In modern urban traffic networks, traffic signals are like silent commanders, coordinating the safe passage of tens of thousands of vehicles, bicycles and pedestrians every day. These seemingly ordinary light-emitting devices contain complex engineering design, psychological principles and traffic management wisdom. In this paper, we will analyze the basic function, design logic and future development direction of traffic signals, revealing their core role in ensuring road safety and optimizing traffic efficiency.
traffic lights
The core mission of traffic signals is to safely guide vehicles, bicycles and pedestrians. According to different control logics, there are two main types: fixed-time control and dynamic control, which play their respective advantages in different scenarios.
This type of signal light changes color at fixed intervals through a timer, and the time setting range is as short as 30 seconds and as long as several minutes, with the specific duration determined according to the historical flow data of the intersection. The core device is an electromechanical signal controller that precisely executes the preset color hold time through moving parts and a dial timer. For example, at intersections in suburban areas with stable traffic flow and obvious peak and trough patterns, fixed time control is often used to simplify management and reduce maintenance costs. The advantage of this type of signal is its simple structure and high stability, while the disadvantage is that it cannot flexibly respond to sudden traffic changes.
Dynamic traffic light control system relies on advanced detectors to sense the current road conditions, and once congestion is detected at the intersection, it will automatically adjust the length of the signals in each direction to realize efficient traffic diversion. The system architecture is extremely complex and consists of multiple components such as a main controller, control circuits, timers, clock signal generators, decoders, decoder driver circuits and digital display decoder driver circuits. The components work together: the detectors transmit real-time traffic data to the main controller, which then synchronizes the processed commands to the other signals.
Taking a city trunk road during the morning rush hour as an example, the dynamic signal can extend the green time in the congested direction while shortening the passing time in the direction of less traffic according to the feedback from the sensors. However, due to the high cost of sensors and intelligent control systems, this type of system is currently more common in the core roads of large cities, while small and medium-sized cities are limited by funding and technology, and the popularity is relatively low.
| Type | Key Advantages | Installation Cost | Maintenance Cost | Suitable Scenarios |
| Fixed-Time Control | Simple structure, high stability, easy maintenance | Low | Low | Suburban intersections with stable traffic, non-core roads |
| Dynamic Control | Real-time traffic adaptation, reduced congestion, improved efficiency | High | High | City centers, intersections with traffic fluctuations, main roads |
The classic combination of red, yellow, and green has its roots in history and psychology, and has become the universal language of traffic worldwide.
The tradition of red as a warning color dates back to the early days of railroad signaling, when trains traveled at high speeds on the tracks in the 19th century, and to avoid collisions, railroads used red arm signals to indicate “No Passing” because red was highly recognizable over long distances and in inclement weather. 1920 saw the design of the first traffic light system in Detroit, by U.S. police officer William Potts. In 1920, American police officer William Potts designed the first set of traffic signals in Detroit, directly following the red and green color scheme of the railroad arm board signals and adding yellow as a transition signal. London experimented with color-coded signals as early as the 1860s, and these early practices gradually made red, yellow, and green the international standard for traffic signals.
Red: Red light has the longest wavelength (about 620-750 nanometers), travels the farthest of any visible light, and the human retina is sensitive to red light, so it is visible at long distances and in bad weather. At the same time, the color red is naturally associated with “danger” and “forbidden” in human culture, and drivers will instinctively slow down and stop when they see a red light.
Yellow: A transitional color between red and green, its wavelength (about 570-590 nanometers) is second only to red, transmitting the signal of “ready to change”, allowing 3-5 seconds of reaction time for drivers to avoid rear-end accidents caused by braking sharply.
Green: With a wavelength of about 500-560 nanometers, it is in the middle range of visible light and symbolizes “safety” and “passage” in human visual perception. The visual soothing sensation brought by green light helps to reduce drivers’ anxiety and maintain smooth traffic flow.
Whether in Asia, Europe or the Americas, the meaning of red, yellow and green is always unified: red light stop, yellow light warning, green light line. This standardized design greatly reduces the communication cost of cross-country travel. In the U.S., traffic signals have a vertical layout of “red on green off”, so that even color-blind people can judge the signal status by its location; while in Japan, some signals include flashing modes to further enhance visual cues.
traffic lights
Circle: The most common basic shape, suitable for ordinary intersections, the non-directional design indicates “omni-directional control”, i.e. vehicles and pedestrians in all directions are required to comply with the signal at the same time.
Triangular / Rectangular: Triangular and rectangular signals are more likely to attract attention on complex roadways or in areas with poor visibility. For example, rectangles are often used for countdowns (e.g., the “progress bar” signal in Tianjin, China) to help drivers anticipate whether they have enough time to cross an intersection by displaying the number of seconds remaining, while triangles are often used as a warning sign to alert drivers to slow down.
Arrow-shaped: At lane-splitting intersections, an arrow-shaped signal clearly indicates one-way passing privileges, effectively reducing lane conflicts. For example, when the green light of the left-turn arrow is on, only left-turning vehicles are allowed to pass, and straight and right-turning vehicles need to wait.
Different countries and regions adjust the shape of their signals according to local needs and cultural habits. In Canada, the red light is designed as a square, yellow light as a diamond, and green light as a circle, which strengthens the recognition through the difference of contours, and even color-blind people can judge the signals by their shapes; some cities in Germany use signals with pedestrian patterns to help children and tourists understand the meaning of the signals quickly; and in Holland, the special signals for bicycle lanes are often designed with bicycle patterns to emphasize the importance of cyclists.
To ensure the consistency and standardization of traffic signals worldwide, the International Electrotechnical Commission (IEC) has developed standards such as EN 12368 and EN 50556, which specify the shapes, sizes and luminance requirements of signals. For example, equilateral triangles are required for warning signals, circles for directional signals, and arrows for directional instructions. The implementation of these standards enables drivers around the world to read signals quickly when traveling across borders, significantly reducing the risk of accidents caused by misunderstandings.
traffic signal light
Properly set traffic signals not only prevent collisions, but also significantly improve the efficiency of road traffic. On urban arterials, computer algorithms synchronize and coordinate signals to create a “green band” – when vehicles are traveling at a specific speed (e.g., 40-60 km/h), they can continuously encounter green lights, avoiding repeated stops. By implementing the “Green Wave Program”, New York City in the United States has improved the efficiency of some road sections by 35% and reduced the average waiting time of vehicles by 40%; Shenzhen in China has also reduced the congestion index during peak hours by 22% through an intelligent signaling system. In addition, optimized signal timing can also reduce vehicle idling emissions, helping to improve urban air quality.
Although signals are an important safeguard for road safety, when they malfunction, intersections can be thrown into chaos in an instant. Data shows that accidents are five times more common when signals malfunction than in normal conditions. 2022 saw 17 accidents in two hours in a US city due to extreme weather conditions that led to widespread signal failure. In order to reduce the risk, modern transportation systems have introduced multiple protection mechanisms: backup power supply can maintain signal operation for 4-8 hours in case of power outage; remote monitoring system detects the status of the equipment in real time and alerts the police as soon as abnormalities are detected; and some cities are equipped with a manual Override function, which allows the traffic police to manually intervene to switch signals in case of emergency.
traffic signal light
With the development of Internet of Things (IoT) and Artificial Intelligence (AI) technology, traffic signals are being upgraded from “independent individuals” to “intelligent network nodes”. In the future, signal lights will have three core capabilities:
Vehicle communication: through V2X (Vehicle-Circuit Collaboration) technology, signal lights can send information such as “time remaining on the green light” and “congestion at the intersection ahead” to vehicles equipped with intelligent terminals in advance, so as to assist drivers in adjusting their speed or planning their routes.
Data linkage: Access to the city traffic big data platform, combined with real-time variables such as traffic flow, weather, large-scale activities, etc., to automatically optimize signal timing. For example, when encountering rainstorms, the system will extend the green light duration of the main road to channel emergency vehicles.
AI Prediction: Using machine learning algorithms to analyze historical traffic data, the system predicts traffic trends in future hours and adjusts signal strategies in advance to avoid congestion.
Traditional signal lights use incandescent lamps, which consume a lot of energy and have a short lifespan. The popularization of LED light source reduces the energy consumption of signal lights by more than 70% and extends their lifespan to 5-10 years. Some cities are even trying solar power to realize “zero-carbon signals” in remote areas or temporary intersections. In addition, intelligent dimming technology can automatically adjust the brightness according to the ambient light, avoiding glare to interfere with the driver’s vision, while further saving electricity. The EU plans that by 2030, all new traffic signals will adopt LEDs with intelligent control technology, which is expected to reduce carbon emissions by more than 1 million tons per year.
From the manually controlled signals of 1920 to today’s intelligent and connected systems, every evolution of traffic signals reflects mankind’s relentless pursuit of road safety and efficiency. In the future, with the deep integration of 5G, AI and new energy technologies, these street “sentinels” will no longer only be the maintainer of order, but also become the key nodes of the smart city, using technology to redefine the relationship between people and the road. Whether it is the basic function of guaranteeing life safety, or the innovative design of integrating culture and technology, traffic signals will continue to guard the smoothness and peace of every road in the world.
