It seems obvious. You arrive at the checkouts and see one line is much longer than the other, so you join the shorter one. But, before long, the people in the bigger line zoom past you and you’ve barely moved toward the exit.

When it comes to queuing, the intuitive choice is often not the fastest one. Why do lines feel like they slow down as soon as you join them? And is there a way to decide beforehand which line is really the best one to join? Mathematicians have been studying these qu