In the summer of 2000, a couple thousand people walked over the brand-new Millennium Bridge in London (pictured) at once, causing it to sway dangerously from side to side. The structure wobbled not because so many people were on it, but because they fell into lock step with each other and the motions of the bridge itself. In a new study that used multiple modeling approaches to capture pedestrian-bridge interactions, researchers found that pedestrian bridges don’t become more unstable as more people walk over them. Rather, things break down when the crowd exceeds a certain limit, the team reports today in Science Advances. When the number of people walking on a bridge passes this threshold, the force of their collective footfalls pushes the bridge from side to side in sync with the bridge’s minute oscillations and exacerbates the swaying. But, there’s a simple fix: Change the bridge design to make it heavier, longer, or stiffer, the researchers say. That will put the bridge’s natural motion outside of pedestrian influence. The team hopes the work will inform new bridge design, potentially saving lives and millions of dollars.