Would I be able to read a tag attached to a vehicle's license plate via a buried antenna?
The answer would depend on the type of tag used. A passive HF tag probably could be read by a loop antenna buried in the road. If you were using a passive UHF tag, it would likely be more difficult.
The reason is that an HF reader's loop antenna creates an electromagnetic field that is like a balloon. If the loop antenna in the reader can couple with the loop antenna in the tag, they can communicate. With UHF RFID systems, the reader antenna emits a stream of energy, kind of like how a flashlight emits light. The tag on the license plate would be positioned vertically, so the waves of energy coming from a reader antenna in the road would mostly pass by the tag antenna, making it difficult to read. An antenna facing the car license plate would read the tag more easily.
If you wanted to bury tags in the road, it would be better to put the tags under cars, rather than on their license plates. Having said that, there might well be an omnidirectional passive UHF tag and reader that could work for your purposes. You would need to test some options with a skilled systems integrator.
—Mark Roberti, Founder and Editor, RFID Journal