How Does A Traffic Light Detect That A Car Has Pulled Up?

To install an inductive loop, workers lay the asphalt and then come back and cut a groove in the asphalt with a saw. The wire is placed in the groove and sealed with a rubbery compound. You can often see these big rectangular loops cut in the pavement because the compound is obvious.

Inductive loops work by detecting a change of inductance. To understand the process, let's first look at what inductance is. The illustration on this page is helpful.

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Here you see a battery, a light bulb, a coil of wire around a piece of iron (yellow), and a switch. The coil of wire is an inductor. The inductor is an electromagnet.

If you were to take the inductor out of this circuit, then what you have is a normal flashlight. Close the switch and the bulb lights up. With the inductor in the circuit as shown, the behavior is completely different. The light bulb is a resistor (the resistance creates heat to make the filament in the bulb glow). The wire in the coil has much lower resistance (it's just wire), so you'd expect when you turn on the switch that the bulb would glow very dimly. Most of the current should follow the low-resistance path through the loop. What happens, instead, is that when you close the switch, the bulb burns brightly and then gets dimmer. When you open the switch, the bulb burns very brightly and then quickly goes out.

The reason for this strange behavior is the inductor. When current first starts flowing in the coil, the coil wants to build up a magnetic field. While the field is building, the coil inhibits the flow of current. Once the field is built, then current can flow normally through the wire. When the switch gets opened, the magnetic field around the coil keeps current flowing in the coil until the field collapses. This current keeps the bulb lit for a period of time even though the switch is open.

A traffic light sensor uses the loop in that same way. It constantly tests the inductance of the loop in the road, and when the inductance rises, it knows there is a car waiting.

Inductive loop systems are commonly used thanks to their simplistic nature. There's much less chance of a breakdown compared to expensive and complex digital sensors, but this simplicity can also be a drawback. All the induction coil "knows" is whether or not a car is currently parked on top of it. This is the main reason that the light may fail to change in a timely manner if a car doesn't pull all the way up to a stop.

Lighter vehicles like motorcycles may also fail to trigger the inductor with their weight alone, making them a hassle for bikers during low-traffic hours. Digital sensor systems do away with these problems, and they allow transportation authorities to log countless hours of traffic data that can be used for future planning of routes and city projects.

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