How Does a Traffic Light Work?

While waiting at the Wonderopolis bus stop, we overheard a conversation between two dogs about the traffic at the nearby intersection:

Dog 1: So let me get this straight, Charlie. Humans drive these cars around and stop for no apparent reason just because there’s a red light hanging in the air?

Dog 2: I know it sounds crazy, Rex, but I’ve observed the cars here for a while and that seems to be what happens. Stay here and watch with me for a bit.

Dog 1: Look at that, Charlie! You were right. That car was driving, and even though there are no other cars around, it stopped when the light up there changed from green to yellow and then red.

Dog 2: It’s a mystery, Rex. Why would they do that? It’s like they have nowhere important to be. It’s as if they have all the time in the world and are never in a hurry.

Dog 1: People are so strange. I’ll never understand them. Oh well, I need to go home and chase cats and bark at leaves.

Dog 2: Look! That car is coming this way. Let’s chase it!

Rex and Charlie ran off, barking and causing a commotion. We’re not sure if they caught the car or what they would have done if they did. However, their conversation made us WONDER about the traffic lights that hang above intersections. How exactly do they work?

While traffic lights may remain a mystery to dogs, human drivers appreciate their presence in regulating traffic on busy streets. Without traffic lights and stop signs, navigating a big city would be quite hazardous.

As you may already know, most traffic lights have three colored lights. The green light indicates that it is safe to proceed through the intersection. A yellow light (also known as a caution light) warns drivers that the light is about to turn red.

When drivers encounter a yellow light, they understand the need to slow down and stop at the intersection. The red light signifies that drivers must come to a complete stop and wait for a green light before proceeding through the intersection.

Although most traffic lights have a similar appearance, they can operate in various ways. The most common traffic lights use timers. Depending on the traffic levels at a particular intersection, the traffic light will cycle through green, yellow, and red at regular intervals, ensuring a consistent flow of traffic in all directions. Timer-based systems are effective in busy areas with heavy traffic.

In areas where traffic is sporadic and unpredictable, timer-based systems are not as efficient. For instance, in rural areas, a timer-based system may unnecessarily halt drivers when there is no traffic. In such situations, sensor-based traffic signals optimize traffic efficiency by functioning only when there is traffic.

Instead of timers, “smart” or “intelligent” sensor-based traffic signals rely on a system of sensors to detect the presence of vehicles. The types of sensors used may vary based on location and technology. Some systems use lasers, rubber hoses filled with air, or video cameras to detect cars.

Another popular type of sensor is called an inductive loop system. These systems rely on a coil of wire that is embedded in the surface of the road. The wire detects changes in magnetic fields when vehicles (large metal objects!) are stopped above them.

So how do sensor-based systems optimize traffic flow? In a rural area, for example, the main direction of travel can stay on green to allow the majority of vehicles to pass through quickly. When a vehicle on a side road arrives at the intersection, a sensor will detect it and change the lights to allow traffic on the side road to pass through. This way, traffic on the main road can flow continuously unless traffic on a side road appears.

Some traffic signals in big cities are advanced enough to let certain types of vehicles control them when necessary to ensure quick passage through intersections. These systems, known as traffic preemption systems, allow emergency vehicles like ambulances, fire trucks, and police cars to manipulate traffic signals as they approach, enabling them to reach their destinations much faster.

Try It Out

Are you ready to go? As soon as the light turns green, make sure to explore the following activities with a friend or family member:

  • Ask an adult friend or family member to take you on a field trip to a local area with multiple traffic lights. Park your vehicle and spend some time at each intersection with a traffic signal, observing how it works. Can you determine whether it’s working on a timer or a sensor? If it’s a sensor, how is it triggered? How can you tell? Have fun observing traffic signals in the wild!
  • Inductive loop sensors work by detecting changes in magnetic fields caused by the metal in cars. Do you think these sensors will become outdated as more and more cars are made out of plastic and composite materials? Why or why not? Based on other types of sensors you’ve seen developed and implemented in recent years, what types of sensors do you think will be most suitable for traffic signals in the future? Why?
  • Do you have a vivid imagination? Imagine that you’re a traffic engineer in the year 2099. Regular vehicles like cars, trucks, and motorcycles are still in use, but there are also autonomous (self-driving) vehicles and even flying cars. What type of traffic signal system would you create to manage traffic involving all these different types of transportation? Ponder on it and then write a short story or draw a picture that explains the type of traffic signal system you would use in the future!

Wonder Sources


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