Here´s how Q-free works

The rectangular Q-free tag (1) that is glued to the inside of the windshield is a modern SAW tag.

The little plastic encasement is the supporting girder in this payment system. The tag is "passive", in other words it doesn´t contain a computer or a transmitter and needs no electrical power. Nevertheless, the tag does have the ability to reflect radio signals.

A radio signal (2) is sent from an antenna at the toll booth and strikes the tag as you pass. In a fraction of a second the tag converts the signal and reflects a code number back to the recording unit at the toll booth (3). This number tells the computer that it was your car that passed through. Your tag code is then sent to the central computer (5) that maintains your Q-free account and makes sure your bill (6) is sent out on time.

A video camera (4) is also included in the system, so that the license plate can be photographed if a vehicle without a valid Q-free tag passes by. Theoretically, there is no speed limit for the Q-free system, but a good estimate of a practical maximum speed is subsonic speed. In Lisbon, for example, cars have passed at 240 km/h, and are still properly identified.