Divided, Together


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I’m at Atlanta-Hartsfield on a busy Thursday evening, and my goal is to walk a few dozen yards from Simply Books across the “A” concourse’s main hallway to the Chick-Fil-A. It’s a short but not a trivial journey — the wide hallway is packed from edge to edge with a swiftly moving river of thousands of travelers headed both left and right. It reminds me of that chaotic moment between classes in high school when everyone rushing to get to their next class before the bell rings, although with less flirting and more grim expressions.

Closest to me a thick stream of people is headed to the right, toward Gate 34, and beyond that a counterstream has set up headed in the opposite direction toward Gate 1. Drawing on experience in whitewater kayaking, I ease into the flow moving to my right and ferry across the current, crossing diagonally as I move ahead. I hit some crowd turbulence in front of restrooms, and use this to double back, picking up the counterflow, and eventually eddying out in the food court now on my right.

Academics have been studying crowd dynamics and collective motion for about a half-century. This arose partly because crowd dynamics sometimes go bad — notably at soccer matches in England, at pilgrimages in Saudi Arabia, and at rock concerts everywhere. Mayhem erupts, order breaks down, people get panicky, and deaths result — such as when more than 1,400 died in a pedestrian tunnel in Mecca in 1990.

Crowd researchers today often study this through computer simulations of crowds to better understand and better predict. Each person becomes a particle, like an atom moving independently but within a larger mass.

Each of these atoms is programmed with certain shared traits that govern interactions. With some cultural variations, each atom tries to keep a fairly standard distance from others as it moves, with that distance narrowing as the density of the crowd increases.

Edward T. Hall wrote in his 1966 book The Hidden Dimension that in most public realms — say, on a sparsely populated sidewalk — we Americans generally incline to maintain about 12 feet between ourselves and others. When density increases, we’re willing to go down to about four feet before we start feeling confined and agitated. So, essentially, we like to maintain a bubble of about two feet as we move through masses of other people.

When we walk down a crowded city sidewalk or a busy airport concourse, we seek the most efficient route while maintaining this bubble. This collective movement often naturally and economically breaks into two streams, one flowing each direction, like opposing schools of fish. (Americans tend to stream to the right; the left-driving British have a weak proclivity to veer left, but still often head to the right, which may explain why London streets often seem so vexing for Americans to navigate.)

At intersections and pinch points, these tidy streams are disrupted, and new patterns emerge. Researchers have found that syncopated flows often occur at busy pedestrian intersections, such as where two concourse corridors converge. One stream, say, from east to west will dominate until the pressure on the stymied south-to-north stream exceeds it, whereupon it will surge like a squeezed balloon, and halt the east-west flow until pressure builds again crosswise and another rebalancing occurs. Under less dense circumstances, sort of zig-zaggy stripes will emerge as the two crowds interweave and each person briefly tacks slightly away from their destination, as I did to get to the Chick-Fil-A in Atlanta.

This modeling mostly assumes each pedestrian is moving as an individual and seeking the most efficient route. But that’s often not the case in the real world. We often travel in groups — we’re less lone atoms than parts of molecules. Studies have found that a majority of pedestrians — about fifty percent on a weekday afternoon, or seventy percent on weekends — are part of a group, such as couples, families, or co-workers headed back from lunch.

The socially preferred shape of these social molecules in motion is typically a straight line moving in the direction of travel — in others words, we like to walk several abreast since that makes communicating easy. Larger groups, it’s found, tend to atomize into groups of two, three or four people when walking down streets; more than that makes conversing as a group difficult.

But as crowds grow more dense maintaining this formation becomes trickier, and these units often bend to form a “V” — with one person taking the lead, and others trailing slightly behind. The group can still talk, although not optimally, and it can still make forward progress, also not optimally — a linear formation would face less oncoming resistance. It’s a compromise, and one that changes pedestrian dynamics and modeling. (How? “These groups constitute medium-scale aggregated structures and their impact on crowd dynamics is still largely unknown,” notes a 2010 study.)

Groups of pedestrians also frequently form ad hoc pods and divide labor under certain circumstances, not unlike an ant colony. Sociologist Jon Wagner noted in 1981 that if you’re walking alone on an empty sidewalk and reach a crosswalk, you’ll often stop and gather several pieces of information: you check the walk/don’t walk sign, you consult the light for automobile traffic to see if it’s about to change, and you look to see if approaching cars are beginning to slow, signaling an opening for a crossing.

But if a group of pedestrians is gathered at the corner, it turns out that we often split up tasks up among specialists and form an impromptu squadron. The walkers on the “front line” watch the lights, and another group will scan for approaching cars. “The person in the backfield does not look at the signal or the traffic conditions,” Wagner wrote, “but looks straight ahead at the back of a front-liner,” awaiting the signal to cross.

“The back-fielder often relies completely on the judgment of a front-liner,” Wagner noted. “In the roles of light-watcher, street-watcher and backfield, anonymous individuals have worked out a collective solution to one small part of their common fate.”

Not everyone enjoys being part of a moving crowd — we all know someone who will avoid crowds, for any one of several reasons. There’s simple claustrophobia, or a more narrow concern about blocked exits, or general social anxiety, or a fear or being mugged or catching a strange disease. Demophobia comes in many flavors.

I tend toward demophilia — I love being a tiny part of a larger flow. (Exception: I hate being stalled in a huge group, as when waiting for the gates of a concert to open. I like to keep moving and often scan ahead for blockages; non-moving crowds are my kryptonite.)

When weaving through a group of strangers, I love the micro-transactions — the fleeting eye contact, the shoulder pullbacks at the last second, the use of the eyes or slight turn of the head to convey direction to avoid the awkward sidewalk pas de deux.

I like the stimulation that comes from the constant processing of highly local data, of recalculating microroutes and velocity to make headway most economically. (On crowded sidewalks, I’m a curb-darter, and like to use curbside planters and awning posts like screens in basketball.) I love being the Atomic Pedestrian, a unit of one, moving within a fluid social matrix.

What’s the opposite of walking through a crowd? You could make the argument that it’s sitting on an island far from the sight of land.

But I’d argue that a more telling opposite is being stuck in a car trying to exit the parking lot of an octaplex after three hit films just let out. Everyone is trying to squeeze though a single exit, and make the same left turn.

Stuck here, we lack movement, we lack alternatives, we lack even passing human contact. We’re encased in our hard carapace, resolutely looking ahead, tapping the wheel, fearful that any eye contact with the person in that Ford Explorer to our left will weaken our resolve and let them edge in front of us.

It’s not people around us. It’s just machines. We “‘thingify’ the person” in other cars, said psychotherapist Barry Markell in a story about road rage. That sounds about right.

Walking advocates often refer to our becoming bipedal, and highlight our first steps millions of years ago across the empty African savannah. But less about how we’re also a social animal, and we often walk in concert. How we mass and respond when moving in crowds is just as vital a part of daily life.

At Atlanta, I finished my chicken sandwich and started off to my gate. I fell into line and moved with the flow. Obstacles formed: lines extended out into the terminal walkway from Gate A-12 and the nearby McDonald’s; couples stopped the middle of the concourse to double check their boarding passes, people pooled around the departure boards, forming a shallow peninsula that extended into the concourse and created a temporary pinch point. I chose an inside route, cutting through a seating area.

Being apart while being together, always moving — it’s a large part of what makes me feel alive. Moving on foot, with and through a crowd, can be a wonderful thing. It reminds of us that we’re human. • 23 May 2014