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Hank: If you’re stopped at a red light on a totally empty road, that can feel a little silly... You still have to patiently wait there for it to turn green, even though everywhere you can see no cars, for miles. But depending on what you’re driving and the kind of traffic light, the light might not actually know you’re there. Some traffic lights have fixed timing, but others use sensors – like cameras, or electronics below the pavement – to know where you are.

Many traffic lights run on fixed time intervals, like every 30 seconds. But engineers have done some clever things with that. In urban areas, for example, all the lights on a busy street might be synchronized to turn green one after another, so a driver never needs to stop – a strategy called the green wave. Other lights adjust their timing depending on the situation – like if a pedestrian presses a crosswalk button, or by using different kinds of sensors to detect drivers.

Cameras, for instance, are inexpensive and easy to install. They’re normally mounted near the traffic lights to get maximum vision. So, when you’re waiting at one of these lights, you can make sure you’re far enough forward, and try adjusting your vehicle so the camera senses your motion.

The other common type of traffic sensor is underground, and sometimes you can tell by little grooves in the pavement. The sensor is called an induction loop, and it’s basically a big loop of wire that has electricity running through it. And when electricity is flowing through a wire, it creates a magnetic field.

When a conductive object – like your car’s metal frame – moves through this magnetic field, the field actually affects the electrons inside the car. The magnetic field causes a small electric current inside the metal of the car – called an eddy current – that runs in the opposite direction of the current in the ground. And here’s the thing: this eddy current also creates an induced magnetic field.

Then, the induced magnetic field affects the original underground current. So the current in the ground affects the current in the car, and then the current in the car affects the current in the ground. The computer in the traffic signal detects the changes that happen to the current in the ground, and realizes some conductive metal vehicle must be waiting there.

That’s how it knows to turn the light green for you. If you’ve been waiting for a while and you’re worried the induction loop isn’t detecting you, you could try checking that you’re in the center of the loop. If you’re riding a motorcycle, which has less metal than a car, some people recommend putting down a kickstand, so more metal is slightly closer to the loop. If you are riding a bicycle just get out and push the button... or just run the light, because there’s nothing you can do.

So yeah, waiting for traffic lights to turn green can be annoying. But knowing how they work might help you make sure the computers know that you’re there.

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