The Etiquette of Bench Seating
"As the park fills, especially on a beautiful day, people gravitate to the benches, staking their claims as seating space becomes scarce. The act of choosing a park bench can be done without much thought, it but more often is done in accordance with an etiquette that defines the situation for everyone involved. Typically, visitors refrain from sitting on a bench already occupied by a stranger when alternative seating is available. This rule may not apply when the person has an ulterior motive. Most often, if there is a nearby empty bench, the stranger will sit there rather than impose himself and violate the other person's space. As a rule it is not until seating is visibly scarce that habitués will crowd others on a bench. In these circumstances violations may be viewed as excusable or appropriate by those being joined.
"Evident here is a strong concern about sending an ambiguous, unwelcome, or hostile wrong message. If someone violates this rule, the person being joined can get the impression that the person taking the seat is trying to move in on him or her, for, if this were not the case, why wouldn't the person take one of the available seats on another bench? When the seat is taken in spite of the availability of other seats, the person being joined has reason for concern. If the violator is considered an unacceptable seatmate, the person may try to discourage him or her, or simply leave. The action serves as evidence that something is amiss. If the person is deemed attractive, he or she may well be excused and even encouraged, with the un-offended party showing approval by responding with small talk."