Chairs may seem like an overlooked item in an everyday setting, but it is something that is so engrained in our subconscious that we instinctively go for a place to sit whenever we can. This is why seating in a public space is so important to consider and must not be overlooked. I have observed at Town Square in Las Vegas over three days and have come up with a theory that people will more than likely sit in a chair if it meets three criteria, and that is that the chair and it's environment must be comfortable, there must be an implicit authorization that someone may sit in that chair, and the people must be able to customize the space that they occupy.
Comfort is something that is a little subjective among a large group of people, so one thing that may be needed is a variety of seating options to allow for a variety of people to be comfortable. The other thing is that each chair must fully support the person who is sitting down for an extended period of time. This means that the backrest must push on the lower back, must allow for the space for the butt on the seat, and must allow for some space between the back of the persons knee and the front edge of the seat. The environment must also be comfortable, and in Las Vegas that means cool. Shade is a must since the shade can decrease by up to 15 degrees. So in the hot day shade will make a space more comfortable.
Implicit Authorization is also a must. At Town Square on Sunday September 30, 2012 there was an event, Ride for Kids, that had tents setup with chairs underneath it. This was setup some distance away from the stage and between the edge of the tent and the stage was open green space with no one in that space. Not a single person was either standing or sitting in this open space. I believe this was because it wasn't implicit that this area belonged to the people, the public. What was given to the people was the area under the tents, so that's where the people stayed. Other instances that have been observed by my colleagues where people wouldn't venture into "unknown" territory where it wasn't normal for someone to sit in a particular space. So the space must be designed to give people the idea that they may sit in that space.
Personalization is also a must because without this element people won't be able to arrange the space to their comfort level, whether for a private conversation or for a large open group, to that of single people sitting alone. They must be able to move chairs around and essentially make the space their own. They may, as seen in a video from William Whyte, only move the chair an inch, but by moving it they have made it their space and not someone else's space.
So a person will more than likely not sit in a chair if it does not follow these three criteria. They would much rather go sit somewhere else. I have noticed in Town Square that there are multiple places that may meet 2 out of the 3 criteria and yet no one is sitting in those spaces. There is a higher likelihood that someone will sit in a chair if it is comfortable, they have the authorization to sit there, and they have a way to personalize the space such that it is theirs.