Are there are cameras inside the zone tracking cars to bill them if you are already in the zone, or if cameras only track entry to the zone? (i.e. cameras only on the border). If someone happens to evade the cameras, do they catch them eventually just by traveling within the zone? I believe London for example has internal zone cameras.
The purpose being, first of all, to ensure that people do not somehow evade paying just by operating solely within the zone, defeating the purpose of reducing traffic. And secondly, to stop people from engaging in loophole seeking behavior.
I hope that loopholes and people defrauding the system (license plate obscuration, etc) are quickly caught and penalized. You would hope that if a car enters or is detected with invalid plates, it triggers an automatic report to police nearby to follow up. Otherwise, like so many things (it seems now) we just throw our hands up at people who evade the rules and charge those who follow them. (my comment spurred by an NYT article about how people might scam the system)