Technology Insight - Inductive Loops
Hey everyone! In this Technology Insight episode of ITS Now, we are looking at Inductive Loop vehicle detection, which is probably the most important individual technology used in ITS applications, because it is used so widely in applications as diverse as traffic signals, incident detection, vehicle classification, presence detection etc. I think it is probably worth stating that these are inductive rather than induction loops, which they are sometimes erroneously referred to as. The later are actually used to wirelessly transfer energy, so within the ITS sphere, these are starting to be used for wirelessly charging electric vehicles, but no you will not be able to charge your shiny new EV off the detection loops at a set of traffic signals! Anyway, I’m Alistair, and this, is ITS Now. Inductive loops have been utilised extensively since the 1960’s across a wide range of uses, including traffic signals, vehicle count sites, access control and incident detection. Although newer technologies have been introduced over the years, loops are still considered to be highly reliable, and are the standard against which other methods of detection are compared with. Many will be familiar with the outline of vehicle detection loops in our roads, a thin black line in the tarmac. Loops come in a variety of shapes and sizes depending on their use and the standards of
the country being used in, but include rectangles, chevrons, circles, diamonds and even butterflies! In the UK, the requirements for inductive loops is set out in the Manual of Contract Documents for Highway Works, or MCHW for short, Highway Construction Details ‘G’ Series drawings. The set of 32 drawings provide details of the various construction elements required for different inductive loop detection applications. Further technical guidance and links to other documents is given in MCH1540, the specification for the installation of detector loops. Also, the Federal Highway Administration Traffic Detector Handbook covers this. To install a loop, a slot is cut in the required shape and dimensions for the desired detection functionality. A connecting slot (cut back) to the edge of the carriageway is also required to allow the loop to be connected. It is essential to ensure that the
slots are clear of debris before the cable is installed and there mustn’t be any water in the slot when they are backfilled, this is normally done using a handheld leaf blower. A specialist insulated flexible wire (Hypalon) is then wound around the ‘loop’ slot 3 or 4 times (dependent upon the size, required sensitivity and function of the loop) then the ends (loop tails) are typically wired back through the cut back to a jointing chamber situated in the verge or footway. It is important to ensure that the loop tails are twisted, so that the tails do not detect vehicles or other metallic objects passing in their vicinity.
The slot is then backfilled to protect it from damage, often part filled with an epoxy resin, then finished with a bitumous hot-pour sealant on top. Although inductive loops provide a reliable means of accurately detecting vehicles, their design and installation needs to be carried out carefully to ensure that the equipment provides a reliable level of service. An assessment of the carriageway condition should be carried out to ensure that it is good enough for loops to be installed in to. Evidence of issues
such as ‘tram lining’ (where the road surface has been compacted by the wheel tracks of vehicles), differential subsidence, heave or cracking and breakup of the wearing course may result in a reassessment of the method of detection to be used or for the carriageway to be re-surfaced. It is worth noting that in some locations, including the US, some loops are pre-formed and installed in the roadbase, before the wearing course, the top surface of tarmac, is rolled out over them. Stick down loops are also sometimes used in situations like concrete multi-story car park structures, where it isn’t possible to slot cut the loops into the carriageway surface. However, these wouldn’t stand up to enough wear for highway applications. The wire in the pit is jointed to a feeder cable, this was traditionally done using a chemical or heat-shrink joint, but increasingly this is now done using a lightweight plastic bottle joint. The feeder cable is fed through the duct system to the TSC or equipment cabinet where it is connected to a detector pack. The feeder cables contain twisted pairs of cables,
usually in an earthed steel wire armoured sheath to provide both mechanical protection and to minimise electrical interference. Once switched on, the loop coil emits an inductive field, which is disrupted when a metal object passes through it, such as a car or lorry. The detector pack processes information regarding the changes in the signal it receives and when an anomaly is detected, an output is generated to indicate the presence of a vehicle. (The process involved is essentially similar to that used by a handheld metal detector, but in this case the sensor is buried in the ground and is affected by metal objects passing overhead). Although this technology has had widespread use since the 1960’s, it is still seen as the baseline that other detection technologies are measured against.
To show you how this works, I have set up an example to demonstrate an example of a single loop. So, what we've got here is this loop of wire going round. I've set them around four cones, so it's reasonably taught, and this is a three flex ordinary electrical wire, so it isn't what you'd use in an actual installation, that would use something called Hypalon wire, which is a quite a thin, black, flexible wire, which would actually be laid into the slot. Most loops use three turns in them. So that's the the
loop itself and then here you see the loop tail, this black and white cable which is twisted, and this would normally go back into a joint in the verge, in the chamber in the verge and then take them back using a loop feeder cable, back into the controller cabinet or into a roadside cabinet of some sort, with a detector pack in. So, here we've got two four channel detector packs, we're using this one and you can see the lights on here, which have been switched on so that you can see them. So, we're only using one channel at the moment, so this loop is actually coming into channel one here, and we'll see that going in a minute. These are standard types of formats that are used for detector cards, so the interface on the back then going to a traffic signal controller or a count site, or whatever, those bit patterns are all defined, so you can actually use different brands of detector packs, without any hassle. So, here I've just got this on a power supply, but normally, as I say, this would all be within a roadside cabinet. So, anyway, we've got the loop tail, so what I thought I'd do here, I've got an electromagnetic detector, it seems to have just faulted! Here we go, right, so here you can see how those lights are continuously monitoring, so that shows that there's an electromagnetic field here that its detecting. If we go and do the same thing with the loop tail now, as you can see that doesn't happen,
and that's because of the twist on here, so that's why it's so important that the the tails from the very point where at the corner of the loop, is that they are twisted all the way back in the cutback slots, all the way into the chamber, to where they're actually jointed, because otherwise, they'll pick up either vehicles going over the cutback or just random noise, which means that the whole installation would be no good basically, it would be unreliable, it would be double counting, adding extra vehicles for other lanes etc. Anyway, so how does this work, so it's basically like a metal detector, only instead of looking for metal in the ground, we're looking for a car basically, going over the top of this, so what I'll do is see that LED coming on, as I push this through this field, and this extra light comes on, you can probably hear it as well, clicking on and off. And that's all there is to it, this is more or less the the size of an ordinary Chevron Loop. I've just set it out square because it was easier here, but obviously the shapes differ, you can get round ones and different sizes and shapes etc, but that's the basic way in which these things work. Next, we’re going to look at an installation of motorway Incident detection loops to show you how these are typically installed. You will see that this consists of two loops in each lane, with cut-backs to the verge to allow these to be terminated.
The installation of an inductive loop will normally require the lanes either side of where a loop is being installed to be closed. This is because the footprint of a loop stretches across most of the effected lane and the slot cutting machines are also quite wide as well, so additional clearance is required to protect the installation operative whilst undertaking these tasks. On multi lane installations like the example we are going to show you, the cut-backs from the outer lanes also cross the inner lanes to reach the termination point in a chamber in the verge, so it isn’t really practical or safe to try to maintain a live lane, so this example was done under a full closure. It can therefore be difficult to get an opportunity to replace loops, so regardless of use, these are often done at night when traffic volumes are lighter but adding considerably to the cost.
As we saw in the video, testing of the loops is essential. The generic test requirements for loops is given in drawing G6, although many highway authorities have their own versions of this. Tests are normally undertaken at the end of the loop tails before they are terminated to the feeder cable, then once this is done they are also tested at the far end of the feeder cable in the equipment cabinet where the detector cards are housed. The electrical tests include measuring the resistance and inductance of the loop then loop and feeder cable, followed by a series of resistance measurements to earth. The length of the loop tails and feeder cables are also measured to ensure that the results of the electrical testing is accurate. Measurements of the distance between the leading and trailing edge of the loop, and if installed as a pair of loops in a lane, the distance between the two loops may also be required to ensure that the system will interpret parameters such as vehicle speed and vehicle length correctly.
The actual location of the loop may also need to be measured, MOVA loops for example, it is important to record the distance to the traffic signal stop line because the dataset needs to know what this to accurately time the signal phases. The From Stop Line distance (referred to as FSL for short) is measured from the nearside leading-edge corner of the loop, not the distance between the nearest edge of the loop to the stop line. Once installed, terminated and operating, the setup of the detector card needs to reflect the requirements of the particular use. Different manufacturers have varying means to set these, but typically on four channel cards, each channel is set to minimise cross talk between adjacent channels, and the sensitivity of each input is adjusted to minimise false triggers, whilst also allowing more difficult to detect vehicles, such as bicycles, to be detected reliably if needed. It is also essential to ensure that the loops are terminated into the correct channel inputs of the detector cards. If there is a road or lane closure, it might be possible to drive a vehicle over each loop in turn to verify this, but if the site is open to traffic, it is much harder to achieve accurately. In this situation, it is necessary to have a ‘spotter’ located
adjacent to the loop being verified, and using a handsfree mobile phone or radio, tell the engineer at the cabinet when vehicles are actually passing over the loop. The engineer will then need to ensure that the operation they are seeing on the detector card panel matches the information the spotter is calling out to them. If it doesn’t, they will need to investigate what is going on. Please remember to click on the Like and Subscribe, to keep updated on ITS Now! At the corners of loops, a small intersecting slot, called a crosscut, is often used to reduce the strain on the loop cable by removing the sharp internal corner that could possibly damage the Hypalon cable. It is important that the crosscut is accurately located at the junction point of the two side cuts and doesn’t leave a small area of tarmac between them, these are referred to as ‘floaters’ because over time there is a tendency for these to break away and cause a pothole to form at the corner point of the loop.
One of the weakest parts of a loop installation is the transition point, where the loop tails go from the carriageway surface to the verge or footway. It has been common in the past to take the loop tails back to the jointing pit by cutting them directly through the kerb or into the verge, but this can leave the tails vulnerable to damage and weaken the kerbing. The design should therefore incorporate a reliable means of making the transition from the carriageway into the verge or footway, such as using a carriageway loop tail box. These are used by cutting the loop tail directly into a ductile iron box set into the carriageway surface. The tails are then routed through a flexible conduit
in the bottom of the box, which connects to a pit in the verge, by running under the kerb. The tails are then jointed in the pit to the loop feeder cable. Whilst loops are being slot-cut, it is essential to ensure that the slots are cut to the required depth, contractors may try to cut these too shallow, in order to minimise the time required. A number of issues can occur if the slots are not deep enough that would lead to premature failure: • the backfill is more likely to come away, exposing the loop cable, which will result in the cable being worn and broken • the loops will be destroyed by ‘planing out’ during resurfacing works During the life of a loop installation, damage to the road surface can result in the loop not working. Damage tends to be caused by two issues, the first are utility companies,
who are notorious for trenching through loops, whilst carrying out works. The second is caused by the deterioration of the carriageway, breakup of the surface will result in the loops becoming exposed and damaged, and tram lining (differential compaction) can result in the loop wire becoming stretched and subsequently breaking. Installed properly, inductive loops will provide a reliable, long lasting and effective means of detecting vehicular traffic. They don’t suffer from the
obscuration or climatic issues that effect most forms of verge mounted above-ground detection. I think this episode shows the huge array of information that relates to an individual topic area such as indictive loops, in reality there is probably enough content to make a whole series on this subject alone. If you did find this useful, have a look at our books in the publication page of ITSNow.org. Thank you to Crown Highways for their assistance with making this episode, if you’d like to be involved in a future Technology Insight episode, please feel free to get in contact. Could I ask you to please subscribe to our channel, this really will help us to bring you further videos about ITS, thank you to everyone who has, so far.
Thanks for watching, see you next time.