Toll signs on 101 report your transponder setting
Here's a little bit of Silicon Valley local flavor for a change. We have this freeway that runs from San Jose to San Francisco called US 101, or around here, just "101". (Southern California has it too, but they call it "the" 101. That's how we detect invaders.) Anyway, this thing is usually eight or more lanes up and down the peninsula, and it's had a "diamond" or car pool lane on the #1 (innermost/"fast") lane for many years. You had to have a certain number of people in the car (or some other conditions met) during certain hours (AM and PM rush, weekdays), or you'd be in for a hefty ticket.
This started changing recently, and now parts of the corridor have turned it into a toll lane all day on weekdays. This is a deal where you can drive in the lane and pay for the privilege if you're by yourself, pay a discounted fee if you have one other person with you, or it's free if at least two other people are riding along (plus a bunch of other variations I won't get into here - motorcycles, clean air vehicles, you name it).
How does the system know what to charge you? Well, you either have an old-style toll transponder that always bills you as a solo driver, or you have a new-style transponder with a 1/2/3+ switch that tells the road how to handle you.
Yes, the billing is entirely a function of whether you have that thing in your car, and how you configured it for that particular driving session. You could totally lie to it and set it to "2" for a discount or "3+" for the complete freebie ride. But, then you have to worry about getting caught and getting a ticket.
Think about what this does to enforcement. Now, instead of just "is there a vehicle in the lane", "is it the right time during a weekday", and "how many people are in the car", now it's also a matter of "do they have their transponder set properly".
How do you suppose they're going to tell what position you had that thing set to when your car rolled under the reader? The answer turns out to be relatively simple: they put a sign over each lane, and it just flashes up a "2" or a "3" depending on what it heard from your transponder's response.
Check this out.
This is a scene from Mountain View where a car just passed underneath a reader and the "2" lit up a second or two later. If there's nobody else in that car, CHP could roll up on them and give them a ticket.