This is fascinating! I'm very tempted to build a "early warning door bell". Being the only house on our road, it'd be fun to try and set up a few of these to detect when a car takes the exit from the bigger road ours connects to, and give a heads up that visitors are coming!

There will be a few false positives from farmers coming to work the fields by our road, or people looking to hike in the nearby forests, etc. but it could be pretty fun nonetheless!