Interacting pedestrians in a crowd spontaneously adjust their footsteps and align their respective stepping phases. This self-organization phenomenon is known as synchronization. However, it is unclear why and how synchronization forms spontaneously under different density conditions, or what functional benefit synchronization offers for the collective motion of humans. Here, we conducted a single-file crowd motion experiment that directly tracked the alternating movement of both legs of interacting pedestrians. We show that synchronization is most likely to be triggered at the same density at which the flow rate of pedestrians reaches a maximum value. We demonstrate that synchronization is established in response to an insufficient safety distance between pedestrians, and that it enables pedestrians to realize efficient collective stepping motion without the occurrence of inter-person collisions. These findings provide insights into the collective motion behaviour of humans and may have implications for understanding pedestrian synchronization-induced wobbling, for example, of bridges.

人群中互动的行人自发地调整他们的脚步并调整他们各自的步态。这种自组织现象称为同步。然而，尚不清楚为什么以及如何在不同密度条件下自发形成同步，或者同步为人类的集体运动提供什么功能益处。在这里，我们进行了单行人群运动实验，直接跟踪交互行人双腿的交替运动。我们表明，同步最有可能在行人流量达到最大值的相同密度下触发。我们证明同步是针对行人之间安全距离不足而建立的，并且它使行人能够实现高效的集体踏步运动，而不会发生人与人之间的碰撞。这些发现提供了对人类集体运动行为的见解，并可能对理解行人同步引起的摆动（例如桥梁）产生影响。