That annoying person walking down the street with their head buried in their cellphone—don’t lie, sometimes it’s you—isn’t just slowing themself down. They’re also hamstringing other people’s ability to move quickly and efficiently, according to a new study.

Like flocks of birds or schools of fish, human crowds self-organize to move safely and efficiently. (Think different lanes of people walking down a sidewalk.) But such efficiency is only possible when people can anticipate others’ moves.

To test how a phone tapper might mess with this flow, scientists set up a straight corridor on a closed-off street for two groups of 27 pedestrians to walk down in opposite directions. They tracked each person’s path of travel, measured their walking speed, and calculated the groups’ order parameter, which measures where and when straight lanes formed. The researchers also tasked three participants in one group with typing on their phones as they strolled, and ran three different experimental conditions where the distracted participants were in the front, middle, or rear of their walking group.

The video above shows what happened with the inattentive individuals in front: Lanes both behind and opposite them developed slowly and—when compared with the middle and rear positions—several seconds later than they would have if all pedestrians were alert.

As expected, phone users had trouble anticipating the movements of others because they couldn’t see the crowd around them. But the researchers also found that alert pedestrians, especially those moving toward distracted ones, had to make sharp turns or step out of the way to avoid collisions, they report today in Science Advances. The volunteers were unable to anticipate the moves of the distracted walkers, even though they could devote their full attention to the paths in front of them.

It turns out that it doesn’t just take two to tango—but also to walk down the street.