That actually seems to be a thing: https://arstechnica.com/information-technology/2015/12/new-open-source-license-plate-reader-software-lets-you-make-your-own-hot-list/

You might not need to even roll your own rasp pi with camera module; you can use an off-the-shelf outdoor camera streaming to a network location and do the processing inside.

It would be much more local network traffic, but only local where it doesn't cost anything. There are cameras with local (I think) AI for vehicle detection, which might be a good-enough improvement...transmit only frames containing vehicles which would be a broader set but still smaller than streaming everything.

Either way, what you're describing sounds like a fun project.