How Do Traffic Lights Work?

Have you ever pulled up to a red light, only for it to suddenly change to green? It seemed like it was waiting for you, and in most cases, it probably was. Most traffic lights come with digital sensors that allow them to monitor vehicles. Once they detect a vehicle stopping at an intersection, the traffic lights change the light from red to green, helping traffic move smoothly.

Do All Traffic Lights Have Sensors?

No, not all traffic lights have built-in sensors. Some operate using a built-in timer that switches the traffic light according to the set timer, which can vary depending on the time, location, and even major events. Some cities run on a 100-second cycle, which refers to the time it takes for the traffic light to go from green to yellow, to red, to green again. Other cities might adjust the timer during rush hour or on days with major events, such as sports games.

What Are the Different Types of Traffic Light Systems?

There are two main types of traffic light systems:

Fixed-Time

As its name suggests, fixed-time traffic lights operate under a set timer. Once the set time is up, the controller switches the light from red to green or vice versa, regardless of current traffic conditions.

This makes them great for busy intersections with consistent traffic in all directions but not so much for intersections with sparse or unpredictable traffic.

Sensor-Activated

This includes inductive loops and other sensor-activated systems. Nowadays, some sensor-activated systems are advanced enough to change the traffic light when they detect emergency vehicles, such as fire trucks, ambulances, and police cars, passing by.

You can often find this type of traffic light system in the suburbs, small towns, and intersections with inconsistent traffic.

What Is an Inductive Loop System?

Most traffic lights use a sensor called the inductive loop system. Inductive loops, sometimes called induction loops, are coils that are embedded into the road’s surface. They constantly test the inductance of the loop in the road. What exactly does this mean? When a car stops on the loop, the car’s metallic body increases the inductance of the loop. This causes a change in the current flowing through the sensor’s circuitry, activating the traffic signal controller, which then changes the traffic signal from red to green.

To install an inductive loop, construction workers first lay the asphalt near traffic lights. After the asphalt dries, they cut it with a saw, place the wire in the groove, and then seal it with a rubbery compound.

Pros of Induction Loops

Inductive loops aren’t affected by the weather, and they typically aren’t affected by false negatives. They’re also able to cover a great length of road.

Cons of Induction Loops

While induction loops are one of the most reliable sensors out there, they aren’t perfect. They don’t always detect bicycles, motorcycles, and other lighter vehicles.

What Are Other Types of Traffic Light Sensors?

How do traffic light sensors work? Well, it depends. Most traffic lights use different types of sensors to make sure they can detect vehicles as accurately as possible. Instead of being embedded into the pavement, these sensors are usually installed near the traffic lights. Here are examples of other types of traffic light sensors:

Infrared Sensors

There are two types of infrared sensors installed in traffic lights: active and passive sensors. Active infrared sensors shoot a beam of infrared light onto the road. When a vehicle stops in the light’s path, it breaks the beam and alerts the sensor that a car is waiting for the light to turn green.

In contrast, a passive sensor detects temperature changes in the intersection. It picks up on the heat of an engine when the car nears the stoplight.

Microwave Radars

Instead of an infrared beam, a microwave sensor generates a magnetic field. When a vehicle enters this field, it disturbs the field and its magnetic waves. The sensor picks up on this change and switches the light from red to green.

Video Camera Systems

Some traffic lights have video cameras that monitor and detect traffic at intersections. These cameras are usually mounted onto traffic light poles and come with special software that detects cars.

Unfortunately, bad weather, headlight glare, and shadows can all affect their accuracy, making them the less popular choice. However, you can still find video camera systems used in the intersections of gravel and uneven roads.