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Gantry Gatso

M25 Front View.    Copyright © Steve Warren.

Above: Gantry Front View

M25 Rear View.  Copyright © Steve Warren.

Above: Gantry Back View

Yet another development - Gantry Gatso. This is basically a complete Gatso camera but mounted on a gantry. It has the same box on top of the pole as a normal Gatso, and hence works the same (radar, 800-frame roll of film. There are ruler markings on the road as well; the only difference is that the speed settings change with the variable speed limit.
The first picture is of what you will see looking forward, second is the view behind.

About one gantry in five has the facility to take live equipment, and the boxes on the other gantries are dummies. On live gantries they have mounted one radar per lane, but only one camera per gantry which is swapped from lane to lane periodically. When the speed limit changes so will the set speed that the Gatso will trap you at.

The system was developed to regulate the speed and flow of the Traffic during peak times, and in spite of predictions by some experts, it has improved the number of vehicles per hour through the section at peak times by ironing out some of the stop/start effect.

How the system works.

The CMIIS is/was a pilot scheme for a variable speed limit GATSO camera system put up for tender by the Highways Agency and won by SERCo in conjunction with GATSO. SERCo is a group multinational company, and various
different companies within the SERCo group were involved in the production of the unit, which encompassed mechanical engineering, hardware and software. Development started in August of 1994 and finished mid-1995.
The idea of the CMIIS was to increase traffic flow by slowing traffic coming into a gridlock or traffic jam to below the speed of the vehicles leaving the jam, hence dispersing the bottleneck and keeping the traffic moving. Being a pilot scheme, the idea was to see if this actually worked, and reports from the field indicate that in many ways it doesn't - the average speed of vehicles travelling along the section of the M25 covered by these cameras is lower than that of other sections.

However, many drivers report that they feel less stressed and traffic flow is smoother with less abrupt near-emergency braking, so it is perhaps a partial success.

The system was never intended to be a "speed trap" to simply prosecute drivers travelling over the national speed limit, however an amendment to the specification called for it to have this ability, and it subsequently became clear that the Police were more interested in this ability than its original purpose of smoothing traffic flow. It should be noted as a result that the system also enforces the national speed limit of 70mph as well as any lower limit that may be set. Therefore do not assume that because no signals are displayed you are safe from a possible ticket if you are over the limit.

The system is essentially an unmodified radar GATSO system (the type you're used to seeing already) with additional hardware and software for the variable speeds. The radar unit is a special narrow beam version devised for this application to make it lane-specific.It is housed in a bigger than standard box to accommodate the extra
hardware, and is mounted on a gantry over one lane of a motorway, looking down the motorway lane (i.e. at the backs of the cars).On the other side of the gantry (the side the motorists see as they drive towards it) is a matrix board that displays the speed limit. The box in which the system is housed (the enclosure) is made of double-skinned stainless steel and is locked with two high-quality Chubb locks. It also has an anti tamper intrusion system. The glass over the camera, flash and radar apertures is bullet-proof (although presumably not paint-ball proof). The system can enforce a number of speed limits. That is to say, it has a fixed number of speed limits rather than being constantly variable. The speed limits are 0 (a.k.a. red X or "lane closed"), 20, 30, 40, 50, 60 and NS (the national speed limit, currently 70mph). Each speed limit also has a user-settable threshold, so that the actual speed at which an offence occurs is the sum of the speed limit and this amount (e.g.. a 40mph limit with a 10mph threshold means it will enforce at 50+mph.). Generally the threshold is no less than 10%+2 of the speed limit, but this is merely a guideline and it can be anything from 0-99mph, and is entirely at
the discretion of the operator. The speed limit set is read directly from the same electrical input that sets the speed limit displayed to the motorist on the matrix board. There is also a fibre optic sensor on each bulb in the matrix board that is turned into an electrical signal so that the system can verify that the matrix board is actually displaying what it has been told to display.

When a new speed limit is posted, the CMIIS software does the following:
1, Starts a timer for a user-set "grace period", to allow the motorist time to react to the new limit. This is generally set at 5mins.
2, Checks that the posted limit is valid (i.e.. is one of RX, 20, 30, 40, 50, 60 or NS)
3, Checks that the optical output from the matrix board is exactly the same as the electrical input. In other words, if even one bulb that makes up the display of the speed has blown on the matrix board, it is invalid and the system will never
4, Performs a whole raft of crosscheck's, internal checks, etc., any of which can invalidate the system and cause it not to enforce. This includes an anti-tamper check on the enclosure.

Assuming that no fault conditions occur, and the speed limit doesn't change in the meantime, once the timer expires the camera system is instructed to start enforcing the enforcement speed (i.e.. the limit + threshold). Until such time as the speed limit is changed, or a fault occurs, or the camera runs out of film, it will take 2 photographs of any offending motorist in the same way as the ordinary GATSO cameras do. This is all handled by the GATSO system - the CMIIS simply tells the
camera to enforce, and the existing radar GATSO system does the rest. If the camera runs out of film it will enter "dummy mode" whereby it flashes twice to simulate taking 2 photographs (and make the motorist think they have been

The system is incredibly fault-intolerant. Any number of minor discrepancies (like that blown bulb) can cause it not to enforce. In fact the software and hardware are constantly at war with each other - the hardware is always trying to shut the system down and the software trying to convince it not to. It’s a wonder it takes any photos at all to be honest. It therefore follows that if it does take a photo its pretty certain it's valid!
When a photo is taken, a load of data is written to a log file on a PCMCIA flash RAM card. When the film is collected from the gantry, this card is also collected and replaced, and is an additional crosscheck to the information recorded on the film. If you've never seen a GATSO picture, the date, time, current speed limit, vehicle speed and camera location are all optically superimposed on the bottom of the picture, and this data is recorded in the log file too. The date and time are accurately maintained from the Rugby atomic clock (GPS was too new then). The source of the set speed limit is of no concern to the CMIIS. The Highways Agency was at the time evaluating a system called MIDAS which would use inductance loops in the road to measure traffic flow and automatically reduce the speed limit as traffic density increased, but this was entirely independent of the CMIIS. The CMIIS only cares about what the matrix boards are displaying, and the mechanism used to set the matrix boards is of no concern. Anyone who tells you that the CMIIS measures traffic flow and sets its own speed limits is misled.

The GATSO camera system used by the CMIIS follows the same rules as the ordinary radar camera system:
1, It is only calibrated up to about 130mph. However, exceeding this as an attempt to avoid prosecution is probably a bad idea.
2, It can distinguish vehicles travelling as close as ½ metre apart at about 90mph. This was determined by Police drivers driving closer and closer together at Millbrook Proving grounds. Very scary.
3, Two photographs of a receding vehicle are needed for it to be legally admissible. The camera system is capable of taking single photographs of approaching vehicles, but these serve only to worry, upset (and night-blind) motorists. Unless the law has changed since 1995, these photos are not legally admissible, and hence you cannot be prosecuted as a result of them.
4, The length of time it can enforce for is limited by an 800-shot bulk loader cartridge of 35mm film. This means the system can record an absolute maximum of 400 offences (2 photos per offence), not counting test shots taken during startup.



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