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First, the facts. later, the personal observation ("The good stuff").

This interesting device is similar to a revolving door or turnstile in some of its structure, but has a very different function.

It has a central vertical axle, with vertical rows of horizontal bars on it. It can be passed by pushing on the bars and rotating them ahead of you on the axle, like a revolving door. It has walls on two sides, so it can only be passed through along one line. Along one of the walls, it has a vertical row of bars, similar to the rows on the axle, but staggered vertically by the space one bar takes up, so the rows on the axle can pass it.

Assuming that the gate can only rotate in one direction, then a person can only pass through it in one direction, because if they try to go in the wrong direction, they will be trapped between one of the rotating gates and the stationary row of bars.

I've seen them used instead of turnstiles in many public facilities, such as New York City subway stations, and at the entrances/exits of the Bronx Zoo.

Here's a front-view diagram:

The gate rotates towards you on the left and away from you on the right.
___________________________
 |          | |          |
 |========) |R|          |
 |          |O|          |
 | (========|T|========) |
 |          |A|          |
 |========) |T| Walk     |
 |          |I| into     |
 | (========|N|========) |
 |          |G| this     |
 |========) | | side.    |
 |          |C|          |
 | (========|O|========) |
 |          |L|          |
 |========) |U|          |
 |          |M|          |
 | (========|N|========) |
 |          | |          |
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Now, the anecdote. Ever since I was little (probably since I was five or six and first saw a revolving gate at the Bronx Zoo) It's always seemed to me that these things were designed with the intent that anyone trying to go through the wrong side of the gate would, because of the stream of people traveling through the other side, be pressed into jam.

(That last line used to end "...pressed into a jam-like substance." I had originally intended to write "pressed into jam." because it has a nice ring to it, and I don't know why I originally wrote something stupid-sounding instead. I like to say "Pressed into jam." referring to people.)
Nifty, asterphage, I used to work for the company that made these.

These gates are called NYCT High Barrier Entry/Exit Turnstile (HEET). In the NYC Subway system, this gate combines the Rotogate Barrier (which you drew a nifty picture of, asterphage), a Gate Controller, a Swipe Read/Write unit, and a set of vacuum flourescent displays (VFDs).

The Rotogate is designed to prevent fare evasion at unattended stations. These units accept magnetic farecards, tokens or MetroCards.

The HEET gate can be set up as an exit gate with no farecard required. It monitors the passengers with sensors, which is reported to the Station Controller (SC).

All data from passenger movement and fare cards are captured and stored at the gate. This information is sent to the SC when the gate is polled. A single HEET gate can store up to three days of data.

When a farecard is presented and validated, the gate keypin retracts and allows a single passage.

Description            Specification

Material               Type 304 stainless steel
Fare Media Acceptance  Farecard, Metrocard, Tokens
Farecard capture       No
Farecard Read/Write    Swipe
Patron Displays        Vacuum Flourescent Display (2x20)
Operating rate         30 passengers/minute
Barrier type           Rotogate
Anti-Passback          Yes
Counters               Electromechanical and electronic
Communications         RS422 serial
Reliability            120,000 Mean Time Between Failures
Maintainability        0.5 Hrs Mean Time To Repair
Power                  120VAC, 60Hz, 5 Amps
Battery backup         Yes

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