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I'm not sure if this the right place to ask, but I think it's still related here.

Increasingly, I've been noticing more and more rectangular cut-throughs in the tarmac of UK roads. Usually, they are before traffic lights, and located in pairs, about 1 meter apart. I believe they simply cut the tarmac with a circular saw to create a large enough rectangular wire frame, and install an RFID reading coil in the cut-through, then patch it over. And there's always a single cut that goes to side of the road, which must be a way to connect an antenna cable to whatever electronics are installed at the road side.

I think it's somehow related to whatever is being called Smart Motorways, but I can't find any mention of RFID technology use. Also, I tried googling it, but can't find any articles talking about such cutouts.

I was driving recently and on one stretch of motorway undergoing Smart technology installation it was said that they are installing vehicle recognition technology. That only confirms my suspicion of RFID use.

Thanks to everyone who can share any info.

psmears
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zmechanic
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    Do you have a citation for why you suspect it to be RFID (specifically radio frequency identification)? The use of coils to simply detect large pieces of metal (i.e. cars) based on simple changes to inductance/loading is already well-understood and well-used by traffic engineers in the US. In my home area we don't even call that a "smart road" but just a "road" with a sensible traffic light that only cycles when a vehicle is waiting at red. – nanofarad Apr 20 '22 at 21:29
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    It is only a vehicle presence detector, no RFID identification. May be used to switch the traffic lights to green only when a vehicle is waiting. – Uwe Apr 20 '22 at 21:30
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    You're seeing an upside-down metal detector to sense the presence of cars for the traffic lights. The transponders that actually read data e.g. from toll tokens/stickers look like little plastic cones or plates that are above or to the side of the roadway. – vir Apr 20 '22 at 21:32
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    RFID would require each vehicle to have a transponder, such as with the E-ZPass system.

    https://en.wikipedia.org/wiki/E-ZPass

     – GodJihyo Apr 20 '22 at 21:48
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    It's possible your region is adopting magnetic loop sensors now, but these are a very old technology. The wiki article says they started using them in the 1960s. Certainly ubiquitous in my area the last 30 years. – user1850479 Apr 20 '22 at 21:57
  • Note that tire pressure sensors have unique IDs, so vehicles could, in theory, be uniquely identified, even without "special" transponders. Which wouldn't make much sense without some database matching those IDs to vehicles, or owners, however. – Guntram Blohm Apr 21 '22 at 09:50
  • "Smart" Motorways are a separate thing. From what I've seen, they mainly involve the creation of a high risk of death by opening up the hard shoulder to traffic. I'm not sure why this was ever permitted, or why anyone ever thought it was a good idea. – Aaron F Apr 21 '22 at 15:28
  • Electroboom did a video about this a while ago. – rodrigo Apr 22 '22 at 08:54
  • Tyre pressure sensors can and have been used to log vehicles when doing traffic surveys, providing transit times through a certain region but without using ANPR or other identification means linking the sensor to the vehicle is hard. – uɐɪ Apr 22 '22 at 10:33
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    @user1850479 - Yep, they've had these in the US since 1968, at least. – Hot Licks Apr 22 '22 at 21:38

3 Answers3

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They are inductive loop sensors, similar to a metal detector. They sense when a vehicle is at the light and alter the light's timing accordingly. You are correct, they cut into the road surface and put a coil of wire in, the cut going off to the side of the road is simply a cable to connect it to the light controller.

Some smaller vehicles such as motorcycles can have trouble tripping them, I've heard of people putting magnets on the bottom of their bikes but I don't know if that actually works or not.

Smart motorways also make use of these for vehicle detection, along with RADAR. See this article for more information.

For identifying individual vehicles either an RFID system where each vehicle carries a transponder can be used, such as E-ZPass, or ANPR/ALPR can be used to read the vehicle's plate using cameras and OCR technology.

GodJihyo
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  • Interestingly, the article says that radars are seemingly cheaper to install and maintain, but cutouts are almost predominantly being used. – zmechanic Apr 20 '22 at 21:52
  • +1 And, when used in pairs as described in the question, they also work with "red light advance" cameras and speed cameras. – devnull Apr 20 '22 at 21:52
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    @devnull, yes, speed can be measured, but how to catch the offender if there's no camera to do ANPR? Unless they issue RFIDs to every vehicle once they install all the antennae. – zmechanic Apr 20 '22 at 21:58
  • @zmechanic The cost of either kind of sensor seems to be a wash in practice and highly depends on traffic, weather damage to pavement, pavement quality, etc. On high quality roads with competent installers the loops can last almost forever, or at least as long as the surface of the road. On softer, more worn out roads, they get destroyed awfully quickly, and the camera and radar sensors are a win. The radar and camera also provide more information for traffic monitoring, research, etc. – Kuba hasn't forgotten Monica Apr 20 '22 at 21:59
  • @zmechanic Sure. As I said, they are used with cameras for these two purposes all over the city where I live. There is quite enough resolution/image quality to perform ANPR. – devnull Apr 20 '22 at 22:03
  • @devnull the thing is, in London area they are used without cameras. – zmechanic Apr 20 '22 at 22:08
  • @zmechanic About 20 years ago they tried to use the same type of sensors to measure traffic flow (without cameras) for a "smart city" project. It lasted a few years and later the system was replaced by another based exclusively on cameras. And now they are trying to use also these cameras to catch other infractions... – devnull Apr 20 '22 at 22:10
  • The tarmac at intersections is often quite deformed because of all the braking and accelerating. How do these wires deal with it? Are they made out of thick steel to resist the forces? – Michael Apr 21 '22 at 08:18
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    They are most likely inductive, not magnetic. – kackle123 Apr 21 '22 at 21:40
  • @zmechanic - The radars are cheaper to install and maintain but are a recent development and so loops are still much more common. In the UK motorways have a loop in each lane approximately every 500m and used to monitor the traffic flow. An example of the radar detector is the AGD 343 which replaces a set of loops in a number of lanes. The loops are fragile and prone to breakage due to the heavy traffic loads that pass over them. – uɐɪ Apr 22 '22 at 10:25
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It is not RFID, those coils are basically metal detectors.

See page 59 of this https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/851465/dft-traffic-signs-manual-chapter-6.pdf for some discussion in the context of the control of traffic lights.

Vehicle recognition typically means ANPR (Number plate reading cameras).

Dan Mills
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  • I don't think it's entirely correct. Before the rollout of this cutouts, most UK traffic light junctions were and still are equipped with motion detectors. They are small boxes mounted on top of the traffic light. And they work perfectly fine. More over, those cut outs are in the motorways stretch, which has no traffic lights at all. I can imagine they can use it to measure the average speed of traffic flow, but why not to use cameras? Still need electricity and using AI they can achieve more with cameras then just with dumb "metal detectors" – zmechanic Apr 20 '22 at 21:44
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    Could be privacy issues if the camera feed is being recorded or transmitted; maybe the motion detectors don't work as well as advertised or cost more to maintain; maybe they're reaching the end of their service life and they decided to use a new system. – vir Apr 20 '22 at 21:52
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    @zmechanic Camera systems work extremely well when the camera housing and processing electronics are top-notch. The inductive loop sensors are more immune to environmental concerns that the cameras have real trouble with: accumulation of pollution on the window, obscuration by precipitation/fog. Microwave radar sensors are slightly less robust than the processing electronics of the inductive loops. The loops had a long time to mature. Their electronics are solid now – not so when they first came out. – Kuba hasn't forgotten Monica Apr 20 '22 at 22:00
  • @zmechanic, induction loop detectors have been used in the UK for longer than microwave (radar) detectors - certainly since before I started driving in the 1980s. Perhaps you didn't notice them if your area previously installed the loop before laying the asphalt surface, rather than laying the surface first and then cutting into it as now? (I suspect the latter approach is more reliable, as it avoids heat damage to the coil, but it's more labour-intensive, and therefore more expensive) – Toby Speight Apr 21 '22 at 07:17
  • Cameras have lenses and England has weather, two things that absolutely do not play together well. Also, sensors away from intersections often are used to modify the timing of lights at the next intersection based on upcoming traffic. – AnalogKid Apr 21 '22 at 11:54
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The cut-outs you see on motorways are inductive loops used in the MIDAS scheme (Motorway Incident Detection and Signing). The original system used the loops to continuously monitor traffic speed and density - if these met certain thresholds, signs would be set to tell to drivers that there was congestion ahead and to slow down. The scheme evolved into Smart Motorways, where if congestion was beginning to occur, speed limits would be reduced with the hope that the congestion would not get worse. On some motorways, the hard-shoulder would be opened to provide an additional running lane. See https://en.wikipedia.org/wiki/Motorway_Incident_Detection_and_Automatic_Signalling

Andrew Battye
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