I'm guessing nothing, unless you did it in front of some sort of law enforcement person, or there's a camera. With the cop, I would expect immediate negative reinforcement. If there's a camera, I'm guessing you'd get a ticket in the mail along with a copy of the photo of your car, you lawbreaker you ("you" in this case being the same "you" that's running the tolls).
They have cameras there that capture your liscence plate number. Then, they send the bill + the fine, to whoever the car is registered to. My parents had an ezpass, but the velcro had melted off the glass, so my dad was holding it up to be read. He dropped it as he was going through. Then ended up getting a fine in the mail for $250. Yes, they still had to pay it, even though they have the EZPass.
Call them, tell them you thought it was in the car, but it wasn't. They should let you off with only paying the toll.
A little "did you know.." about the Ipass. If you don't have all of the license plates to all the cars you drive they will fine you. The pass follows the plates, not you. Which seems silly to me, but that's the rule. My dad got $500 in fines when dh was driving his car. The dropped it when we explained that it was in the car, being driven by dh. They said you have to register the plates of any car you might use it with.
They will send you a bill. You have to pay the toll plus a fine for each time you go through. It may take a little time-but it will come. I think they wait until a certain amount of time has passed and then send you one big bill.
Unfortunately, I have been there, done that!
A friend got a bill for a few thousand dollars. She went through it both to and from work for a while. Man, was her DH pissed!
In my area, it's called an EZPass. Basically, it's a device you attach near your rearview mirror, which is scanned when you go through the "EZPass lane". They then bill you every few months for the tolls you went through. It's the quick lane. While everybody else is waiting in line, collecting their change, and throwing it out the window, or handing it to a person, those with EZPass are gliding right through.
They will send a bill - my sister has an IPass and something was wrong with the transponder, but she was able to get out of the ticket since she sent back the broken transponder and thet confirmed her story.
Actually you do not have to register all of your license plate numbers with IPass but if you do and the transponder doesn't work while you are in one of your registered cars they can deduct the toll from your account after the fact.
I'd fess up - those fines can get pretty pricey!
My husband worked near Chicago for a few months and went through the IPass lanes a few times completely by accident, because the freeways were so torn up, and the signs were actually wrong, and he couldn't figure out which lane was which. We don't have any device or whatever you need. There were cameras. We keep waiting for a ticket or something in the mail, but so far nothing has happened, and this was in April.
If you're doing it that often, might be worth it to get the I-Pass. Some tolls are half as expensive, and I think you get the initial $10 fee back when you're done with the thing. What with all the open road tolling that's opening up now, it makes sense.