Thanks for the replies and feedback.
I do understand that magnetometer are not necessarily designed to detect cars, but thats doesn't mean they wont and to be fair thats half the fun, making something do what it wasn't initially intended to do
. Using something like a magnetometer also allows for a much smaller install, i.e. small box on a wall.
This isn't my application but if you had a long drive, you could build a RPI magnetometer, power it with a battery, connect it to your WiFi and every time a vehicle comes up / onto your drive, be it family or delivery you could send a command to another device that rings a bell, not the best use of this solution but you get the idea.
To make this work, consider that a magnetometer is just a sensor that detect the Earths magnetic field and being able to detect a car ( or anything else thats metal ) is actually very straight forward. If i cant find a sensor that has a variable sensitivity I'll just built the sensitivity into the code.
Th easiest way to see this work, and presuming you have a smart phone ( majority of smart phones use a magnetometer for the compass and to detect magnetic North ). Install a magnetometer app, when installed stand still ( away from anything metal ) and watch the reading, it will be anywhere around 20 - 60 uT, now walk toward something metal like radiator and you will see the readings will go bonkers. So if you have a base line ( e.g 40 uT ) and you know what type of reading you would get if a small car were to pass ( e.g 200 uT ) you just code in something that says if a uT reading is > 200 add 1 to a file and thats how you detect and report on things.
Nothing is impossible, you just need to do a bit of head scratching.