I’ve seen the number of induction coils increase drastically over the years in The Netherlands. They are quite easily recognizable, as there’s typically a solar panel on a pole, that appears to provide power to the system.
Considering there’s a wide variety of vehicles on the road, surely each variant must have (slightly) different characteristics when passing over the coil; especially when in a specific place at a specific time.
And given that they are situated at highway exits (see picture) or after entrances, and road users unable to exit and enter elsewhere, it would be trivial to track the bulk of a vehicle’s trip.
This in context of ALPRs (in different forms) being in place at strategical locations (large junctions or at bridges or tunnels, and parking), and the address of vehicle’s owner; you’d be able to connect the dots, and end up with a pretty complete picture.
Each detector seems to have two coils (see picture), which appear to be an emission coil and a reception coil; similar to metal detectors with a DD coil. Typically these coils slightly overlap to increase detection sensitivity; but this doesn’t really seem necessary when a massive block of metal hovers directly over the coils. Each coil likely consists of multiple turns of wire, that sit on top of another inside a groove cut into road surface; which should increase the inductive properties.
The magnetic field also induces Eddy currents in non-ferrous metals (meaning currents running through metal within the field), which by themselves generate opposing magnetic fields, depending primarily on the material’s conductive properties (highly conductive materials yielding stronger magnetic fields). These fields change the phase of the received signal, which in metal detectors can be used as an indicator for material properties: primarily the type of metal and its mass.
If the same were possible on the traffic detectors, or even a signature of metal concentrations, I believe you’d have a pretty robust identifier for vehicles.