Presence Detection of Factory Machines for Their Traffic Management
I've used RFID sensors and tags in a product. The RF tags we used are for short-range stuff but the UHF tags are used for detecting objects from further away. So this is detecting containers on lorries from 15 m away, the same sort of thing you're looking at.

The UHF readers are off-the-shelf and readily available. I don't know if you could mount the equipment above or to the side of the vehicle detection zone - you obviously want to be close enough to not falsely detect the vehicle behind. But you are getting a code from each tag so you can tell vehicles apart. 

I don't know enough about your application but obviously the RFID route needs the compliance of the vehicle i.e. they have to agree to carry it and they might lose it. The metal-detecting route doesn't have that disadvantage. I'm sure you've already weighed these factors up.