Hills number plates pioneered the idea of using rfid in a number plate. The latest offering is the "Hiltek" design plate with the numbers on the front, leaving the back to contain "electronics". There seems no reason why a simple id tag should be used, more complex ones are available in the same size. Besides, number plates will soon have to have the VIN number of the vehicle on them as well.
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The visible vehicle Identification Number (VIN) would be displayed on the Number Plate. We have built this facility into our latest version of our software and will be available for installation when the commencement date is given.
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Electronic tagging of number plates offers possibility of making a limited improvement in advance of a full system of electronic vehicle identification system (EVI) and DVLA concluded that, “it is evident that technological solutions exist, and could be put into operation in the not too distant future”. As part of the Hiltek plate development we have developed e-plate, which is, a fully compliant number plates containing an active RFID tag and it is available for demonstration and installation now.
http://www.hillsnumberplates.com/dvla_cons.html