I think that more sinister than the license plate readers, and they are certainly sinister enough, are the Bluetooth sniffers that are springing up all over.
I was in Michigan in June and saw them all over as well as in Indiana and Illinois. I saw them in Ohio last month as well.
What about Wisconsin, are you getting them there too? Here is what they look like though there are other formats as well:
You may also see them as diamond shaped panels mounted with traffic lights:
These capture the MAC address of every bluetooth device that passes them and stores it in a database.
Thus they can tell not only that the car passed, assuming it has a Bluetooth in it. They can also tell who was in the car, by reading cellphone MAC addresses.