How do locals know which supermarket queue will suddenly speed up?
A queue is moving toward a future, not displaying a present.
Most shoppers count people. Experienced shoppers often evaluate transactions.
The hidden mechanism is remaining workload. A queue's future speed depends less on how many customers are present and more on how much work remains before each customer leaves.
Imagine two checkout lines. One contains six customers carrying only a few items. Another contains three customers with overflowing carts and complicated purchases.
A second-order effect develops because many shoppers focus on visible queue length. This behavior creates opportunities for those paying attention to less obvious signals.
People often think queues are made of people. Operationally, queues are made of unfinished tasks.
