Now consider an equally large crowd, but imagine they consist of a highly trained military company. When the fire alarm goes off, instead of each individual going straight for the door and attempting to shove each other out of the way, they all immediately form a single file line down the center of the room, each taking their place based on where they started, no one "cuts in line", everyone moves at a quick but controlled pace and never any faster than the person in front of them.
The desire for speed overwhelms the desire to avoid collision and the blob people jam up against one another -- just as salt can jam the shaker even though the hole is bigger than the largest grain. The room takes longer to empty even though everyone tries to move faster -- handfuls of people escape in bursts between clogging events.
Say every car is as close as is safe to the car ahead, in every lane, and everyone is moving at a constant rate. Now what happens if one car wants to change lanes? Since the cars are all as close as can be already, there is no possible way that the car can change lanes smoothly, because someone is going to have to slow to let them in, and they will have to slow to make the merge. Now the following cars in both lanes have to brake. And since the cars behind them were already as close as possible to them, those cars also have to brake. And since the cars behind them... you get the idea... the stopped cars now travels back through the traffic in a wave.
Most fire codes require that the pathway to an emergency exit be kept wide open, but according to researchers in Japan, placing an obstruction next to an exit may actually help crowds of people to get out of a room more efficiently.
Researchers found that when people bottleneck near an exit, they start to jostle each other for position. The jostling acts much like friction, slowing down the rate at which people can exit. Introducing a strategically-placed obstacle near the exit can reduce the number of people pushing for the exit, speeding up the rate at which people can pass through.
"We found that we can evacuate faster if we put an obstacle at the suitable position in front of the exit," said Daichi Yanagisawa, who lead the study from the University of Tokyo in Japan.