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I suggest you contact Tönnjes, which has developed a highly regarded passive UHF tolling system based on readers and software from Kathrein (see RFID Breaks Speed Records for Tolling Solution). I also suggest you reach out to FEIG Electronic, which offers a passive UHF reader used in many toll-collection systems. You would need to purchase passive UHF tags designed to work on windshields, and make sure they are tamper-evident.
Founder and Editor