Thursday, 16 February 2017

Inside Our Streets: Wireless Magnetometers

This is the first of an occasional series which gives a little introduction to some of the "things" we put in, on, or under our streets to help them function. This week, wireless magnetometers as used with SCOOT.

I covered SCOOT in a previous post which referenced magnetometers, but to recap, the devices (specifically the wireless kind) are gradually replacing loops cut into the road surface to detect traffic (motors and cycles). 

By loops, I mean wires which are cut into the road surface (see image below) which count the flow and/or speed of passing vehicles. They can be set up to detect cycles, although there is the odd complaint about junctions where this doesn't work. Loops work by vehicles affecting the inductance of a tuned electrical current as they pass over them (more here).

Traffic loops - lower image in red to highlight them.
Area circled yellow is an inspection pit.

There are a few issues with loops;
  • They are cut into the road surface which creates the potential for water to get in if the sealant to the cut fails. This can lead to potholes.
  • Where a road is resurfaced, the loops are often destroyed by the planing machine as they are usually installed at around 40-70mm deep.
  • The loops need to be directly connected to the traffic signal system. This means pits at the side of the road (see above, circled yellow) and extensive ducting where lots of detection is provided (on higher speed approached for example).
  • Road workers are exposed to safety risks during installation in terms of traffic, dust from cutting and noise (even though this will be controlled with traffic management and protective equipment).

Wireless magnetometers are installed in the centre of a traffic lane by core-drilling a hole, putting in some resin bedding, inserting the unit and then topping up the hole with more resin. Siemens has a natty little installation guide here. The installation still involves road workers being out on the road but it's a much easier and quicker task.

OK, lets look at a wireless magnetometer up close and personal;

The little circle of resin is the clue.

Zoomed in. It's 100mm in diameter.

Here's a unit removed from the road, encased in resin.

The depth of the hole was about 70mm.

I've removed the resin from the top of the unit. As the
resin is poured in two parts (bedding then topping) it's
fairly easy to prise open.

Here, the magnetometer is removed showing a plastic
shell in which it sits and which is in turn bedded into
resin poured into the cored hole.

The magnetometer is about 70mm square and 50mm
in depth. When it is set into its resin bed, it is about
6mm below the road surface.

Here, I've removed the top to show the internals. From
here on, it's all sealed in clear silicone.

Now don't worry, I haven't gone out and prised a magnetometer out of the road, this one was removed prior to some resurfacing and was replaced, although they can be reused. A planing machine would destroy the unit easily, but it is best to pop them out first!

In terms of operation, you may have noticed additional boxes being added to certain traffic signals at junctions. These are called "access points";

These are different from microwave vehicle detectors (MVDs) that you see aimed at traffic. Access points essentially collect the detection data being transmitted by the magnetometer which has an on-board battery (8 to 10 years life) which runs a low power transmitter. In some circumstances, the magnetometers "talk" to repeater units which allow detection a long way before the junction as typically, magnetometers will be within 40 metres of an access point.

Access points can handle multiple magnetometers and so when taken as a whole, we can construct some very clever arrays of detection approaching, within and leaving junctions. This is very cool stuff, especially where with have an urban traffic control system such as SCOOT which runs traffic signals on an area (and city-wide) basis. For those interested in cycling, the technology is being used now. The image below is on Cable Street in London;

The magnetometer is circled in yellow and the access point in red. In this location, cycle demand is actually detected by another sensor on the traffic signal and so the magnetometer is being used to detect flow into the junction. However, we could as easily place a magnetometer well in advance of the stop line and trigger demand before people get to the signals so that a green may come immediately - something we haven't really got to grips with yet.

So there you have it. A little plug of happiness making our streets run that little bit more intelligently.


  1. what (roughly) is the detection radius on them?

    1. The detection radius is 2m long by 1.3m wide.

  2. Very interesting. Thanks! I'm old enough to remember the predecessors of loops: a transverse raised rubber channel, connected presumably to a pneumatically operated switch that would be triggered by a vehicle passing over it. As a cyclist when lights failed to change on a quiet night the technique was to dismount and stamp on the rubber channel to get some movement. On rare (similarly quiet!) occasions a vehicle would reverse and then move forward again to get the damned thing to register.

  3. I good article about the wireless vehicle detection. I would just add that the detector you highlight on Cable Street is not a magnetometer but in fact the radar version of the product specifically to detect cycles. The radar version uses the same wireless infrastructure and interface cards in the traffic signal controllers as the magnetometers that are used for vehicle detection but can accurately detect cycles that have less ferrous materials.

    1. Ah, thanks for the clarification; it's good for me to learn new stuff too! - probably something like this;

    2. Hi Yes that's the one TfL have successfully trialed the M100BR sensor in a number of locations for cycle stop line and also advance SCOOT detection in cycle lane approachs about 30m from the stop line. They hope to install more in due course to allow cycle demand to be adaptive rather than just timed phases.

  4. We use a bright red housing for our magnetometers. This highlights them to our road resurfacing team, whom we've made aware of this. That way, they don't get destroyed by someone being over-enthusiastic with the planing machine!

    We don't use them in cycle lanes, though. We use piezo-electric loops which, according to the manufacturer, are guaranteed to detect bicycles and only bicycles. Ours count cycles in both directions and insert a demand to the traffic signal controller when a bicycle travels towards the signals. They are about 20-25m back, and currently they don't insert a hurry call so that a green appears as soon as possible, although they certainly could.

    Piezo-electric detectors can distinguish all types of vehicle (not just cyclists) - currently testing them for bus priority (in a general traffic lane).