Any magnetometer suitable for an electronic compass (i.e. sensitive enough to easily detect the Earth's magnetic field) will detect vehicles a few meters distant. The vehicle will introduce easily detectable variations in the direction and magnitude of the local field. I used this magnetometer https://www.sparkfun.com/products/7915
to make a "driveway alarm" which warns us of visitors.
So, i can "easily" use this magnetometer to detect if a car is over the sensor? My intention is to use arduino just to build a prototype so I can develop my final product.