As you may know, a turnstile is a type of gate that lets in one person at a time, and is most commonly used at entrances to public services like trains or events.
More info. here: http://en.wikipedia.org/wiki/Turnstile
I regularly use the local train system (BART) in San Francisco and realize they have a pretty good turnstile system. Some of the nuances show how much design and thought has gone into the design (or is it just a weird side-effect?).
While visiting Chicago, I used the CTA (train system) and noticed the design of their turnstile. To begin with, their entry card seemed unintuitive to me since the card had an angular cut on one edge, which one would think is the leading edge, but the broader edge of the card needs to be inserted (vertically) in the reader.
I realized both systems had a (few steps) distance between the reader and the gate. This obviously provides a kind of buffer so that even if a person fumbles or takes longer at the gate, the person behind can still use the reader.
BART is really nice because it prints the dollar amount remaining on the ticket; you insert the ticket at one end and it travels and comes out at a slot near the gate a little further down, which also adds a buffer to the ticket removal action. The downside to this is that if a ticket fails, the person has to backtrack from the gate, but I guess the probability of this is low compared to the benefit provided.
Also noticeable is that BART turnstiles are bi-directional and although at any given moment they operate in only 1 direction, they are switchable between entry and exit mode to accommodate rush hour patterns.
The BART system is sometimes confusing since exits also need a swipe with the same ticket used for entry and also machines inside the exit only charge the amount required to get out and dispense the rest as change. So, I pity the person who needs a needs to add a charge of 30 cents and only has a $10 bill; it’s $9.70 back in quarters.
Again, this is a choice between efficiency and probability.
That’s all I can think of for now…will observe and add more later…