Variable Speed Limits on the M1
Discussion
I was driving down the M1 on Tuesday night and saw what I have seen once before but assumed it was an error. 4 Lanes of motorway, with steadily reducing speed limits across all lanes. 60 at the first gantry, 50 at the next, then 40.. then the next gantry the inside lane was 20, and the other 3 lanes were 40.
As I said I've seen this once before, but on this Journey i saw it on two occasions on different sections of the M1. Both times there seemed to be no reason at all for the speed reduction. After the 20 40 40 40 gantry the next one took us back up to NSL.
A couple of thought... I didn't think it was possible to have different speeds on the same carriageway? Also, 20 on the M1 seems like a really stupid idea!
As I said I've seen this once before, but on this Journey i saw it on two occasions on different sections of the M1. Both times there seemed to be no reason at all for the speed reduction. After the 20 40 40 40 gantry the next one took us back up to NSL.
A couple of thought... I didn't think it was possible to have different speeds on the same carriageway? Also, 20 on the M1 seems like a really stupid idea!
TheBALDpuma said:
I I didn't think it was possible to have different speeds on the same carriageway? Also, 20 on the M1 seems like a really stupid idea!
It is possible to have different speeds, but normally as a prelude to some sort of incident. 20 would only be used just before a single lane or full closure.It's entirely possible that a closure was in the process of being lifted
The last two days on my journey home on M1 south the gantries have been at 60, then 60 queue after junction, then 40 (the only actual queue being caused by everyone braking down to 40! ), then NSL at the next gantry. Nothing to report on either occasion. This was approaching and after Jct29.
I can't believe any queue would have disappeared in the few minutes it took to get there.
It's a regular event in both directions elsewhere on the M1 between 28 and 33.
I'd say at least 50% of tbe time I see absolutely nothing that could have caused it before the NSL kicks in.
I just wonder how small an event it takes to trigger these limits and if there was an event how long does it take to remove them.
Does it need 'permission' or is it automatic?
I can't believe any queue would have disappeared in the few minutes it took to get there.
It's a regular event in both directions elsewhere on the M1 between 28 and 33.
I'd say at least 50% of tbe time I see absolutely nothing that could have caused it before the NSL kicks in.
I just wonder how small an event it takes to trigger these limits and if there was an event how long does it take to remove them.
Does it need 'permission' or is it automatic?
MikeB444 said:
I just wonder how small an event it takes to trigger these limits and if there was an event how long does it take to remove them.
Does it need 'permission' or is it automatic?
MIDAS is automatic. The lack of a queue means it's working as expected . . . The traffic flow rates are calculated by a very complex algorithm and when they're exceeded they set the signals (speeds) which sequence the signals to put the messages up. Does it need 'permission' or is it automatic?
The timer for MIDAS to remove sign & signal settings was recently changed from 4 minutes to 2.
chopper602 said:
It is possible to have different speeds, but normally as a prelude to some sort of incident. 20 would only be used just before a single lane or full closure.
It's entirely possible that a closure was in the process of being lifted
So you’re saying that lane 1 could be 20mph, and lanes 2,3 & 4 could be 40 (or more)?It's entirely possible that a closure was in the process of being lifted
In my previous life ‘at the pointy end’ this was not possible, and a 20 limit was only ever set for ‘Oncoming Vehicle’, a truly horrendous scenario.
chopper602 said:
MikeB444 said:
I just wonder how small an event it takes to trigger these limits and if there was an event how long does it take to remove them.
Does it need 'permission' or is it automatic?
MIDAS is automatic. The lack of a queue means it's working as expected . . . The traffic flow rates are calculated by a very complex algorithm and when they're exceeded they set the signals (speeds) which sequence the signals to put the messages up. Does it need 'permission' or is it automatic?
The timer for MIDAS to remove sign & signal settings was recently changed from 4 minutes to 2.
littleredrooster said:
So you’re saying that lane 1 could be 20mph, and lanes 2,3 & 4 could be 40 (or more)?
As a prelude to a closure or similar you could, but would never be a differential of more than 20 mph. So you wouldn't get 20 60 60 60 for example. You, also, cannot get 'islands' where the speeds would be 20 40 20 40 for example, it's all built into the signal rules (which are very complex and I won't bore with !)jm doc said:
I would suggest the lack of a queue would most likely be because there was never a queue in the first place. For "very complex algorithm" substitute "let's make something up" and is just there to justify the extortionate cost of the installation of the utterly useles and frankly dangerous system.
The problem is that no queue doesn't tell you anything. There could be no queue because the system has managed the situation and the ambient traffic flow effectively or there could be no queue because the restrictions imposed were totally unnecessary for the situation and the ambient traffic flow.Tbh I have no idea how the system compares with the flow rates etc of not having it but it does have a well deserved reputation for being wrong which makes it dangerous because of the cry wolf effect. Last week I went down the M23 and the signs were reporting an obstruction and lane 1 was closed the whole way from the M25 to Gatwick. Nothing there I could see nor in any of the refuges. Oh and as usual a number of vehicles including a couple of HGVs driving in lane 1 through the red Xs without a second thought.
Somewhatfoolish said:
In some countries (like Russia) differential fixed limits are even possible in different lanes.
Whereabouts was this on the M1 btw? I have seen it but only near junctions.
Somewhere between Leeds and the M25 Whereabouts was this on the M1 btw? I have seen it but only near junctions.
was late evening and I can't remember exactly where they were. One was definitely by a junction, the other I can't be sure.As others have said above the left hand lane before an exit with lots of off-going traffic can trigger this to keep the lane moving.
By virtue of not seeing a queue it shows that it is working in preventing a queue.
It will immediately go to NSL alongside the exit to make it safer for on-coming traffic at the same junction.
By virtue of not seeing a queue it shows that it is working in preventing a queue.
It will immediately go to NSL alongside the exit to make it safer for on-coming traffic at the same junction.
LukeBrown66 said:
I am convinced at busy times these messages are simply speed control, no accident, no issue.
Of course it does not matter too much, but I do have my doubts about all of thee incidents because frequently you drive by and nothing has happened.
Clearly the smart motorway stuff has two basic functions. Speed reduction in busy times to increase traffic density and thus flow is one.The other is lane control for incidents (needing speed control as well).Of course it does not matter too much, but I do have my doubts about all of thee incidents because frequently you drive by and nothing has happened.
There is no need to put made up messages up saying incident, debris etc when there isn't.
Clearly there are other things it does including reducing noise and pollution.
Sadly it's not a trusted system as we've all been through them many times and seen no incident. Doesn't mean there hasn't been one, but intuitively it feels somewhat inept.
littlebasher said:
Happens all the time around Sheffield.
The lowest limit is usually reserved for the gantry that has a speed camera. Comical really
The reduced limits for ‘air quality’ are pretty much a permanent fixture on the M1 by Sheffield now. The other one I see a lot is a reduced limit southbound past Meadowhall and over the viaduct at j34, followed by a few miles of blank gantries (which I assume technically means the lower limit is still in place) and then miles later a gantry with a NSL sign. The lowest limit is usually reserved for the gantry that has a speed camera. Comical really
This usually happens when there are signals set on a slip road just after the gantry, it's a glitch in the system, I try to amend any signals I see to a lane divert arrow instead but it's not always possible. Note confirmed signals are only set when officers have "eyes on" an incident not just for fun, we would be in very serious trouble otherwise.
TheBALDpuma said:
Somewhatfoolish said:
In some countries (like Russia) differential fixed limits are even possible in different lanes.
Whereabouts was this on the M1 btw? I have seen it but only near junctions.
Somewhere between Leeds and the M25 Whereabouts was this on the M1 btw? I have seen it but only near junctions.
was late evening and I can't remember exactly where they were. One was definitely by a junction, the other I can't be sure.Years ago I messaged Highways England about something similar.
20 / 40 / 40 / 40 on gantry and also 40 / BLANK / 40 / 40
I was told that it was impossible that the lane had its own speed limit and the actual rule and common sense would tell you was to drive at the lowest speed displayed.
They also said that the gantry signs would never increase or decrease by more than 10mph between gantry’s. Yet I always see them showing 50 when it’s currently NSL - 20 mph difference. Or 40 when it’s currently 60 and the same going up too. Gantry shows 40 or 50 and then the next is NSL.
Blank gantry are the worst as technically you have to be at the last speed shown but then you get people joining who aren’t subject to the same limit as you who are ‘speeding’ but they’re not or those who see a blank sign and choose to speed. So now I just follow the rule that if there is a junction between the last speed limit and the new blank limit then I’m safe to do NSL as I could have come on that junction.
20 / 40 / 40 / 40 on gantry and also 40 / BLANK / 40 / 40
I was told that it was impossible that the lane had its own speed limit and the actual rule and common sense would tell you was to drive at the lowest speed displayed.
They also said that the gantry signs would never increase or decrease by more than 10mph between gantry’s. Yet I always see them showing 50 when it’s currently NSL - 20 mph difference. Or 40 when it’s currently 60 and the same going up too. Gantry shows 40 or 50 and then the next is NSL.
Blank gantry are the worst as technically you have to be at the last speed shown but then you get people joining who aren’t subject to the same limit as you who are ‘speeding’ but they’re not or those who see a blank sign and choose to speed. So now I just follow the rule that if there is a junction between the last speed limit and the new blank limit then I’m safe to do NSL as I could have come on that junction.
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