Francis Ching, in his book on Building Illustrated, first and fourth edition,  classifies  stairs under special construction. Egress is a means of exiting a place and so, it follows that egress is implied departure from a building or enclosure, or it could be open, depending on context. But in Ching’s notes, egress is specific to exiting  a building in case of accidents or hazardous conditions to humans.

Stair design in general is considered as special construction for two reasons, first it is internal and it is listed together with spaces that have overlapping areas with mechanical, electrical services or multiple servicing.

Stairs are a means  to circulation up or down  for occupants( in an enclosure).  Constant foot traffic and movement must be ensured to safety by the stairs themselves. If people are moving, the generalized strides up/down must be comfortable and safe. It is for this reason that stairs and egress stairs are designed with safety fire codes and comfort.

In times of general fear and panic people behave like water, constantly rushing without regard to path. Trampling and mob behaviour are displayed in such times; fires, hazards and others. In this way, egress staircase must ensure that foot traffic is not choked or endangered.

Knowledge of how to design staircases are important also because of the occupants. If a design is a safety hazard it cannot be implemented or in more  dire cases, an architectural designer will be sued for the hazard caused and may lose livelihood and or license to practice profession.