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.
Masses of metal that vary per vehicle, and could therefore result into different characteristics measured. I understand they are also used at traffic signals, but these are nowhere near any such infrastructure.
This will not be able to differentiate if it was VW hatchback A that entered the highway at ramp Z or Peugeot hatchback B entering from ramp X. It also cannot tell a VW up from a Skoda CitiGo or any of the bazillion similar design and mass vehicles between different brands and models. This is completely useless as a surveillance technology there are way too many variables introducing noise that you don't have the ability to filter on because signatures will be way too similar between thousands of vehicles using hundreds of on/off ramps.
Must be the lizzard then, it's the only explanation.