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.
Modern cars and the devices people carry in them emit all kinds of identifying information constantly. There are off the shelf boxes available to collect this information from every car/truck as it passes. No need to get fancy with induction coils if tracking is your goal.
My understanding is the coils are really used for measuring the flow of traffic.
There’s a lot of older cars on the road (including mine), and I’m typically not carrying any devices on me. I’ve read some articles that discuss such technologies, but there’s no clear indicators of them being around roads I’m utilizing; not to the extend of these induction coils for example. Cameras installed with the supposed intent of monitoring traffic, are also increasingly used in criminal investigations; so it’s not unthinkable data from these sensors might be approached similarly.