How do the M25 gantry cameras work?
Discussion
AFAIK basically standard gatso so will flash twice.
Road speed is measured and computer controls speed limit as it feels appropraite. Activitates speed limit in 10mph steps. Gatso pre-programmed at this speed plus X. Each speed limit has its own X. THere is also a pre - programmed time limit from change of limit to camera's becoming live.
Road speed is measured and computer controls speed limit as it feels appropraite. Activitates speed limit in 10mph steps. Gatso pre-programmed at this speed plus X. Each speed limit has its own X. THere is also a pre - programmed time limit from change of limit to camera's becoming live.
bluepolarbear said:
AFAIK basically standard gatso so will flash twice.
Road speed is measured and computer controls speed limit as it feels appropraite. Activitates speed limit in 10mph steps. Gatso pre-programmed at this speed plus X. Each speed limit has its own X. THere is also a pre - programmed time limit from change of limit to camera's becoming live.
That's the thing I never see any of these flash (and you'd expect to see these for miles) - passed one tonight at what thought was X% over limit (was knackered after driving up for the Northants PH run!) and no flash?!
No, not SPECS if they're the one's that measure over a distance
bluepolarbear said:There are very good reasons to believe that while the minimum 'on paper' time for a camera becoming live or using a new catch speed, after a speed limit change, is something like 10 seconds, in practice a 1 minute delay is used.
THere is also a pre - programmed time limit from change of limit to camera's becoming live.
Going back a bit now, but I used to go under them at well over the ton late at night and never set them off. But when the reduced speed limit is in operation, they are armed - my brother got done twice for relatively minor excess speeds.
Maybe someone has more up to date info, but I never 'managed' to set them off when there were no traffic density speed restrictions in force, despite trying quite hard (foreign registered car
)!
Maybe someone has more up to date info, but I never 'managed' to set them off when there were no traffic density speed restrictions in force, despite trying quite hard (foreign registered car
)!nel said:
Going back a bit now, but I used to go under them at well over the ton late at night and never set them off. But when the reduced speed limit is in operation, they are armed - my brother got done twice for relatively minor excess speeds.
nel, right, I too thought they were only armed when the speed limit sign is lit but do they flash (or are they perhaps infrared), cause this one didn't?!
Horses mouth stuff now - I wrote the control software for this system about 10 years ago.
Here is a reproduction of a short essay I wrote on the subject:
So there you go.
>> Edited by JonRB on Sunday 5th December 19:48
Here is a reproduction of a short essay I wrote on the subject:
JonRB said:
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. However, it is unclear in what capacity the system is now being used.
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. 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 (ie. 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 (eg. 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:
a) 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.
b) Checks that the posted limit is valid (ie. is one of RX, 20, 30, 40, 50, 60 or NS)
c) 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 activate.
d) Performs a whole raft of cross-checks, 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 (ie. 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 caught).
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:
· It is only calibrated up to about 130mph. However, exceeding this as an attempt to avoid prosecution is probably a bad idea. :-)
· 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.
· 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.
· 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.
So there you go.
>> Edited by JonRB on Sunday 5th December 19:48
amg merc said:
Going back a bit now, but I used to go under them at well over the ton late at night and never set them off. But when the reduced speed limit is in operation, they are armed - my brother got done twice for relatively minor excess speeds.
nel, right, I too thought they were only armed when the speed limit sign is lit but do they flash (or are they perhaps infrared), cause this one didn't?!
Nope - have only seen them flash for other drivers going the opposite way. My late night forays at speed have never provoked a flash.
Pigeon said:May be wrong but think that now, Home Office guidelines specify one minute. Edited to add - that's a minimum.
JonRB - am I reading correctly that during the 5-minute period after the speed limit change, no enforcement photos are taken at all?
>> Edited by turbobloke on Monday 6th December 00:23
Pigeon said:You are correct that during the "grace period" no photos are taken, but it is not necessarily 5 mins. In fact, I think I should amend my essay to remove that figure since it probably bears no relation to today's reality any more.
JonRB - am I reading correctly that during the 5-minute period after the speed limit change, no enforcement photos are taken at all?
Thanks for your comprehensive response JonRB - it never fails to amaze me the breath of knowledge and experience contained within the PH membership!
I read your article with interest and can only asume that, as there was no flash (dummy or otherwise, though i wouldn;t know), the system above my lane was in error mode - of course, the real test will be whether anything drops throught my door in the next few days! ;-)
I read your article with interest and can only asume that, as there was no flash (dummy or otherwise, though i wouldn;t know), the system above my lane was in error mode - of course, the real test will be whether anything drops throught my door in the next few days! ;-)
Yes, it is a bit silly isn't it? 
Having said that, the camera itself is something expensive like a Hasselblad (sp?), so it could just be to protect the camera from damage.
There were a few other quirks that drove me up the wall. The guy who designed the hardware certainly made my job more difficult. The RS232 data cable between the GATSO camera and the micro-PC running my software went through the main power distribution box with no screening other than provided by the standard cable and the GATSO system didn't support handshaking, flow control or repeats. So if there was any sort of data corruption I couldn't say "sorry, can you say that again?" to the GATSO despite the cable going through an awful lot of RF interference in the distribution box. Madness!
Still, it somehow seemed to all work, but could have been designed slightly better, I reckon.
All seems a depressingly long time ago now.
Having said that, the camera itself is something expensive like a Hasselblad (sp?), so it could just be to protect the camera from damage.
There were a few other quirks that drove me up the wall. The guy who designed the hardware certainly made my job more difficult. The RS232 data cable between the GATSO camera and the micro-PC running my software went through the main power distribution box with no screening other than provided by the standard cable and the GATSO system didn't support handshaking, flow control or repeats. So if there was any sort of data corruption I couldn't say "sorry, can you say that again?" to the GATSO despite the cable going through an awful lot of RF interference in the distribution box. Madness!
Still, it somehow seemed to all work, but could have been designed slightly better, I reckon.
All seems a depressingly long time ago now.
When I say "standard cable" it may of course have been extremely expensive cable - I have no idea.
I just thought it was exceeding crap design that it shared a distribution box with AC power and other sources of RF interference.
For all I know it could have been an extremely elegant design and I certainly didn't get any data corruption, but it seemed a bit strange and counterintuitive to me.
I just thought it was exceeding crap design that it shared a distribution box with AC power and other sources of RF interference.
For all I know it could have been an extremely elegant design and I certainly didn't get any data corruption, but it seemed a bit strange and counterintuitive to me.
Interesting stuff John.
One question - I understood that the cameras sent their data electronically to some central control centre to save the bobbies having to climb the gantry and replace the film/reset the camera etc. (I guess this would mean digital film too). From what you're saying these Gatsos are basicaly no different to ones we see elsewhere ...
One question - I understood that the cameras sent their data electronically to some central control centre to save the bobbies having to climb the gantry and replace the film/reset the camera etc. (I guess this would mean digital film too). From what you're saying these Gatsos are basicaly no different to ones we see elsewhere ...
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