M25 variable speed limit Gatsos

I rarely use the M25 but this week had to go to Sussex twice which means using the section of motorway between the M4 and A3 which has the variable speed limit gantries. These are situated every few hundred yards in some places which must make it the most densely populated speed camera infestation in the world.

What I couldn't quite understand from my limited 2 day experience of them is their strange behaviour which seemed to be more about raising revenue from speeding fines than any logical speed requirement for the road conditions.

One morning going anticlockwise in steady albeit dense traffic flow, no roadworks or queues, four consecutive gantries in the space of only a mile or so had the following limits. 70 (box with diagonal line through it), 50,

60 and then 70 again. There was no obvious reason to have to slow down but everyone had to nail the brakes on regardless.

On the way back last night at 8 pm in very light traffic, a completely clear road and pleasant sunshine one lone gantry had a 50 limit on it with absolutely nothing around to suggest why other than force us all to test either our brakes or our wallets.

All I can say is thank god I work from home and don't have to use the accursed road every day. I just wonder what people who have to use it all the time think about whether these things really are switched on just to raise revenue sometimes.

My legal question would be if one could prove the speed limit sign had been left on in error rather than for a specific safety purpose would the speeding ticket be valid?

Secondly, does anyone know if the usage of these variable speed limit signs has to be logged so that every use has a reason noted against it?

Reply to
Dave Baker
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Why would the reason be obvious? The speed limits are for traffic congestion control miles ahead of you. If you don't notice a reason that means they are actually doing their job.

If you need to 'test your brakes' you aren't driving safely.

I agree.

Yes. Whether or not the system was left on or not, you were speeding you get fined.

Reply to
CheekyMonkey

It would be interesting to know the procedure adopted to have the limits either switched on /off or altered .Who decides ? Is it done by humans and as you ask,do they keep a log of when and why ?.

Reply to
Stuart B

May be worth asking in uk.rec.driving - .maintenance is definitely the wrong place.

cheers, clive

Reply to
Clive George

You're wrong. See below.

You were still moving. That's why they're there and shows that they were working. Before that, you'd have been crawling along in 1st gear at best.

If the sign is up, the ticket is valid.

Reply to
Conor

The message from "Dave Baker" contains these words:

They've been like that for years, long before the speed cameras were installed.

Reply to
Guy King

The message from Stuart B contains these words:

More aposite, however the system is controlled, is it heuristic?

Reply to
Guy King

I see a couple of people have said that if there is a restriction displayed, then you'll get a speeding fine if you break it.

I seem to recall that a couple of years ago a case was appealed and a ruling made that there had to be a reason for the restriction or it was voided and the driver had his speeding conviction overturned.

I'm sure one of our legal bevers can access a law database and find the exact details for this one. If it helps I also seem to recal that the driver making the appeal was a senior police officer (aren't they always?) but it did actually go to the appeal court.

Reply to
Martin

The system is called MIDAS, and uses sensors/cameras for input, and the speed limits for the output. Human operators oversee the computer, and can manually adjust it.

Reply to
CheekyMonkey

ISTR in the distant past someone contested the variable limit and it was ruled that the operators have no right to arbitrarily set limits. The fact that they are still going means maybe some further ruling was made or whatever...who knows. Would be good if someone could dig up the details.

Reply to
Aidy

The speed limits are set by automatic equipment that uses traffic density to predict traffic conditions and set speed limits intended to keep traffic moving freely. They cannot be left on in error. The Police wanted the option of overriding the system, particularly in the case of accidents, but were refused.

The main failing with the system is that the lowest speed limit it can set is 40 mph while maximum road capacity for moving traffic is achieved at around 20mph. It can, therefore, fail to stop preventable stoppages when traffic is at its heaviest.

Colin Bignell

Reply to
nightjar

Perhaps that means those variable limits can not bring penalties, however they are very effective. It may be counter-intuitive that lowering the maximum speed increases the average speed, but it really does work well.

Tony

Reply to
Anthony R. Gold

If you complain to what used to be the 'cones hotline' number they will tell you (eventually) why they were on.

Reply to
Scott

I do not know that this is definite but I have read that the limits are set to match the speed of the traffic a mile or two ahead, and that it is done semi-automatically.

I used to use the section in question before the variable limits were introduced and I must say I think they are a fantastic improvement and to me do seem to work quite well.

(But then I tend to obey the limits, keep my eye on the far distance as well as middle and near, don't do unnecessary braking, and don't think that all speed limits are a revenue earning exercise)

Reply to
pete

"nightjar .uk.com>"

Maximum capacity is achieved between 30 and 40 - nearer the latter if people follow a little too closely.

More than this and the inter car gap has to increase by more than the increased throughput for the speed.

Less and the length of the vehicles starts to predominate and the throughput falls because of the slower speed.

If you google you will probably come up with something.

Reply to
R. Mark Clayton

The message from "R. Mark Clayton" contains these words:

There's also the problem with shockwaves. The analogy with fluid flow in a pipes was given considerable treatment in New Scientist some years ago. Like fluid flow, all is well up to a critical speed, beyond which minor turbulence (like someone braking when they crest a hill and see a stream of tail-lights ahead) causes the flow to stall. I'm sure we've all had situations in heavy traffic where there's no apparent cause for the sudden queue, it just happens and just as quickly evaporates.

It's well proven that lowering the speed of flow to somewhat below the critical speed allows smooth flow to predominate, instead of turbulent flow. That's what variable speed limits seek (and generally succeed) to achieve.

Reply to
Guy King

"Dave Baker" wrote in news:f7npiq$fp8$ snipped-for-privacy@news.datemas.de:

And as you noted the traffic kept flowing, those of us who have to use the M25 on a regular basis will (I'm sure) agree that teh variable speed limit section is one of the few sections of this particular motorway that rarely come to a grinding halt and especially during the day the average speed along this length is probably higher than elsewhere.

The reason for this is because the controllers look many, many miles ahead and by using the variable limit reduce bottlenecks by preventing fast- traffic running in to the back of slower moving traffic and then breaking hard - causing a knock-back effect.

It really does appear to work .. most of the time, and when it doesn't it's usually because of traffic conditions ouitside of the variable zone. As for "authority" to change the speed limits etc, completely unrequired, the control room staff have a free hand. Records, yes and the cameras are automatically keyed in to the speed limit.

Reply to
Periander

Is it called MIDAS because it's run by donkeys or because it's especially good at collecting money?

-- x If you have been, were you touched? /|\

Reply to
Theo_Delight

I would be very surprised if they can, mince I would be surprised if your recollection is correct.

Reply to
Alex Heney

I'm sorry that is baloney.

There is some fat moron sitting at the contol room randomly pressing buttons on a whim for his own sexual pleasure.

We have this shit idea here on the M42 in Solihull, and it has not helped congestion at all, and in fact has caused it when the limits are randomly changed when there is no significant traffic and no particular need.

There could be no traffic and free flowing, and then you hit a queue when the limit is suddenly reduced to 50. And then the limit goes up to 60 and as you speed up, the next gantry says 50 so you slow down, and everyone is braking and speeding up for bloody miles.

This poxy idea is up there with the classics such as traffic lights on islands, and hatching four lane carriageways to make them two lane.

dg

Reply to
dg

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