tailgate sensor

In security, piggybacking, similar to tailgating, refers to when a person tags along with another person who is authorized to gain entry into a restricted area, or pass a certain checkpoint. It can be either electronic or physical. The act may be legal or illegal, authorized or unauthorized, depending on the circumstances. However, the term more often has the connotation of being an illegal or unauthorized act.
To describe the act of an unauthorized person who follows someone to a restricted area without the consent of the authorized person, the term tailgating is also used. "Tailgating" implies no consent (similar to a car tailgating another vehicle on a road), while "piggybacking" usually implies consent of the authorized person, similar to a person giving another person a piggyback on their shoulders.
Piggybacking came to the public's attention particularly in 1999, when a series of weaknesses were exposed in airport security. A study showed that the majority of undercover agents attempting to pass through checkpoints, bring banned items on planes, or board planes without tickets were successful. Piggybacking was revealed as one of the methods that were used in order to enter off-limits areas.

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