10:48
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The driver doesn't need to know what scheme or Speed Limit Order applies. All the driver needs to be given is adequate guidance of the speed limit; the signs do that. Well! They are supposed to.

The Speed Limit Order only becomes relevant to the driver if it is not properly made in a way that causes the variable limits to be unlawful. It's up to you if you wish to have a ready reference about the speed limit order on each road as you drive but I would think that your progress would be so impaired that a speed limit would not assist.

Speed limits are, in my experience, clearly indicated to the driver by permanent signs showing the start and end of a controlled section of motorway and by the illuminated signs therein. What more do you need?
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The Rookie
post Mon, 16 Jul 2018 - 11:59
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You still haven't even bothered to try and answer the question under discussion which was in the absence of an NSL sign, where can the limit be considered to end....or does the order that was in relation to this stretch mean it ends at a blank sign which some believe it can be read as stating (I disagree).

Your posts to date have been 'stating the bleedin' obvious' and not actually moving the discussion anywhere at all.


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notmeatloaf
post Mon, 16 Jul 2018 - 17:10
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QUOTE (DancingDad @ Mon, 16 Jul 2018 - 11:46) *
What's that all about ? The gantries are only a few hundred yards apart, why suddenly stick a 60 between 40s ??

Remember the stop start queues on the M25 before VSL, and the constant rear shunts?

This is the "clever part" of VSL. It monitors traffic using loops and when it detects bunching (and resultant lane changing) it reduces the speed limit behind so that following vehicles don't end up braking to a stop. The fact it varies from gantry to gantry is proof it is working, by slowing you to 40 at one gantry it means traffic is free flowing.

Seeing as when congested traffic naturally bunches a constant speed limit would still result in stop start traffic.
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big_mac
post Mon, 16 Jul 2018 - 17:39
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QUOTE (notmeatloaf @ Mon, 16 Jul 2018 - 18:10) *
This is the "clever part" of VSL.

I don't think 40, 60, 40 in the space of a few hundred yards is particularly "clever". It may be better than nothing, but there still seems to be plenty of room for improvement.

I don't go that way so often, but it was quite common on the M62 to have no end of limit sign. There were times when I would leave the motorway and rejoin, rather than be the only one obeying the last limit sign.
I can't think of a good reason why the system should allow a blank sign where the previous sign was displaying a limit.
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DancingDad
post Mon, 16 Jul 2018 - 19:19
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QUOTE (big_mac @ Mon, 16 Jul 2018 - 18:39) *
QUOTE (notmeatloaf @ Mon, 16 Jul 2018 - 18:10) *
This is the "clever part" of VSL.

I don't think 40, 60, 40 in the space of a few hundred yards is particularly "clever". ……...

Nor do I.

In fact the times I have seen it, I reckon it's because the "clever" part of the system has pressed the wrong button.
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notmeatloaf
post Mon, 16 Jul 2018 - 20:11
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How else would you prevent bunching? If you remember M25 pre-VSL that was rubbish.

Stop start traffic isnt efficient.
Plus when traffic stops lots of people change lanes, which makes things worse.
Clearly letting people drive along at their own speed wasn't sorting the above.

Personally I dont really see the problem. Mostly people stick to the limits and so you just go with the flow off traffic. Maybe other people find different but I find it very easy to go from 40 to 60 to 40 in my car over the space of a kilometre, plus there are sodding huge signs in the sky to tell you what the limit is.

Plus believe it or not before they spent a few £bns on it they did a shedload of research to check it worked, then trialled it on the UKs busiest motorway.

But the facts the limits change in a variable speed limit zone must surely be a bad sign too, apparently.
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DancingDad
post Mon, 16 Jul 2018 - 21:42
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The theories behind VSL are partly based on fluid mechanics, faster you try to squeeze a fluid through pipes, more turbulence you get, slow it down and the flow is smoother.
I haven't a problem with the theories, simply the application.
If the 40-60-40 was with gantries a kilometre apart would have little issue. But on the M6 through Brum they are far closer.
Traffic is slowed to 40 progressively, fine, unless an emergency, should avoid the need for sudden braking.
Then, 2 to 300 yards past the 40 gantry, the sodding great sign in the sky says 60.
So traffic speeds up, some will be in cars that can easily accelerate from 40-60 in a few seconds, some will not, some drivers will anticipate the 60 and put boot down the second they can read the sign, others will wait till they pass underneath it. There will be the normal MLOC drivers in lane 2, more wary in lane 1, speed kings and frustrated MLOC drivers in lane 3 and as VSL is operating, plenty driving down the hard shoulder, often faster then lane 1 is moving.
Some of the faster are changing lanes, some are booting up behind the slower and trying to move them by tailgating.
That's fine, it will sort out in a few hundred yards but whoops, the next sodding great sign in the sky says 40 well before it has sorted out.
Brakes on lads and the whole system has to go in reverse, except this time with no gentle slowing, just fast pedal on to fast pedal off in the blink of an eye and with traffic far more bunched then it was before the VSL or at the previous 40 sign.
If that is the way the system is designed, someone in design needs to get out in the real world.
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Figaro
post Mon, 16 Jul 2018 - 23:05
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I’m with you there DD. Good summary of behaviour. Inconsistency in the VSL in Midlands is common and frustrating. In my opinion if one gantry is lit the whole series should be lit to either repeat or change a reduced limit or go back to NSL.
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notmeatloaf
post Tue, 17 Jul 2018 - 18:43
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It's not based on fluid mechanics, it's based on traffic modelling. Fluid doesn't compress/bunch.

There is an element of slower speed = more throughput but whether or not you can see it from the windscreen it is significantly more nuanced.
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DancingDad
post Tue, 17 Jul 2018 - 19:23
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QUOTE (notmeatloaf @ Tue, 17 Jul 2018 - 19:43) *
It's not based on fluid mechanics, it's based on traffic modelling. Fluid doesn't compress/bunch...…..


You may want to look up items like Kinematic Wave Theory relating to traffic flow.... stems from work back in 1955 and models traffic on how water behaves.
There are many other theories used and various computer models and algorithms that predict flows around bottlenecks, traffic lights, junctions et al.
Many have probably superseded the original work, especially those based on real time observations and measurement.
But doesn't change that fluid flow dynamics were the basis for many traffic schemes.
Certainly were for the original controlled motorway proposals IIRC, loads in the papers at the time explaining how faster was actually slower and the like. M25 by Heathrow back in the 90s ?

This post has been edited by DancingDad: Tue, 17 Jul 2018 - 19:24
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