The e-TAG electronic tolling is based on radio-frequency identification (RFID) transponders using the DSRC protocol. The system uses electronic transponders provided by Kapsch (called e-TAGs) mounted on the inside of the vehicles’ windscreen. Gantries constructed over each carriageway record registration plates and detect the e-TAGs. As the vehicle continues along the toll road, additional gantries monitor the distance traveled. Associated software then determines the toll amount payable, which is automatically deducted from the prepaid account associated with the tag. Where a tag is not detected, the vehicle’s registration is recorded using video tolling technology which incorporates an automatic number plate recognition system and is checked against a government motor registration database.
The excerpt above is culled from Wikipedia
