I’ve heard a lot about size sensitive speed cameras that can tell the difference between cars and trucks etc and flash according to the vehicle and its respective speed limit. What confuses me slightly is the ability of such cameras to tell the difference between a 7.5ton truck and a 12 ton truck, which could be very simlar in size, though the 7.5 tonner can ofcourse go 10mph faster on all national speed limit roads, yet whenever I’ve driven a 7.5 tonner past endless speed cameras, doing 50 on a single A road or 60 on a dual carriage way, I’ve never been flashed and was wondering how these cameras actually work?
The registration of the vehicle will tell the operators if it;s 12 tonnes or 7.5 tonnes. This could be checked automaticaly at the ca,era site, but I think it’s done at base once the photos developed, Not sure whether the camera checks by height or axle sensors in the road.
I thought they were weight sensitive not size sensitive, I could be wrong though
simon
SimonRS2K:
I thought they were weight sensitive not size sensitive, I could be wrong thoughsimon
So did I, I thought there was something under the road just before the camera to do this.
A303 has cameras that can tell the difference,there is 1 inparticular on a steep hill so if you are heavy you try to get a run up,camera is half way up hill and i have seen it get trucks going over 40 
This is definetly news to me, regarding the cameras being weight sensitive. I’m wondering that the two on the A15 between Lincoln and the M180 are because I don’t remember them ever digging up the road or seeing any sort of working going on there and I went down there every day in the period when they were both installed. One of these cameras got my dad doing 49mph, so maybe its set just for HGV’s? Unless whatever weight sensors they do have, don’t involve the need for roadworks, or I just missed them doing it.
I can’t see how they can be weight sensative, surely an unladen 12 tonner weighs less than 7.5 tons.
The link below claims GATSO’s can differentiate but doesn’t say how:
There are both weight and size (height) sensitive cameras.
Think of it from the ‘cash machine’ operators point of view; it’s better to have a photo of a large vehicle (for example on an A road) going over 40mph that might (after a VRN check) get some money in their coffers than to have missed the opportunity altogether.
If the DVLC computer comes back with data that shows the vehicle is a <7.5t type and therefore not breaking the 40mph limit, it’s no great problem for the ‘cash machine’ operators to disregard that particular photo.
I don’t know for sure how they know the difference but I do know that it would be a simple process to get it to know a large vehicle from a small vehicle.
They operate by emmitting a beam of energy, if there is nothing there the beam keeps on travelling, if there is something there the beam is reflected back to the camera and a calculation is made to test the speed.
The camera just has to be able to analyze the amount of reflected energy to calculate the size, the larger the vehicle the more energy is reflected. ( That’s how they knew the size of bomber raids in the second world war ).
As I said I am not sure but the technology to do this has been around for years and as far as I can see there are no marks that show the road has been dug up in front of most cameras that catch you from behind.
I believe speed cameras work on a magnetic field. The larger the target the slower the limit, If the camera takes a picture of every large vehicle doing over 40, it doesnt take Einstein long to work out if it is a 7.5 tonner or an Artic.
Dont forget Long Vehicle Signs differ between light trucks and top weight trucks
bikemonkey:
A303 has cameras that can tell the difference,there is 1 inparticular on a steep hill so if you are heavy you try to get a run up,camera is half way up hill and i have seen it get trucks going over 40
the one at the hill is at the Ilminster part as is well known down this way and yes it has the two speed setup
The thing is, I often drive a 7.5 ton vehicle at 50mph down a single carriage way road and I have NOT been flashed at all, its not like I’ve been flashed and they’ve found later that it was a 7.5, I have never been flashed in the first place. Obviously a camera can tell between a 7.5 and a 44t truck but how does it tell between a 7.5 ton truck and a smallish rigid that can only do 40mph?
It’s a while since I’ve seen any of these now but there are some size related Gatso’s around that have a second sensor mounted on top of the camera. How it works is that it sends another beam but this second one is directed across the road at 90degrees to the road. It goes something like this.
If the 90deg beam isn’t being reflected then the camera sits watching for the higher speed limit. If vehicle higher than the sideways sensor goes past then the camera drops the trigger speed to the lower one. The thing to remember is that the vehicle normally (with the exception of truvelos) passes the camera before it enters the speed beam.
This does mean that drivers of larger vehicles get flashed irrespective of the weight of the vehicle but this error is supposed to be checked by the processing authority before the penalty is issued. The last time we had this discussion a chap said he’d been sent one for a 7.5tonner doing 50 on a national-derestricted road. His boss sent the form back together with a copy of the V5 and ticked the ‘not guilty, take me to court’ box. Nothing more was heard.
I don’t think it would be too difficult to hide this second sensor within the main body of the camera so that the casual passer by wouldn’t be able to tell whether it was a height sensitive camera.
Just going slightly off this topic, I was on the A13 Eastbound on Friday near Leamouth Road / Canning town. As I approached the camera on my side it flashed a vehicle on the opposite carriageway. There was nothing else on the road 
It seems that 8 wheel tippers are exempt from the cameras in East London though 
I pulled into a layby on the LLanelli-Swansea Road where a mobile camera was…
Not wishing to cause any upsets, i went and chatted to the officer and explained i needed to park for a 45min break, he replied no probs driver, i will reposition my van…which he did, if i had then moved my truck to obstruct him, i would get a ticket.
We got onto the topic of HGV’s and speeds…He told me the black strips you see in the roads work on air pressure. All vehicles will compress these strips, they work out a vehicle by its weight and the fact an artic will have several “footprints” as it crosses the strips.
The local council will then use this data to position its “safety camera”…
I’ve also seen a camera flash on the opposite side of the road, with no traffic. These are supposedly set up on a timer, to operate at randon. This “fools” the drivers into thinking the camera is active…
I was once speaking to a guy who was a service engineer for speed cameras. He told me if you look on the back of a Gatso camera you will see two holes with a slider between them. This is a height sensor.This is coupled to weight sensors in the road.If the slider is in the up position,the sensor is disabled.
Problem is ,you need pretty good eyesight to see if the slider is up or down as you approach the camera,but thats a whole different subject 
I don’t know about timers setting camera’s off Logistics Loader, but I have noticed quite a few camera’s flashing when vehicles are exceeding the limit in the opposite direction. It happens frequently on the A40 near to Hangar Lane and is most prominent at night when there is often spells of inactivity in the direction the camera is facing and the camera’s seem to be triggered particularly by vans travelling in the opposite direction (because they are higher perhaps?).
I think all prosecutions from camera’s in this state should be cancelled as they are obviously not working correctly. 
Highlander:
I was once speaking to a guy who was a service engineer for speed cameras. He told me if you look on the back of a Gatso camera you will see two holes with a slider between them. This is a height sensor.This is coupled to weight sensors in the road.If the slider is in the up position,the sensor is disabled.Problem is ,you need pretty good eyesight to see if the slider is up or down as you approach the camera,but thats a whole different subject
Old Wives’ Tale I’m afraid.
As someone said above, they can tell the difference between trucks and cars, but only on some of the gatso’s, not all of them by any stretch.
They do work via the induction loop in the ground at the side of them. If you look on the ground next to the gatso’s on the A556 at Tabley you’ll see like a grid on the road. If your wagon “foot print” exceeds 7.5 tonnes across the loop then that’s relayed back to the gatso and when you go past it measures if you’re exceeding the goods vehicle speed limit for that road.
There are only a couple of the gatso’s on the entire stretch of the A1 that are weight sensitive. The rest are just set for normal national speed limits. The same applies on the A420 from Oxford to Swindon; I think there’s only one which is weight sensitive. The ones on the A14 at Cambridge don’t have them but I did see a Dutch wagon get flashed today going the other way; must have been going some! 