Life Safety, Means of Egress, and Human Behavior
The egress system
Means of egress has three parts: exit access (path to the exit), the exit (protected, e.g., enclosed stair or exit passageway), and exit discharge (path to the public way). Every component must be continuous, unobstructed, and of sufficient capacity.
Occupant load and capacity
Occupant load = floor area ÷ occupancy load factor (e.g., 100 ft²/person business, 15 ft²/person unconcentrated assembly, 5 ft²/person concentrated). Capacity is width-based: 0.2 in. per person on level components (0.15 in. where sprinklered and smoke-protected), 0.3 in. per person through stairs, and door clear width is measured with the door at 90° open. Two exits are required once occupant load exceeds the threshold for the occupancy (commonly 49–50 for many business/mercantile cases); additional exits are triggered at higher loads. The loss of any one exit must not reduce capacity below the required minimum.
Geometry limits
Common path of travel, travel distance, dead-end corridors, and remote-exit separation (typically one-third or one-half the diagonal) are the usual failure points in plan review. Exit width is not additive across components - the narrowest component governs.
Human behavior
Panic is rare; people typically delay, investigate, and use familiar routes. Pre-movement time often exceeds travel time, particularly where alarms are ambiguous or occupants are asleep or in assembly settings. Defend-in-place and staged evacuation are valid strategies where construction and compartmentation support them.
Worked example
250 occupants in a sprinklered business area: level egress width = 250 × 0.2 = 50 in. total clear width, and stair width = 250 × 0.3 = 75 in. Split across two exits, each exit must be adequate to carry its share and the redundancy rule must still be satisfied.
Traps
Mixing inches with feet, using unsprinklered factors, forgetting stair capacity factors, and treating areas of refuge as exits rather than as protected waiting areas.