Drive your Model T through a major intersection a hundred years ago, and you'd likely encounter a policeman directing traffic. Ten years later, that officer would've been replaced by a traffic light. As the number of cars on the road increased, cities needed a way to keep cars from crashing into each other. With its dogmatic procession of green, yellow, and red, the automated traffic light did the trick.
A few adjustments were made in the decades that followed, such as pedestrian-walk sensors, but the basic concept remained the same: Signals were for safety's sake. But today's traffic engineers are starting to give the humble signal new responsibilities—programming them to not only react to the flow of traffic, but also to predict driver behavior. The signals of the not-so-distant future may help cities cut congestion without adding lanes or building new roads.