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How traffic lights know when to change

Traffic lights are not guessing. They can detect cars with buried loops, cameras, radar, infrared, and even connected-vehicle data, then feed that info into a controller that decides what gets green next.[2][3][4]

  • 44 Output Networking Intelligent <strong>Traffic</strong> <strong>Signal</strong> <strong>Controller</strong> With <strong>Cabinet</strong> <strong>Traffic</strong> Light <strong>Controller</strong> Shenzhen
  • r/pics - My job. Sometimes people ask me what is inside a traffic signal cabinet.
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The classic sensor is the inductive loop: a wire coil cut into the road that watches inductance change when a metal vehicle stops over it. It is simple and reliable, but it can miss cars if they do not pull far enough forward.[2][3]

  • Infographic showing how GLIDE works with two cars on road apart
  • How Do Traffic Light Sensors Work?
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Other detectors cover different gaps. Cameras can see multiple lanes and measure counts or speed, radar works above ground and is less weather-sensitive, and some systems also use geomagnetic, laser, or infrared sensing.[3][4][13]

  • Types of Traffic Cameras
  • Red Light Cameras
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Fixed-time signals follow preset schedules. Adaptive control is different: it keeps updating green, yellow, red, cycle length, splits, and offsets from live sensor data, often every few minutes or even every cycle.[4][9][15]

  • Adaptive Traffic Control
  • Adaptive Traffic Control
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Coordination is what creates the green wave: multiple intersections share a common cycle and offset so a platoon can move through a corridor with fewer stops. It can fail when demand is uneven, pedestrians interrupt timing, left turns complicate phasing, or traffic patterns are too unpredictable.[1][18][19][33] Which part surprised you most?[6]

  • Cars navigating traffic with help from green wave traffic lights
  • a green light and a traffic light
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