Just now I heard a story on WUOM{NPR} stating there are license plate cameras at locations on I-94 and I-75. I didn't hear the whole story but I got the sense that they're experimental.

Google shows many stories about the subject from all over Michigan and apparently these cameras have been in operation for quite a while now. I had no idea this was happening. Justifications given are like Amber alerts and freeway shootings.

So it's not just speed traps. According to some stories, they record everyone's plates, with or without cause.