EZ PASS (the little white box in your car)

Hello,

Here in the USA we have a highway toll system called EZ PASS. (I don't know if it exists in other countries.) Basically in your car you have a small white box you attach to the windshield and when you drive through the right lane at a toll booth, you don't have to stop and pay, the toll is electronically added to your account when you drive through. You can go up to 25 miles an hour on most systems without stopping. I am curious how this works. I don't think it is any kind of radio. How does it work???

And if you don't pay...they take a picture of your license plate and send you a fine in the mail.

thanks

George L.

What is the evidence for or against RF devices as the underlying technology?

no evidence, just a guess. It probably is some kind of RF tech.

If I had to guess, and it is only a guess, it would be the RF equivalent of the magnetic stripe on a credit card or maybe a bar code. A twelve digit credit card number would require 48 bits of data plus some overhead and check bits. You'd have to get it right the first time since there would not be any do-overs. Can you imagine getting a letter asking you to return to a certain toll both to be scanned again?

My evidence for this is the transponder used in aircraft. In response to incoming radar energy, with no information content, the transponder replys on a different frequency with a 12 bit code, assigned by the Air Traffic Controller and set by the Pilot with thumbwheel switches, which uniquely identifies the aircraft to the Air Trafic Controller. In the case of the car and the toll both, the response would be fixed and only respond to interrogation by the toll booth authority.

Gee, do you think spoffing somebody's box would be something folks would do? Naaah, that would be be so wrong!

If you're still wondering about this, here's some links (Googled "automatic toll RFID"): https://www.google.com/search?client=safari&rls=en&q=automatic+toll+rfid&ie=UTF-8&oe=UTF-8