Earlier quoted context omitted.
It basically reduces deadlock chance. When you have one exit and it's blocked due to many people attempting to fit at once, the entire queue is congested. That barrier turns one exit into two exits (even if it's backed by only one real exit). If one is congested, people can still flow through the other one, and vice versa. This explains why the evacuation rate is roughly twice faster. If the barrier is designed to ev…
"And this would be some heuristic which orders people in an unambiguous way, so only one person is attempting to exit at a given moment." Given how badly people behave at four-way stop signs (where there's an unambiguous ordering algorithm that most drivers seem to ignore), I don't think there's much hope for a heuristic ordering working well for panicked crowds at emergency exits.
But it gets worse than that, as pedestrians have priority and can break up the established order. If it's "my" turn, but a pedestrian blocks only my way, someone else can rightly cross out of turn, but then once the pedestrian is clear when does it become my turn again?