A turnstile is a mechanism that allows a location to have control over entry and exit. Most turnstiles consist of a horizontal arm that extends from a post and allows persons to pass through, given the particular condition has been met. To pass through a turnstile, it might be necessary to present an authorized identification or to make payment. Other similar structures keep a count of individuals that pass through. Turnstiles provide security, control crowds, prevent loss control, and control admission and/or access.
Places that commonly use turnstiles include amusement parks, public transportation, airports, and sporting events. The design of a turnstile allows only one person to pass through at a time. The initial function of a turnstile was for farmers, but not animals, to be allowed to pass through. Another use of a turnstile is to maintain pedestrian traffic in a single direction.
Most modern turnstiles are mechanical, which allows the arm to be rotated one direction but not the other. Now, many turnstiles are electronic, and some operate with optical sensors that can scan tickets or badges. If the sensor does not recognize the ticket or badge, an audible alarm sounds.
Turnstiles come in various sizes, from waist height to full height versions that are used for situations requiring maximum security. Variations include a looped cross-arm model that is also good for conditions requiring higher security. A traditional three-armed model is good for tight spaces, and even portable turnstiles exist.
For those in wheelchairs or those with strollers or delivery carts, a variation of the turnstile with a drop arm exists. The American Disabilities Act requires that accommodations must be provided for those with physical challenges to gain entrance, which a traditional turnstile generally does not allow. Some modern turnstiles appear almost like a revolving door, with clear panels as opposed to the traditional metal arms.
Other methods used to direct traffic and control crowds include stanchions, or vertical bars, like those used to designate a lane inside a bank lobby. These posts are often connected by chains or ropes, and some have retractable belts.