Let's take a closer look at the technology and inner workings behind traffic signals and see what really drives one to switch from red to green.​

How do traffic signals work?​

They use a complex system with different components to manage traffic at intersections. The brain of the operation, aside from the engineers who design and oversee it, is the controller. It receives information from detector loops, radar sensors and video cameras to decide the timing of signals at an intersection. Detector loops are underground coils identifying the presence of approaching vehicles. They sense them by detecting the metal, which alters the magnetic field in the coil. Radar sensors measure the distance and speed of vehicles by emitting radio waves and collecting data on the time it takes for the waves to bounce back. Video cameras can continuously capture footage of vehicles, allowing software to interpret the images and monitor the conditions of traffic.

Once the detectors have identified the approaching vehicle, it sends a message to the local controller, which is in a cabinet near the intersection. The master controller acts as an intermediary between the local controller and the traffic control center, the hub for managing the entire signal system. For everything to run smoothly, communication between each element is essential.

“Generally, you have a computer, a device called a conflict monitor which makes sure you don’t have two conflicting green lights displayed at the same time, sensors, signal heads and pedestrian equipment,” said Alan Davis, Georgia DOT Traffic Engineer.

What happens if the signal malfunctions?

Georgia DOT uses a centralized software system to monitor signals around the clock. If one malfunctions, it sends an alert to the computer system, prompting a traffic engineer to address the issue. Automated Traffic Signal Performance Measures are used to monitor trends, helping to identify both current and potential failures. Georgia DOT also has programs to track the time from the moment it malfunctions to the repair of the signal. This ensures contractors and Georgia DOT staff respond promptly to minimize disruptions. “Proactive is the goal,” said Davis. “We try to have it fixed either before it totally fails, or with as minimal impact to the traveling public as possible.”

How do traffic engineers decide where to place a traffic signal?

When determining where to install one, Georgia DOT does an engineering study to determine the most appropriate location. For a signal to be deemed as the preferred solution, it must meet specific criteria known as signal warrants which are further outlined in the Manual on Uniform Traffic Control Devices. The decision to install a signal is based on factors such as traffic volume, pedestrian activity and traffic incident history. While meeting these criteria often justifies the installation of a signal, it is not an absolute requirement, as each situation is evaluated on an individual basis.

What are the types of traffic timing?

Traffic timing can be categorized into four main types. Free timing operates independently, with signals responding to real time traffic demands based on basic control parameters, without coordination with other lights. Time-based coordination involves synchronizing signals to create dedicated green bands for specific movements, with timings derived from modeling programs and analytical software based on set goals and objectives. Traffic responsive systems use a central control to monitor traffic patterns and adjust signal timings for groups of lights according to current conditions. Finally, adaptive timing systems make cycle-by-cycle decisions using computer algorithms, allowing for continuous adjustments based on real-time traffic flow. These adaptive systems rely on programming parameters to operate with minimal manual intervention, ensuring optimal traffic management.​