Safe Speed Forums

Ok, I understand how speed limit signs work (terminators and repeaters), but what is the case with the variable speed limit signs on overhead gantries on the M25?

The reason I ask - last night I was on the M25 and the overhead signs were showing . The traffic was flowing quite freely at about 50 to 55 (perhapsa couple of cars were going faster, but there wasn't a lot of bunching and tailgating going on). As I went under one gantry the signs on the gantry ahead (about 500 yards away?) flashed to , then a couple of seconds later flashed to .

I couldn't see the gantry I was under so I can only assume that they changed around the same time. It was no great trouble to gently slow to 40 before the next gantry anyway, so I was doing 40 (by satnav speedo) by the time I reached the signs. Again, the rest of the traffic around seemed to do the same thing.

What are the 'rules' for these signs? Would I be expected to slow to 40 as soon as possible when I saw the speed limit change - i.e. using brakes - or to do as I did and slow using nothing more than engine braking so that by the time I reached the signs I was within the posted limit? I am assuming that these signs operate as equivalent to terminator signs rather than repeaters, it was only afterwards that I made the mental jump to realise that the signs on the gantry above my head probably changed at the same time.

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COAST Not just somewhere to keep a beach.

A young loner on a crusade to champion the cause of the innocent, the helpless, the powerless, in a world of criminals who operate above the law.

I understand from a Police Officer that there's a 2 minute moratorium on prosecutions after a speed limit change.

I definiately would not brake. Adjust speed gently and smoothly.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

The VSLs that you posted (red ring) are absolutely mandatory. Once these signs have changed you must safely apply correct speed. The speed can change between gantries, and thus you are allowed a certain amount of time.

* If you approach a sign that is 50 or it has just changed to 50, you have time in your approach to adjust your speed. Once you have passed the sign, you have 10 seconds to ensure you are doing the correct speed. After 10 seconds is legally enforcable.

* However a general rule is to allow 1 minute after passing the sign, and after that point it will be legally enforcable.

The M25 VSLs are a complete mess at the moment. I don't know how the limit is set but it can vary rapidly and for no apparent reason from one gantry to the next. Yesterday I passed successive gantires which read 'derestricted', '60' '50' then 'derestricted' again - all in the space of a mile or two.

The limit is often set automatically by the MIDAS (Motorway Incident Detection Automatic System) which detects rates of traffic.

Vehicle detection loops are installed in the road surface to detect slow moving or stationary traffic on the motorway and MIDAS sets message signs and signals to warn traffic approaching the hazard ahead.

This is also installed on the M42, hence the VSLs which, like the M25, is (or will be) a Controlled Motorway. The only difference being that the M42 will have hard shoulder running and the M25 does not.

There are times when the signs and signals will be set by operators working from the Regional Control Centres, e.g. 3 Vehicle RTI in L3 - this will involve constant and accurate signal setting thoughout the incident.

related thread:
http://www.safespeed.org.uk/forum/viewt ... 5048#55048

Didn't you see the original "The Italian Job" film, where the traffic control centre is knobbled by a bad magtape (probably had a recording of Britney Spears on it).
Perhaps the Mafia have come to London to extract revenge...