How Traffic Lights Detect Cars Are Waiting for the Light to Change (2023)

How Traffic Lights Detect Cars Are Waiting for the Light to Change (1)

Has this ever happened to you? You pull up to a red traffic light and it changes to green in a few seconds! How did it detect your presence? Or maybe you've had the opposite experience: You sit at a traffic light for what seems an enormous length of time with no change.

Some lights don't have any sort of detectors. For example, in a large city, the traffic lights may simply operate on timers — no matter what time of day it is, there is going to be a lot of traffic. In the suburbs and on country roads, however, detectors are common. They may detect when a car arrives at an intersection, when too many cars are stacked up at an intersection (to control the length of the light) or when cars have entered a turn lane (in order to activate the arrow light).


Traffic lights commonly detect vehicles using digital sensors mounted on the lights themselves, or through an inductive loop embedded in the surface of the road. Both methods allow the traffic system to keep tabs on stopped vehicles occupying the intersection and help traffic to flow smoothly. However, they achieve this in very different ways.


  1. What Are Inductive Loop Systems?
  2. Other Types of Traffic Light Sensors

What Are Inductive Loop Systems?

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.

How Traffic Lights Detect Cars Are Waiting for the Light to Change (2)

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.


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.


Other Types of Traffic Light Sensors

How Traffic Lights Detect Cars Are Waiting for the Light to Change (3)

In addition to induction methods, you may see traffic lights employing a variety of advanced sensors for vehicle detection. These are typically installed near the lights, and don't require any laying of metal inside the road. Sensing methods you may encounter include infrared sensors, microwave beam emitters, and video cameras.

Infrared sensors come in "active" and "passive" varieties. An active system shoots out a beam of infrared light that stops just where a car might be during a red light. When a vehicle pulls up, the beam is broken and the sensor is able to detect that the space is occupied. (Emergency vehicles like ambulances and police cars have active sensors that can request a traffic light be changed, usually when their lights or sirens are on.)


Passive sensors instead use their infrared sensing to detect the heat that comes off a car's engine. However, other intense heat sources like direct sunlight can cause the system to see false positives.

A microwave sensor works similarly to active infrared, but instead generates a magnetic field around itself. When vehicles enter the field, they disturb it and cause changes in the waves. The sensor is then able to detect these changes and see the vehicle. Microwave emitters are relatively expensive, and don't have the heat contamination issues that you may see in infrared.

Video camera systems are the most complex we see in traffic lights, but also have the potential to be the most effective. Installed on the lights are multiple cameras, similar to those would see on a CCTV installation. These cameras allow multiple, maybe dozens, of traffic stops to be networked together.

They all talk to a server running software designed to identify vehicles, and count the number of them at a stop in real time. It may also be able to distinguish cars from pedestrians. The network takes in all this data and then attempts to run the series of lights at maximized efficiency. Long-term analytics can then be used to improve the system further or figure out where roads need to be modified. However, weather conditions like fog can seriously hinder the cameras' vision.

In order to ensure that traffic stays moving, lights are often installed with two types of sensors, or a sensor plus an inductive loop. If one type of detection becomes infeasible due to outside conditions or a hardware failure, the system is able to fall back on another and avoid a major traffic nightmare.


Originally Published: Apr 1, 2003

Traffic Light FAQ

How does a traffic light work?

There are two ways in which traffic lights work. In big cities, the traffic lights usually operate on timers, as there is a lot of traffic consistently throughout the day. However, in the suburbs and on country roads, traffic lights use detectors. They detect vehicles arriving at an intersection when too many cars are stacked up at an intersection. This detection mechanism controls the duration of the light. It also activates the arrow light when cars have entered a turn lane.

How do traffic lights know when a car is there?

There are different technologies that are used by traffic lights for detecting cars. However, the most common technique is the inductive loop, which is embedded in the road's surface.

Does a computer control traffic lights?

Sometimes, traffic lights are centrally monitored and controlled by computers to coordinate traffic lights in real-time and deal with changing traffic patterns. Timers or sensors are also used to manage traffic flow across a city.

How does a traffic light circuit work?

First, a coil of wire is placed in the grooves with an inductance meter attached to the coil to gauge the coil’s inductance. A traffic light constantly tests the inductance of the loop in the road. When a car arrives in the loop's magnetic field, it increases the inductance, telling the traffic lights that there is a car waiting.

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How Traffic Lights Detect Cars Are Waiting for the Light to Change? ›

Active infrared sensors emit low-level infrared energy into a specific zone to detect vehicles. When that energy is interrupted by the presence of a vehicle, the sensor sends a pulse to the traffic signal to change the light.

How does a stop light know you are waiting for it to turn? ›

A traffic light constantly tests the inductance of the loop in the road. When a car arrives in the loop's magnetic field, it increases the inductance, telling the traffic lights that there is a car waiting.

How do cars know when the light will change? ›

Essentially, a sensor installed in the traffic light “talks” to the car and tells it how much time is left before a red light turns green.

Do traffic lights have sensors to change? ›

Because the wire forms a large circle, these detectors are commonly called “loops.” When a vehicle drives over the loop, detector hardware senses the change in electromagnetic field caused by the presence of metal (from the vehicle) and a request for a green light is queued.

How do car sensors work at traffic lights? ›

Active infrared sensors emit low-level infrared signals into a specific zone. A vehicle is detected when it enters the zone and disrupts the signal. Passive infrared sensors do not emit signals of their own but instead detect the infrared energy emitted from nearby vehicles.

Are there actually sensors at stop lights? ›

Every traffic light signal has either a timer or sensor, which helps it direct traffic flow. In large cities, where vehicles cross road intersections around the clock, traffic is usually dictated by traffic lights that use timers.

Do traffic lights use weight sensors? ›

Myth #3: The Weight-Builder. The amount of weight present at an intersection triggers a green light. Reality: The weight of a vehicle has nothing to do with triggering a green light indication.

How do automatic lights know when to turn on? ›

Automatic lights are enabled through a system of headlight sensors — photoelectric sensors gauge ambient light and relay that information to the electronic control unit to turn the lights on and off as needed. These sensors are usually located on top of the dash where it meets the windshield.

Are traffic lights sensors or cameras? ›

Put another way, they are sensors and wireless devices used for traffic safety and helping traffic engineers. All in all, you shouldn't be alarmed if you see a camera and while you might think the camera on top of a traffic signal is a red-light camera, they're usually not.

Which algorithm is used for traffic signal? ›

The traffic light control system using the fuzzy logic algorithm based on the Mamdani method gives the value of the length of time the green light is on depending on the number of arrivals of a path in the first cycle and second cycle of lights.

Are traffic lights pre programmed? ›

The interval is fixed in pre-timed control and varied in actuated control. In actuated settings, the minimum interval in the minimum amount of time for which a signal will stay green before changing. This can be as low as 2 seconds for local roads, but may need to be up to 15 seconds for arterial roads.

What triggers light sensors? ›

Outdoor motion detection lights can be activated by passing cars, approaching visitors, or even rustling leaves in nearby trees. Sometimes bugs or spiders that crawl across your lights' sensors can even trigger your lights to turn on.

Why do some traffic lights not change? ›

At traffic lights there is often a buried coil of wire just behind the give way line called an induction loop traffic detector that detects large metal objects immediately above, such as a car or truck.

How do smart traffic lights work? ›

Smart traffic lights use data from sensors, cameras, GPS, vehicles, cell phones and other devices to detect patterns of traffic and the volume of vehicles, pedestrians and bicyclists approaching an intersection.

Do they put sensors in the road? ›

Types of Sensors

In-roadway sensors are embedded in the pavement or the subgrade, or they are taped or otherwise attached to the surface of the roadway. Over-roadway sensors are mounted above the roadway or alongside it.

How do red lights know when to change? ›

Active infrared sensors emit low-level infrared energy into a specific zone to detect vehicles. When that energy is interrupted by the presence of a vehicle, the sensor sends a pulse to the traffic signal to change the light.

Are there underground sensors for traffic lights? ›

The sensors are cut into the ground within the lane. “Most lanes have several and some in advance of the intersection. There are multiple levels of detection. At most intersections the front-most ones are important because they are generally more sensitive for motorcycles and bikes,” said Mustafa.

Do sensor lights come on by themselves? ›

Motion sensors lights

That usually means that the light will automatically turn on as soon as this sensor (also called an occupancy sensor) notices you. There may also be a mechanism to turn the light on manually, but not always.

What type of sensor is used in traffic lights? ›

Traffic Sensors (Doppler type)

These sensors use the ultrasonic Doppler effect. They detect vehicles travelling in a particular direction using a change in frequency (the Doppler effect) according to the speed of the vehicle.

Do traffic lights use electricity? ›

LED traffic signal lamps typically use 80 to 90 percent less energy than the incandescent lamps that they replace. (Depending on the size and color, incandescent lamps typically use between 70 to 150 W, while LEDs use just 5 to 20 W.)

How are traffic lights powered? ›

Power sources called LED drivers are used to convert the electric current coming from the traffic control device to suitable voltage and current for LEDs. Power source located in signal module. The body of signal module is like hemisphere.

Should you keep headlights on auto? ›

A: Not all vehicles have automatic headlights, especially older vintages. But I agree that, if your car is so equipped, you should use the automatic setting. Although there are not reminder lights on the dash indicating that the headlights are off, that task often falls to oncoming motorists who flash theirs.

What is the little black thing on top of traffic lights? ›

Some traffic signals have small, black devices that look similar to a camera. These are infrared sensors that are part of a “traffic preemption system” that allows emergency vehicles such as ambulances and fire trucks to safely pass through an intersection.

What are the GREY boxes on top of traffic lights? ›

The boxes themselves are called repeater points and allow the signals to travel to the main access point where data is recorded. Magnetometer systems will typically be used when an inductive loop system, which uses magnetic technology physically installed in the road, isn't an option.

What cameras are attached to traffic lights? ›

Traffic light cameras or red light cameras are installed at traffic lights on major junctions. They can snap any driver running a red light. They're activated when the lights turn red and use sensors to detect - and then photograph - any vehicles that pass through the junction during that time.

Which is the best traffic signal system? ›

Flexible progressive system: In this system it is possible to automatically vary the length of cycle, cycle division and the time scheduled at each signalized intersection with the help of a computer. This system is most efficient among all four systems.

What is the most common type of traffic signal installation? ›

Poles are the most common type of traffic signal support. In a pole installation, the signal heads are suspended from a span wire that is connected between the poles. The poles themselves are typically located on the corners of the signalized intersection.

Which software is used for traffic simulation? ›

Use PTV Vissim to study the driving behavior of such vehicles and their effects on traffic flow. The software simulates autonomous and connected vehicles and displays their specific motion sequences, as well as interactions with other connected and non-connected individual vehicles.

Who controls the timing of traffic lights? ›

Traffic signal engineers are responsible for the design and construction of the traffic signals. They are also responsible for developing the coordinated signal timing plans – the instructions under which the intersection controller operates.

What language are traffic lights programmed in? ›

Traffic-Signal-System Timing Plans

The computer program, written in FORTRAN IV programming language, converts all speed and distance units to travel time units.

Are traffic lights timer based? ›

Traffic signals operate in either pre-timed or actuated mode or some combination of the two. Pre-timed control consists of a series of intervals that are fixed in duration. Collectively, the preset green, yellow, and red intervals result in a deterministic sequence and fixed cycle length for the intersection.

How do you trick an automatic light sensor? ›

Quickly Switching the Motion Sensor On and Off

One of the quick tricks you can try is to quickly turn the motion sensor ON, OFF, ON to override the motion detection and make the light stay on. To go back to motion detection mode, turn the motion sensor off, wait for about 10 seconds, and then turn it on again.

How do you block a light sensor? ›

A simple silicone spray will blind the sensor, and is invisible to the naked eye. It leaves a trace, but it might not be discovered until much later, if at all. Alternatively, you could place glass in front of the sensor when it was off, which would do the same thing.

Can spiders set off house alarms? ›

Has your alarm triggered for no apparent reason? It may be you've been the victim of an 8 legged intruder - a giant spider. Actually, in reality, they don't have to be huge to do this; all it takes is a spider to crawl directly over a sensor to fool it into thinking there's some kind of intruder.

Why are stop lights in Texas sideways? ›

What they're saying: Horizontal lights offer better vertical clearance — leading to less chance of damage to the signal infrastructure — and are less likely to be tossed about by gales, Jack Flagler with Austin's Department of Transportation told Axios.

Are all traffic lights timed the same? ›

An intersection can have different signal timing on different days and times of day to account for different traffic patterns. Most intersections have different signal timing during weekday peak hours (i.e. Monday-Friday, 6 a.m.-10 a.m. and 3 p.m.-7 p.m.) and for special events.

Why are traffic lights sideways in New Mexico? ›

In areas of high winds, horizontal lights above lanes will have more of their surface attached to the gantry which gives better stability and reduced wind resistance; in some American countries, lights are hung from cables and these will swing wildly in strong winds.

How do traffic lights work and who controls them? ›

These traffic lights use an electro-mechanical signal controller, which has movable components and a dial timer. This allows the light to switch and hold light colors for the predetermined amount of time. Dynamic traffic light control systems work by adapting their timing to current traffic conditions.

What is the new traffic signal technology? ›

A smart traffic light is an internet-connected vehicle traffic control system capable of adapting traffic light controls based on information collected from sensors, edge devices, and video systems.

What are two ways that computerized traffic lights can help traffic flow smoothly? ›

They monitor incoming traffic and continuously adjust their timing to keep vehicles flowing as smoothly as possible, communicating with other lights along routes and working together to keep logjams from developing.

Which sensor is used to detect vehicles? ›

FMCW method MW and Ultrasonic sensor enables to detect vehicle approach and presense. Point to point photo beam which is designed for reliable vehicle detection. Measuring difference of earth magnetic field enable to detect vehicle presence.

How do street lights know when to turn on? ›

Photocells detect if light is needed. Photocells are light-sensitive sensors that respond to the amount of light detected. When the light is too low, such as at dusk or under heavy overcast skies, the sensor tells the computing unit within the streetlight to activate the flow of electricity.

How do street lights know when to turn on at night? ›

Streetlights are controlled by sensors that detect when it is dark enough for lighting to be required and switch on the lights.

What sensors are used to turn on street lights? ›

LDR is used to detect the presence of light and turn on and off the street light accordingly. Infrared sensor is used to adjust the brightness of street light.

Are street lights on a light sensor? ›

Photocells detect if light is needed. Photocells are light-sensitive sensors that respond to the amount of light detected. When the light is too low, such as at dusk or under heavy overcast skies, the sensor tells the computing unit within the streetlight to activate the flow of electricity.

Can a laser pointer turn off a street light? ›

You can use a laser pointer to trick the sensor into thinking it is day when it is actually night simply by focusing the beam on the sensor. The street light turns off. The beauty of this method is that it does not harm the street light in any way and the effect is temporary.

Why do street lights stay on during the day? ›

Very probably repair work is being carried out. This is done during the day. The technicians switch on some or all of the lamps in the street. This enables them to see which lamps are not working.

Are street lamps timed? ›

Time Switch For Street Light

As street lights need to be turned on and off regularly, they are usually controlled by time switches, people often called it street light control system.

Why do traffic lights look weird at night? ›

At night, and other low light situations, your pupil dilates (gets larger) to allow in more light. When this happens, more peripheral light enters your eye. This causes more blurring and glare, and makes lights look fuzzier.

Are traffic lights automatic or controlled? ›

Traffic lights are sometimes centrally controlled by monitors or by computers to allow them to be coordinated in real time to deal with changing traffic patterns. Video cameras, or sensors buried in the pavement can be used to monitor traffic patterns across a city.


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