Free-flow tolling is based on a simple principle: the automatic detection of each vehicle, while it is traveling on the highway without stopping or even slowing down, to assign it the amount of toll due.
The gantries above the traffic lanes are equipped with equipment which determines the category of vehicles (cars, heavy goods vehicles, etc.) according to their volume and the number of axles. These sensors cover all lanes of the motorway, including the hard shoulder. They identify the position of the vehicle on the road and its direction of travel, whatever the traffic or weather conditions. The system takes into account the Liber-t electronic toll tag in vehicles equipped with it provided that it is correctly attached to the windshield.