Vehicle Detection Sensor Recommendations

Working on a small home project that detects cars in a big car park in each individual space, for example one pole detects 4 spaces with 4 sensors.

Was thinking of putting sensors on poles to detect each space, and whether a car is in it. Though my ideas of IR or ultrasonic won't work due to the refraction of the signal.

any ideas on what sensor I could use that can be mounted to a pole?

What's the budget?