The light will turn green in the toll booth. There is also an audible noise if you pass through(and the light stays red) You're from Niceville so you probably use this for the toll bridge into Destin correct?How will you know if it worked? I went through a toll once and the xponder did not beep (batteries were ok), they took a picture of my plate and sent me a notice that next time I did that I would be fined a hundred bucks. Beautiful.
You'll also know when the little gate does not go up if you've not paid your bill(for those plazas that have the gate arm)