Millions of drivers pass through toll booths every day. Traditionally, the process is to put some change in a basket, which tabulates the coins and opens a gate to allow the driver through. Today, many local and state traffic agencies have installed or are installing electronic readers that allow drivers to pass through toll stations without coming to a complete stop. The names of the systems vary, but they all work in pretty much the same way.
Here are the basic components that make the system work:
Drivers usually have to pay a deposit to obtain a transponder, which is about the size of a deck of cards. This device is placed on the inside of the car's windshield behind the rearview mirror. A transponder is a battery-operated, radio frequency identification (RFID) unit that transmits radio signals. The transponder is a two-way radio with a microprocessor, operating in the 900-MHz band. Stored in this RFID transponder is some basic account information, such as an identification number.
Antennas, or electronic readers, are positioned above each toll lane. These antennas emit radio frequencies that communicate with the transponder. The detection zone of an antenna is typically 6 to 10 feet (2 to 3 m) wide and about 10 feet long. These two devices, the transponder and the antenna, interact to complete the toll transaction.
Some electronic toll-collection systems may also include a light curtain and treadles. A light curtain is just a beam of light that is directed across the lane. When that beam of light is broken, the system knows a car has entered. Treadles are sensor strips embedded in the road that detect the number of axles a vehicle has. A three-axle vehicle is charged a higher toll than a two-axle vehicle. These two devices are safeguards to ensure that all vehicles are counted correctly.
In the next section, you will see how all of these components work together to detect vehicles and collect tolls.