Circled Loops at Traffic Stops?

Whenever you stop at traffic lights you will sometimes see circle loops that your car stands on.
The actual name for these loops is the inductance loop. Well I think most people have an idea of what it is but If you don't know what it is for,
it basically allows traffic lights to know that there is a car standing at the lights.
This allows signals to turn green to allow traffic to flow.

Now most people think that these inductance loops are pressure-sensitive but that is not true.
These loops act like magnets and whenever a car drives over it the magnetic field changes which allows them to know that there is a car.
The engineers at the city of Surrey told me that in order to test these inductance loops they sometimes throw a sign on top to see if it works or not.
I always thought that these loops were pressure-sensitive. It was pretty cool learning that they are not.