A parking meter is used to keep track of the amount of time a vehicle is parked in a specific area. After inserting money into this device, the commuter has a certain amount of time to keep his or her car parked in the spot before it is necessary to insert more money. If the person fails to insert more money when time runs out, he or she may receive a parking ticket from a police officer commonly referred to as a meter maid.
Traditionally, parking meters were activated by placing coins inside a special slot. While the coin-activated parking meter is still common, there are also versions that accept credit cards or even special cards called smart cards which can be pre-loaded with credit and used to pay for time.
The slot in which coins are inserted into the parking meter is capable of identifying the type of coin that has been inserted. The slot identifies the coins through a combination of tests it has been programmed to conduct. These include testing the weight of the coin, as well as using electrical currents or lasers to test for other physical properties of the metals used to create coins. To simplify the testing process, the majority of parking meters accept only one type of coin, most often the quarter.
After the coin or coins have been accepted, the user must turn a handle or press a key in order to set the amount of time needed. A display or a dial appears on the parking meter to indicate the amount of time the user has for use of the parking spot. As time passes, the meter counts down to indicate how much time remains.
Some more modern designs also contain special sensors. These sensors use laser lights to determine when the parking spot has been vacated. When the parking meter determines the spot is empty, the time remaining on the clock automatically returns to zero. This prevents other people from using the time remaining on a meter after the spot has been vacated. The special sensor also ensures that cars are removed from areas that have a limit on the amount of time a person can remain parked in one spot.