TLDR: Your bicycle rims are primarily responsible for triggering loop sensors. Learn where you should place your bike. If the loop sensor still doesn’t work you should contact your city.

Today I learned how inductive loop sensors at traffic lights work.

The motivation was a traffic light on my route to work that never detects me. Until now I have been carefully positioning my bike into the middle of the loop. I had been assuming that it was based upon the movement of a small piece of steel somewhere on my aluminum bike inducing current in the loop. This concept fits with the fact that cars (large with lots of steel) reliably trigger loop sensors while bikes (small and usually not much steel) often do not. The solution then would be to stick a magnet on your bike. Problem solved right?

This is not how inductive loop sensors work, thanks to this fantastic writeup. They are resonant systems. The loop forms an inductor in an LC circuit that is driven on resonance (at 20-30 kHz usually). Any change in the inductance shifts the resonant frequency. What’s great about this is that your car/bike is part of an oscillator. A car triggers the loop by being large and conductive, not large and magnetic. Your vehicle behaves like a shortened turn in the roadway inductor, increasing the resonant frequency of the oscillator.

Similarly, a bicycle triggers the loop sensor by decreasing the inductance seen by the circuit. The best way to do this is by bringing a large ring of metal to the region with the largest magnetic field. Your tire rims, unless you spent a small fortune on carbon fiber ones, provide the loops. But where should you position your bike? It depends on the way the loop is configured. If it’s a simple square then you should position it directly over one of the long edges, not in the middle of the loop. If you can see two side-by-side squares in the pavement then you should position yourself over the center line where there are two wires carrying current in the same direction. If there are diagonal cuts in the pavement, just make sure that one of your wheels is over one of the diagonal lines. If the first link is too daunting, read this link (or either of these) for short and sweet diagrams.

It is impressive how many people think that run of the mill permanent magnets can trigger inductive loop sensors (myself included this morning). Loop sensors are normally designed to detect an increase in resonant frequency (i.e. inductance decrease). However, some fraction of loop sensors are configured to also respond to an increase in inductance and you can buy magnets on the internet that purportedly trigger loop sensors. So could you just strap a chunk of iron onto your bike frame? Probably not. If you look at the patent application filed by the Green Light Trigger company you’ll see that the magnet needs to be ceramic, i.e. high electrical resistivity. As the excitation frequency increases the apparent inductance of a magnet decreases due to eddy currents which short it out. By increasing the resistivity of the magnet you increase the corner frequency up to which it looks like an inductor to the circuit. Some people on the internet claim that magnets don’t work because only inductance decreases are registered. Others, who are trying to sell you stuff, claim that inductance increases are also detected and their magnets work. Either way, someone on the internet is wrong.

The frequency change required to trigger the loop sensor depends on a threshold that is programmed into the system. There is a tradeoff in setting the threshold - too low and cars in adjacent lanes might trigger a left turn light. Too high and bicycles and motorcycles will not be detected. This means that if the loop sensor is relatively small (< 10 feet long) then it’s just a matter of adjusting the sensitivity. If you have positioned your bike properly and it still doesn’t trigger then you should let your city know so that they can reprogram it. I haven’t had a chance to ride through my problem intersection yet, but hopefully the power of physics will help me next time and if not, the magic of complaint forms on the internet will.