Collection 3: "Simplexity" by Jeffrey Kluger

They were, instead, twice victimized—once by the men who hijacked the planes and took so many lives; and once by the impossibly complex interplay of luck, guesswork, psychology, architecture, and more that is at play in any such mass movement of people.

Fear plays a role, so does bravado, so does desperation.

But so do ergonomics, fluid dynamics, engineering, even physics—all combining to determine which individuals get where they’re going, which ones don’t, and which survive the journey at all.

But so do ergonomics, fluid dynamics, engineering, even physics—all combining to determine which individuals get where they’re going, which ones don’t, and which survive the journey at all.

On an ordinary day, twenty thousand people working in a skyscraper or half a million people in a coastal city occupy the same spot on the complexity spectrum as air molecules filling a room—moving randomly and chaotically in all directions, filling all the available space more or less uniformly.

The best way to understand the elaborate manner in which people move en masse may be to understand the equally complex way water does the same, particularly how it navigates around obstacles or breakwaters.

A foundered boat or a tumbled boulder in the middle of a rushing river turns even the most powerful current chaotic, reallocating its energy into increased swirling and churning and decreased velocity.

A foundered boat or a tumbled boulder in the middle of a rushing river turns even the most powerful current chaotic, reallocating its energy into increased swirling and churning and decreased velocity.

The obstacle keeps you at the top of the complexity arc, preventing you from plunging so headlong to the frozen end.

“You create a little turbulence,” says Santa Fe Institute economist John Miller, who specializes in complex adaptive social systems.

Open a second exit somewhere else and the people would respond appropriately, with half of them choosing that new option and the flow at both doors adjusting itself commensurately.

They would correct for things like accessibility and proximity of the exits—with more of the simulated evacuees choosing the closer, more convenient egresses.

They would correct for things like accessibility and proximity of the exits—with more of the simulated evacuees choosing the closer, more convenient egresses.

Finally, information tends to get distributed unevenly, with some people learning about an emergency first and acting before the others. Once all of this was written into the fluid-based programs, the simulations ran amok.

Programs with such evocative names as EGRESS, EESCAPE, and EXODUS take into consideration everything from the nature of an emergency (fire, bomb scare, blackout) to the season in which it occurs...

Programs with such evocative names as EGRESS, EESCAPE, and EXODUS take into consideration everything from the nature of an emergency (fire, bomb scare, blackout) to the season in which it occurs...

Of course, computers can’t model the existential terror that comes along with any evacuation and programs like EGRESS are thus limited by their very nature.

The improbability of one building suffering the kind of violence that was done to the towers that day—to say nothing of two—called not for a wholesale
evacuation, but for a so-called sheltering in place, taking cover precisely where you are and thus staying safe from the chaos and debris on the street below.

The delay flows back along the queue the same way a ripple propagates through water, or cars entering a highway briefly slow the ones closest to the on-ramp, causing a wave of tapped brake lights that may radiate miles back.

Sequential evacuation is the best way to handle this problem, with people on lower floors leaving the building first and each higher floor following successively.