This would never work in New York City, where subway door operators score extra points for every rider snared between their door-jaws. For the rest of the world, however, these subway signal lights are designed to tell you when to stroll and when to roll.
The concept, created by designer Hyun Ju Park, involves adding a strip of LED lighting to the side of subway doors. They work just like a traffic light does: green means you're good to go, yellow indicates you're running out of time and red tells you that you should already be diving head-first if you're not already.
As a system that moves people, subways benefit from an elastic schedule that varies throughout the day. Even in a small way, something like Park's subway signal light could work to helping manage congestion and keep things moving smoothly.
Down below, you can see Park's design details for his subway signal concept.
Via Yanko Design