How Traffic Lights Work – The Basic Principle


Traffic lights are essential to any road system as they control traffic flow and help keep everyone safe by using specific traffic patterns. But have you ever wondered how they work? Or do all of them work the same? This guide will look at how traffic lights work and some of the different types you might see.

How Do Traffic Lights Work?

It’s a question you’ve probably wondered while sitting in traffic. After all, it doesn’t seem like the most complicated system in the world.

Traffic lights have three main parts: the controller, the sensors, and the lights.

The traffic signal controller uses the information from the sensors to determine when to change the light. The lights are, of course, what drivers see.

There are three colors: red, yellow, and green; each has a different meaning.

Red means stop. Drivers must stop their vehicles before the stop line.

Yellow means caution. Drivers should start to slow down, but they don’t have to stop.

Green lights mean go. Drivers can resume their average speed.

Road cross with cars

Road cross with cars

Do All Traffic Signals Work in the Same Way?

No, there are two main types: fixed time and traffic responsive.

As the name suggests, fixed-time signals depend on a set schedule. The traffic engineers observe the flow of traffic and adjust the schedule accordingly.

For example, they schedule a specific time for each light. When this time expires, the light will change regardless of the traffic volume.

On the other hand, dynamic control signals change depending on the time of day and the actual traffic conditions.

Sensors in the road detect how many vehicles are there and communicate this information to the controller. The traffic control device then adjusts the green times accordingly, depending on traffic volume.

Have you ever been waiting at a stop light, and suddenly the green signal turned on? That’s not a coincidence but rather the result of the traffic controller making decisions based on real-time data.

Smart traffic control system

Smart traffic control system

What Are the Functions of Traffic Light Sensors?

As mentioned, dynamic traffic signals use sensors to detect the presence of vehicles. There are three types of sensors: induction loops, infrared sensors, and microwave radar.

Induction Loops

Inductive loop sensors are the most common type. They are typically a coil of wire embedded on the road surface and detect the presence of a conductive material, such as a car.

When a car passes over the sensor, it causes a change in the electromagnetic field. Once the car leaves the sensor’s range, the magnetic field returns to normal. The controller uses this information to decide when to change the light.

Infrared Sensors

Infrared sensors use beams of infrared light to detect the presence of vehicles. These sensors are typically installed above the road, pointing down at an angle.

There are two types of infrared sensors: active and passive. Active infrared sensors emit a beam of infrared light in a specific area, and if an object interrupts the beam, it means there is a vehicle in the area.

On the other hand, passive sensors detect changes in ambient infrared light. They are sensitive to heat, so they can detect the presence of a vehicle even if it’s not directly in the sensor’s line of sight. When a car passes by, its energy causes a change in the infrared light, which the sensor will detect.

Microwave Radar and Video Cameras Sensors

Microwave radar sensors can detect the presence of vehicles using microwaves. These sensors emit microwaves in a specific area and then detect the reflections.

The microwaves will bounce off nearby vehicles and return to the sensor if any are present. The controller then uses this information to decide when to change the light.

Roundabouts Vs. Traffic Lights

Roundabouts and traffic lights are two different ways to control traffic flow; both have pros and cons. Let’s figure them out.

Traffic Lights Pros

  • Less real estate as they only need space for the poles and wires
  • Easier to understand for drivers
  • Can control traffic depending on the current situation
  • More efficient in busy intersections and heavy traffic areas as vehicles can keep their speed

Traffic Lights Cons

  • More expensive to install and maintain
  • Prone to malfunctioning
  • Require a power source
  • You may wait for the light to change in low-traffic areas, although there is no need for it.
A traffic light with red and orange

A traffic light with red and orange

Roundabouts Pros

  • Cheaper to install and maintain
  • Traffic always flows
  • Better for simple intersections
  • Rear-end collisions are less

Roundabouts Cons

  • More difficult to understand for drivers
  • Take up more space
  • We can’t use it in all intersections
  • In high-traffic areas, they may cause congestion
Green aerial roundabout

Green aerial roundabout


How Heavy Are Traffic Lights?

The average traffic light weighs between 35 and 80 pounds.

How Do Traffic Lights Know When to Change?

There are two main types of traffic lights: timed lights and dynamic lights. Timed lights change based on a set schedule, while dynamic lights change based on real-time conditions.

What Are the Different Types of Traffic Signals?

Traffic signals are classified into the following types:

  • Fixed-time signals
  • Traffic-actuated signals
  • Traffic Control Signals
  • Special traffic signals
  • Manually operated signals
  • Pedestrian signals


This article briefly examines how traffic lights work and shows you the different traffic signals. We also examine the pros and cons of traffic lights and roundabouts and try to clarify some common questions.

We hope this article is helpful. If you have any questions, please leave a comment.

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