How Does a Traffic Light Detect That a Car Has Pulled Up and is Waiting for the Light to Change?

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September 30, 2006 | 5,903 views

Ever wonder why traffic lights methodically change at the time or shortly after you reach a traffic light, especially along suburban and county roads?

This item from, combining two of my favorite interests -- cars and science, explains why.

The most common technique is the inductive loop, a coil of wire embedded in a pre-cut road surface. Simply put, when a car stops over an inductive loop, a circuit is completed, changing the magnetic fields and later the traffic light.

The HowStuffWorks review explains the simple physics of inductance and how coiled wire wound around a piece of metal affects magnetic fields in ways that can monitor and control traffic lights.


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