Evacuation time, flow and stair capacity

How long will it take to get everyone out? Wardens are asked this by managers, by fire departments after drills and sometimes by worried occupants. Fire protection engineers answer it with detailed calculations and computer models, but wardens can understand the main ideas with simple arithmetic. Knowing where the time goes tells a warden where effort pays off: in many buildings, the minutes lost before people start moving, and the minutes spent queuing at doors and stairs, matter far more than walking speed. This lesson introduces the basic model, shows how door and stair widths limit flow, and explains how to use drill timings to improve a plan. It is awareness-level material: it does not replace an engineer's egress analysis.

The three parts of evacuation time

A useful simple model splits the time to evacuate into three parts. Pre-movement time runs from the alarm until people start moving to an exit; it includes recognizing the alarm, deciding to act, finishing tasks and gathering things. Travel time is the time to walk from the starting point to the exit, and depends on distance and walking speed. Flow time is the time it takes the whole group to pass through the narrowest point, usually a door or stair, and depends on the number of people and the effective width.

The simplest estimate adds the three: total time ≈ pre-movement + travel + flow. This overstates the time a little, because people walk and queue at the same time, but it is easy to calculate, it is conservative, and it shows clearly which part dominates. Engineers refine it with the time at which queues form and with separate calculations for each floor and stair, but wardens rarely need that level of detail.

Pre-movement: the part wardens control most

Pre-movement times vary enormously. In a well-drilled office where wardens act at once, most people may be moving within a minute. In a hotel at night, a hospital, a shopping center or an office with frequent false alarms, pre-movement can run to many minutes, and in some fires people have waited ten minutes or more before leaving. Pre-movement also has a spread: a few people start almost immediately and a long tail follows. The last people to start often decide the total evacuation time.

Lesson 6 described how wardens cut pre-movement: visible presence, specific instructions, voice messages, and a culture in which every alarm is treated as real. Every minute saved in pre-movement is a minute saved for everyone, and unlike stair widths, it costs nothing to change.

Travel: walking speed and distance

Building codes limit the travel distance to an exit, typically to a few hundred feet at most, depending on occupancy and sprinkler protection. Unimpeded walking speeds for adults on the flat are around 4 ft/s, falling sharply in dense crowds, in smoke, on stairs and for people with mobility impairments. Some people using walkers or canes move at 1.6 ft/s or less. Going down stairs, an able-bodied adult in a light crowd typically takes 15 to 20 seconds per story, including the landings.

Flow: why widths matter

When many people try to pass through a door or down a stair, they form a queue, and the rate at which they get through depends on the effective width. The effective width is the clear width minus a boundary layer on each side, because people do not walk right up against walls, door frames and handrails. The SFPE Handbook of Fire Protection Engineering gives typical boundary layers, for example about 6 in for walls and door frames and about 3.5 in measured from the center of a handrail. For doors and corridors, a maximum specific flow of about 1.3 persons per second per meter of effective width is widely used, roughly 24 persons per minute per foot. Stairs carry fewer people, around 18 to 20 persons per minute per foot of effective width for common stair geometries, because people slow down and space out on steps.

These are typical values for healthy adults moving in an orderly way. Real flows can be lower when people carry bags, when some need help, when doors swing against the flow or are heavy to open, or when two streams of people merge at a landing. Treat the figures as a guide, not a guarantee.

Stairs in multi-story buildings

In a multi-story building, the stairs are almost always the bottleneck. A stair can hold only so many people at once, and people from lower floors entering the stair must merge with people coming down from above. Studies of merging at stair landings show that the flow from the floor and the flow down the stair share the space, sometimes with priority to one or the other depending on the door position and on who is more assertive. Upper floors can take much longer to empty than lower floors, because their occupants have to wait for space in a stair that is already full.

Using the model to improve a plan

The value of the model for wardens is in showing which change helps most. If pre-movement dominates, work on alarm recognition, warden response and culture. If a single door or stair dominates, look at whether people can be spread across other exits; drills often show that most people use the exit they know best while another stair stands nearly empty. If one floor is slow because many people need help, that is a planning issue for assisted evacuation, not for the stairs. If losing one stair would double the time, make sure the plan, the wardens and the voice messages can redirect people quickly.

Wardens should never change the building, such as by removing doors, propping them open or blocking a stair, to improve flow. Exit widths and door arrangements are set by building codes and fire engineers for fire safety as well as flow. Suggestions go through the person in charge of the plan.

Measuring flow in a drill

Simple measurements in drills give real numbers to replace typical values. A warden with a stopwatch at a stair door can record when the alarm sounded, when the first person entered the stair, and when the last person entered. Another observer at the exit can record when the first and last people left the building. The difference between the alarm and the first movement is a direct measure of pre-movement. Dividing the number of people by the time between first and last through a door gives the actual flow. Comparing drills over time shows whether training and changes are working.

US building codes and NFPA 101 size exits using occupant loads and capacity factors, such as inches of width per occupant for stairs and for other egress components, rather than flow times. Performance-based designs may use engineering egress analysis. Emergency action plans do not usually include evacuation time calculations, but drill records should show how long evacuation took.