Tailgating is when another person, whether an employee or not, passes through a secure door without the knowledge of the person who has gained legitimate access through the secure door. This is a similar concept to when there is a car following closely behind you on the freeway without your permission. You are being tailgated.
Piggybacking is when another person follows through a door WITH the permission of the person who has received access. If someone is hugging you or actually on your back, this is called "piggybacking". Only 2 technologies are able to distinguish between 2 people hugging and 1 large person. Humans and 3-dimensional machine vision systems.
There are various methods of stopping tailgating and piggybacking through secure doors. At mission critical sites, 24/7 armed guards are often employed at significant expense. An alternative to an armed guard may be the installation of a 3-dimensional machine vision system that can differentiate between humans and objects. This is the same proven technology that has been employed for decades in the field of factory automation and robotics. Newton Security's patented T-DAR technology is worlds most reliable system for detecting and preventing tailgating through secure doors.
The two types of entrances that are found in an anti-tailgating system are single doors and a set of 2 doors in a secure space which is called a mantrap.
A single door anti-tailgating system has the ability to alert sound and alarm, alert security personnel to an unauthorized intrusion. Since there is only one door, this cannot be a man trap.
A mantrap is a secured space with 2 or more doors. When 1 person enters the mantrap area the T-DAR 3-D machine vision system performs a dynamic scan, confirming that only one person is in the space. If another person enters the space, they are detected and since there are now 2 people in the space an alarm is sounded. The door on the secure side of space is locked so neither person can enter space. Both people must exit the mantrap before entry process may proceed.
Here is a pretty good demonstrations of a mantrap in action: http://www.youtube.com/watch?v=gYw0rskVHTU