Here is the link to the Traffic Detector Handbook from the Federal Highway Administration: http://www.fhwa.dot.gov/publications/research/operations/its/06108/02.cfm This will explain more about vehicle detection using induction loops.

Here are a few resources:

So the question I have can be broken into parts:

  1. Can a magnet alone (imagine the bike is all plastic) trigger the vehicle detection?
  2. How strong a magnet do I need (dimensions, pull?)?
  3. Are dimensions / shape sufficient by themselves?
  4. Is the grade / pull force important at all for this application?
  5. Significant difference between ceramic, rare-earth, ... for this application?
  6. Let's say I can purchase a magnet, how should I position it on the bike?

There are many different stances on the topic, some calling the magnets just bogus and not working, some swearing by it. I have very little understanding of magnetic field & eddy currents and how a magnet could affect an induction loop, so verbose explanations would help.

Thank you.


I am trying to figure out if sticking a ferromagnetic magnet (rare-earth / neodymium) under a moving vehicle would help at all.

  • it is ferromagnetic,
  • it is a magnet,
  • it moves with the vehicle,
  • it is lower to the ground than where an engine would be.
  • 1
    \$\begingroup\$ The cars are made of steel, not magnets. \$\endgroup\$ – Marko Buršič Apr 13 '16 at 8:49
  • \$\begingroup\$ some magnets are ferromagnetic, right? \$\endgroup\$ – dnozay Apr 15 '16 at 15:19

Probably any magnets do not help at all. The vehicle detection by induction loop relies on variations of inductance by large metal (ferromagnetic) object standing over the loop. The inductance is measured on AC, DC magnetic field from any permanent magnet just falls out of detection.

Briefly, you need a ferromagnetic object of a car size to trig the detector.

  • \$\begingroup\$ what about the claim that a small ferromagnetic object (in our case a magnet) positioned closer to the induction loop wire would make a difference? \$\endgroup\$ – dnozay Apr 15 '16 at 15:22
  • \$\begingroup\$ A permanent magnet has very low magnetic permeability (1.2-3) - compared to normal iron (1000-5000). So its influence on induction loop is much smaller than the influence of the iron piece of the same size and shape. \$\endgroup\$ – Master Apr 15 '16 at 17:13
  • \$\begingroup\$ And the distance between the object and loop is small if it is smaller than the loop size. I guess (it is only guess!) that the loop size is about 1 m or more. So there is a little difference between 0.5 m or 20 cm. \$\endgroup\$ – Master Apr 15 '16 at 17:14

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