In CA, FastTrak (CA's EZ Pass), If you go through without paying (no pass) or no$ in the account, it takes a picture of your plate and sends you a bill. For the Golden Gate Bridge, they plan on taking out the booths and using cameras to bill you. On the newest bridge, they have a system that will read the transponder if you go through the area with no booths under 40 mph.
They also use your transponder to determine speeds on the highway along with the buried coils,