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Clifton, NJ 07011

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New York, NY 90210

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Monday to Friday

How Traffic Signals Work

How Traffic Signals Work

Introduction: (Initial Observation)

Traffic signal lights are vital to controlling traffic in a safe, orderly manner. They let motorists take turns in moving through busy intersections and can enhance safety.

Without the traffic lights system there would be so much chaos in the world. People would be getting into car crushes and fighting. The traffic signal makes life a little more simple. The traffic light saves time. The traffic lights help to control traffic. Sometimes people take the simple things for granted.

Traffic signals are electrically operated traffic control devices, which alternately direct traffic to stop and to proceed. Traffic signals are designed to ensure an orderly flow of traffic, provide an opportunity for pedestrians or vehicles to cross an intersection, and help to reduce the number of conflicts between vehicles entering intersections from different directions.

This project will help you develop a broader understanding about what traffic signals will do and what they won’t do.


This project guide contains information that you need in order to start your project. If you have any questions or need more support about this project, click on the “Ask Question” button on the top of this page to send me a message.

If you are new in doing science project, click on “How to Start” in the main page. There you will find helpful links that describe different types of science projects, scientific method, variables, hypothesis, graph, abstract and all other general basics that you need to know.

Project advisor

Information Gathering:

Find out about traffic signals. Read books, magazines or ask professionals who might know in order to learn how traffic signals work. Keep track of where you got your information from. A good online source of information is Virginia Department of Transportation .

Following are samples of the information that you might find.

Garrett A. Morgan invented the traffic signal during the 1920’s. The very first traffic lights did not even have lights at all. They had signs that said stop and go. Garret A Morgan is not only famous for the traffic signal. In his prime he invented many inventions that helped man kind. For example Garrett invented a breathing device using a fireman’s hood and a series of tubes. If it wasn’t for Mr. Morgan firemen wouldn’t be able to rescue people out of the flames.

For more information about Garret Morgan and his inventions click here.

What is the purpose of traffic signals?

Traffic signals are designed to ensure an orderly flow of traffic, provide an opportunity for pedestrians or vehicles to cross an intersection and help to reduce the number of conflicts between vehicles entering intersections from different directions.

How do traffic signals work?

Initially traffic lights were controlled manually by switches installed in a switch box in each intersection. A policeman was in charge of changing the lights as needed. Many modern traffic light systems still have the manual operation capability for manual control of needed. With this system policemen did not have to stand in the middle of intersection to control the traffic. Soon mechanical timers and electrical clocks were added to automate the operation of traffic lights. A timer clock times how long a light should stay a specific color. Modern traffic lights can be controlled depending on the amount of traffic on a certain road. This is done by putting magnets on the road and on the cars. On the other side of the street is a bunch of sensors that calculate the amount of cars that pass by. These sensors send a signal to the light to adjust it’s timing depending on the amount of traffic.

Traffic responsive controllers change the lights according to the amount of traffic in each direction. These controllers use sensors (inductive loops in the roadway) to detect the number of vehicles and automatically adjust the length of the green time to allow as many vehicles as possible through the intersection before responding to the presence of vehicles on another approach. Although these types of traffic controllers have been in use for many years, a new generation of microcomputer traffic controllers makes the signalized intersection much more efficient, thereby reducing time-consuming delays.

Life before the light

Before the traffic light there were not many cars. So there wasn’t any need for the traffic light. But I’m sure there was still chaos with all the horses. So even many years ago we needed the traffic light. Garrett Morgan was a great inventor, he invented many things. When the traffic light was invented it only had a red stop sign and a green go sign. It was controlled by a simple electric clock. In those days it was the perfect thing there wasn’t that many people with a car. So it was the ideal thing for that time.

Traffic signal controllers

Simple electronic circuits can control basic traffic signals. These electronic circuits consist of electronic timers. Following is a sample.

For those who are interested in electronics and want to know the details of a traffic signal controller board, following is a schematic. We don’t go into details with this circuit because it exceeds the current level of this project. However if you are interested to have some details, send a message to your project advisor.

Question/ Purpose:

The purpose of this project is to understand how traffic signals work and what is the the mechanics behind them. We will also attempt to build a simple (manual control) traffic signal circuit.

Identify Variables:

This project at it’s current form is a display project or an engineering project. So you will not need to define variables or suggest a hypothesis. If you need an experimental project related to traffic lights, click here to see the attachment.


This project at it’s current form is a display project or an engineering project. So you will not need to define variables or suggest a hypothesis. If you need an experimental project related to traffic lights, click here to see the attchment.

Experiment Design:

Design simple manual control traffic signal circuit.

In this experiment you will make a model traffic light system. You will use two sets of lights. Each set will have three lights. You might be able to purchase color LEDs from your local electronic store. LED is Pronounced EL-EE-DEE and stands for Light Emitting Diode. You may also buy plain white lights and just cover them with color paper when your circuit is complete. LEDs almost work like miniature light bulbs with one major difference. In LEDs the electric current must be in a correct direction or LED does not lit. General idea for this design is that the green light of one side will be in the same circuit with the red light of the other side. The orange lights of both sides also will share circuit. We connect all three circuits to a 3 way switch.

When the switch is set to position A, One side will have the red light and opposite side will have the green light.

When the switch is in position B, both sides will have the orange light.

Position C is opposite of Position A.

1.) Take a piece of flat wood, preferably around 10” x 8”.

2.) Mount 3 miniature base in a straight column on the left of the board and 3 miniature base on the right as shown in the figure below.

3.) Place the battery holder onto the board like shown in figure 2 below. Once you have placed it properly, screw it into the board.

4.) Now, you need to begin connecting the wires. Connect one side of all miniature sockets (bases) to the negative side of your battery.

5.) Next, you need to put three screws right next to each other in the middle of the board. These screws will act as a 3 way switch. Name them A, B and C.
From A connect to the red light in one side and green light on the other side. From B connect to two yellow/orange lights and finally from C connect wires to the remaining green and remaining red light. Make sure you carefully remove the insulation plastic for all contact points.

6.) Finally, you need to take a wire from the positive side of the battery and use it to touch A, B or C screws. This should turn on the connected light bulbs.

More advanced Model:

In the previous model (above) the yellow lights in both direction turn on at the same time. In other words the traffic light had only 3 different states.

As the number of cars increased and so did the accidents, many traffic lights are reprogrammed to have four or more states. Following is the drawing for a manual controlled four state traffic light. Please note that to make this model you will also need to use diodes. Diodes are electronic components that allow the flow of electricity only in one direction. The symbol for diode is like an arrow. Any 3 to 6 volt Zener diode will work for your experiment. Zener diodes may be purchased from Radio Shack or other electronic suppliers.

Materials and Equipment:

Material needed for this experiment are:

  1. 6 Miniature socket or base (MiniScience part# MINIBASE)
  2. 6 Miniature 1.2V light bulbs (MiniScience part# E0112)
  3. D size battery
  4. D size battery holder (MiniScience part# MBH1D)
  5. one base board
  6. Some insulated wires
  7. Some screws

Make sure that the voltage of your light bulbs and your battery or battery set are the same.

1.2 Volts light bulbs can lit with a AA or C or D size battery. For 5.5 Volt light bulbs or LEDs, use a 6 Volt battery also known as Lantern battery. For LEDs, you will not use a miniature socket and you need to secure them with other methods.

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.


Not required for this project.

Summery of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.

Additional notes:

The public’s understanding of the function of traffic signals can improve driving habits by reducing speeding and associated traffic accidents. The more drivers know about the operation of traffic signals, the less they will be frustrated when waiting for a signal to change.

Why are traffic signals needed?

As traffic volumes increase beyond the capability of lesser controls such as a four-way stop, it may be necessary to install a traffic signal.

Traffic Signal Equipment

Traffic signals are more costly than is commonly realized, even though they represent a sound public investment when justified. A modern signal can cost up to $200,000. This money pays for:

A Traffic Signal controller

Signal Heads

Vehicle Detectors

Signal Poles and Supports

The Controller is the signal’s brain. It consists of a computer that controls the selection and timing of traffic movements in accordance with the varying demands of traffic as registered with the controller unit by detectors.

Signal Faces are part of a signal head provided for controlling traffic in a single direction and consisting of one or more signal sections. These usually include solid red, yellow, and green lights and sometimes red, yellow and green turn arrow lights as well. The Signal Head can contain one or more signal faces.

Detectors are devices for indicating the passage or presence of vehicles. In the Las Vegas Valley, they may consist of wire loops placed in the pavement at intersections. They are activated by the change of electrical inductance caused by a vehicle passing over or standing over the wire loop. On newer signal installations, video detection is being used. A camera feeds a small computer which can “see” if a vehicle is present.

Special Signal Functions

Traffic Signal Preemption

The transfer of signal control to a special signal operation is called preemption. There are two common types of preemption, based on reason for preemption: Railroad and Emergency Vehicle. Railroad preemption is initiated when a train passes over advance detectors located on the tracks ahead of the railroad crossing. The purpose of preemption is to clear tracks of traffic stopped on them by traffic signals. Emergency vehicle preemption can be used for any authorized emergency vehicle, but normally only for fire engines. The purpose is to obtain a green light for the emergency vehicle as soon as possible or to hold an existing green light. To obtain a green light, existing green lights, including pedestrian intervals, are abbreviated. After the yellow change interval, a green light is given to the approach to be used by the emergency vehicle. The type of signal preemption used for emergency vehicles is a modulated strobe light. Light modulated output of a vehicle mounted emitter is received by a detector at a signal.

Flashing Red

According to the Nevada Revised Statutes, when a red lens is illuminated with rapid intermittent red flashes, a driver shall stop before entering the crosswalk on the near side of the intersection. The driver may proceed subject to the rules applicable to making stop at a four-way stop controlled intersection.

Flashing Yellow

When a yellow lens is illuminated with rapid intermittent yellow flashes, a driver may proceed through the intersection or past the signal only with caution.

Dark Signals

When a traffic signal has gone dark due to power failure, it is considered to function the same way as a four-way stop controlled intersection; and a driver must stop before entering the intersection.

Signal Timing: How they “know” when to change from red to green.

Along major arterials which have several traffic signals spaced at periodic intervals, the traffic signals are usually synchronized to provide coordinated movement along the major street. Traffic signals assign the right of way to various traffic movements for different time intervals depending on traffic flow levels. Pre-timed signals have pre-set time intervals for different times of the day including the morning, noon, and evening peak travel periods. The City has a few pre-timed signals remaining in the outlying areas which have not been connected to the central system.

Traffic actuated signals use detectors on the approaches to traffic signals to monitor and assign the right of way on the basis of changing traffic demand. These signals attempt to assign most of the available green time to the heaviest traffic movements.

Coordination of Traffic Signals

The greatest benefits to the public for each dollar spent on traffic operations improvements come from the coordination of adjacent traffic signals to provide smooth movement of the traffic through groups of signals on an arterial street. The coordination of traffic signals to facilitate smooth traffic flow (progressed movement) along a street is a proven technique. The quality of flow along a street is basically a function of the spacing of the signals along the street, the prevailing speed of traffic on the street, and the traffic signal cycle length. The amount of traffic and the proportion of the green time given to the progressed movements are also important.

Traffic Signal Coordination Goals

Many drivers ask why they have to wait so long for a signal to change. Many of these drivers are waiting to enter a major arterial street from a side street. This is even more frustrating when no traffic can be seen on the arterial. To allow the coordination of the arterial, the side street must wait until the main traffic movement on the arterial has gone through the intersection. It is possible that the arterial traffic can’t be seen immediately, but will soon be passing through the intersection. Major coordinated arterials in the City include Charleston, Sahara, Rancho and Lake Mead.

The goal of coordination is to get the greatest number of vehicles through the system with the fewest stops in a comfortable manner. It would be ideal if every vehicle entering the system could proceed through the system without stopping. This is not possible, even in the well-spaced, most technologically advanced systems. Therefore, in traffic coordination, the majority rules and the busiest traffic movements are given precedence over the smaller traffic movements. The signal cycle lengths for signal coordination ranges between 80 and 160 seconds in order to adequately accommodate the many left turn phases and long yellow, red, and pedestrian crosswalk clearance time required.

Click here to find out more about the Las Vegas Area Computer Traffic System (LVACTS).


Left Turn Traffic Signals

Until recently, drivers have been accustomed to seeing left turn signals where there is initially a green arrow followed by an amber arrow followed by a red arrow. On the green arrow drivers are given the right-of-way to complete left turns free of any other traffic conflicts. The amber arrow warns drivers that the left turn signal is ending. On the red arrow, left turns are not permitted. These type of arrows are helpful, but when there is no opposing traffic they can cause unnecessary delays.

Protective/Permissive Left Turn Signals

Over the last several years, a different type of left turn signal has been implemented at intersections in the City. Under this new arrangement, left turn signals provide the usual green arrow which is usually followed by the normal amber arrow. After the amber arrow has terminated, drivers are now faced with a solid green ball signal.

During the display of the solid green ball, left turns can be made when there are adequate gaps in opposing traffic to complete left turns safely. This new type of left turn phasing is designed to help minimize delay by eliminating the need for the red arrow and allowing vehicles to turn on the green ball after opposing traffic has cleared. By not having the red arrow, motorists do not have to sit and wait to turn left even when there is no opposing traffic, a situation that often occurs during periods of low traffic volumes. The signal still provides a green left turn arrow during rush hours when traffic is heavy, but during off-peak hours, left turning vehicles are not delayed by a red arrow.

Why Doesn’t the City Use Protected/Permissive Left Turn Signals Everywhere?

The City is using protective/permissive left turn signals where drivers can turn left safely because there are gaps in approaching traffic and drivers can clearly see oncoming vehicles.

In order to provide for good signal coordination, protected/permissive signals will not be available at all intersections. Special left turn sequencing is used to improve signal coordination and provide smooth through traffic flow at selected intersections. These are the signals where one arrow comes up at the beginning of green and the opposing left turn arrow comes up at the end of green. At these locations, protected/permissive operation would be dangerous for drivers. There are some intersections where the City could install protective/permissive left turns. However, this requires lengthening mast arms and installing new signal heads on the end of the longer mast arms which costs $60,000 per intersection. The City does not have funds to convert all these intersections to protective/permissive left turns. The City plans to use protective/permissive signals wherever possible as signals are modernized and new signals are installed.