Sensor recommendation
I've used RFID sensors and tags in a product. The RF tags we used are for short-range stuff. The UHF tags are used for detecting objects from further away, such as containers on lorries from 15 m away.

The UHF readers are off-the-shelf and readily available. You will get a code from each tag so you can tell vehicles apart.

The downside is that the readers are expensive. You don't mention a budget so I'll leave you to weigh this up.