Road signs alert motorists when they enter the charging zone. There are no toll booths, barriers or tickets - instead, drivers pay to register their vehicle number plates on a database.
Almost 700 cameras situated at a network of sites take images of vehicles travelling within the charging area. (There is no charge for travelling along the Inner Ring Road boundary itself.) There are 150 static camera sites around the boundary, with a further 53 static sites and 10 mobile camera units within the zone.
The cameras take close-ups of the vehicles' number plates. These are sent to an automatic number plate recognition computer system, which records the date and time the images are taken.
The number plates are matched against the database. Once a vehicle registration number is matched, showing either the charge has been paid or a vehicle is exempt, the pictures are wiped.
Number plates of vehicles for which no charge has been paid by midnight are matched against Driver and Vehicle Licensing Agency (DVLA) records. The computer then sends out penalty notices to the registered owners.