How do Traffic Signals Work (2019)

Traffic signals are described as the primary method for assigning right-of-way at intersections, balancing space, cost, approach speed, cycle time, sight distance, vehicle and pedestrian volumes, and human factors. Urban arterial intersections create interrupted flow and generally define a roadway’s capacity, so improving intersection efficiency and safety is the main lever for throughput. Signals break movements - right, through, left - into phases, often sketched with ring-and-barrier diagrams that show sequences like protected lefts, through movements, and clearance barriers. Timing decisions set green durations to clear queues, yellow length for perception and comfortable deceleration (a common rule is one second per 10 mph), and a brief all-red clearance interval so intersections can empty.

Modern installations use detectors - inductive loops, cameras, radar - to inform a roadside controller for actuated control that adapts phase timing, avoids unnecessary waits, and can give priority to emergency vehicles and transit. Coordination along corridors times greens to move platoons of vehicles through multiple signals, boosting capacity where interruptions are few. City-scale adaptive signal control centralizes detector feeds and applies optimization algorithms to cut congestion but raises cybersecurity concerns. Widespread vehicle connectivity and autonomy could further reduce interrupted control, yet latent demand fills added capacity quickly, limiting permanent congestion relief.