A Guide to Types of Traffic Signals

A Guide to Types of Traffic Signals

Traffic signals are designed to keep traffic flowing in an orderly and efficient manner while helping to ensure both driver and pedestrian safety as they navigate through intersections. The number of crashes that occur as well as the severity of the crashes can be reduced when the correct types of traffic signals are used effectively.  

Traffic signals have come a long way since their first creation in 1868 when they were gas-lit signals changed by police officers to direct horse carriages. Once automobiles were created and began growing in popularity, a better system was needed. The first electric traffic light was created in 1912 to direct traffic in Cleveland, Ohio. Over the years, the types of traffic signals became more dynamic, able to interconnect and handle more than just two-way directions. In 1928, a traffic signal was created that used the honking of cars at the intersection to know when it needed to change, becoming the first actuated traffic light. Computers and other more advanced technologies like pressure plates started to be used in the 1950s. In the 1990s, a countdown timer was developed to help pedestrians. In the 2000s, technology developed to make traffic signals communicate with vehicles and each other more efficiently and effectively, offering traffic designers many different system options to choose from when deciding how to dictate traffic in a certain area.  

Traditional Types of Traffic Signals

Fixed-Time Traffic Signals

A fixed-time traffic signal is a signal that uses a timer to change at predetermined intervals, instead of changing according to traffic movements. They create an organized, predictable traffic pattern in the area they’re installed by using an electro-mechanical signal controller that can be adjusted and uses a dial timer to ensure the signal changes at whatever interval a traffic designer decides what is necessary for that intersection.

The main advantage of fixed-time traffic signals is their initial costs and maintenance needs are much lower than other traffic signal systems. The downside to them is that they can often end up causing unnecessary delays, leaving vehicles to sit at intersections for long periods of time when no traffic is around. This is why fixed-time signals are mostly used in urban areas where traffic is more constant and heavier.

Actuated Traffic Signals

An actuated traffic signal changes according to traffic movements instead of at fixed intervals of time, using sensors to detect traffic. Their purpose is to make adjustments that will allow traffic to flow quickly and efficiently, instead of stopping the flow of traffic unnecessarily. There are many types of detectors that can be used to communicate with an actuated signal’s control system. There are detectors that can be embedded in the road to detect when vehicles drive over them, like pressure plates and inductive loops. There are also detectors that rely on overhead devices that watch traffic from above, like non-video and video sensors.

There are many benefits to these types of traffic signals, as they can best keep up with traffic flow and help alleviate traffic jams. These work best in suburban or rural areas where traffic patterns change drastically over the course of the day. The downside to this type of traffic signal is the upfront and maintenance costs for both the detectors and the control systems.

Pre-Timed Traffic Signals

Pre-timed traffic signals are somewhat like fixed-time traffic signals as they have their signal changes set ahead of time. However, pre-timed traffic signals can be set to run in phases so that their changes depend on the time of day. For example, a fixed-time traffic signal might change every 60 seconds all day long, whereas a pre-timed traffic signal might change every 60 seconds in the busiest hours of the morning and afternoon but change every 120 seconds at the less busy times of the day.  

These work better than fixed-time traffic signals as they can use collected data to adjust their times throughout the day, helping traffic flow better. However, they still have limitations and aren’t often used in busy areas. A pre-timed traffic signal can work well on an average day, but on days where an event like a concert or a traffic accident occurs, things are thrown off and they are no longer effective.

Adaptive Traffic Control Systems (ATCS)

Adaptive Traffic Control Systems (ATCS) use advanced technology like sensors and algorithms for data analysis to optimize traffic flow by adjusting signals in real-time. These are meant to be able to respond to the constant changes in traffic conditions to keep traffic moving quickly and efficiently. These work by collecting data from detectors like loops embedded into the pavement, cameras, radar, and connected vehicles, putting this data through their algorithms, and then adjusting traffic signals in real-time while also using the data to predict future traffic patterns.

The largest benefit of ATCS is how quickly they can adapt to handle real-time traffic issues. One of their best features is their ability to detect emergency vehicles, changing signals to allow the vehicles to move quickly and safely through the intersection. They can also adapt depending on the types of traffic moving through the area at any given time, such as different types of vehicles, cyclists, public transit, and pedestrians.

SCOOT (Split, Cycle, and Offset Optimization Technique)

SCOOT (Split, Cycle, and Offset Optimization Technique) is a popular type of traffic control system that optimizes the timing of traffic signals at intersections based on real-time traffic patterns. This system operates on the three principles of Split, Cycle, and Offset.

Split is the allocation of green time for different traffic movements through an intersection. The split is adjusted based on real-time traffic demand in a SCOOT system so that green time increases or decreases depending on high and low-demand traffic movements.

Cycle is the total amount of time a signal takes to complete one full sequence between green, yellow, and red. SCOOT makes sure the cycle length changes depending on traffic patterns to ensure optimal traffic flow.

Offset is the time difference between the main traffic signal and the traffic signals working adjacently to it. SCOOT can work to keep the offsets coordinated so that traffic can flow continuously through multiple lights at a time.  

For SCOOT systems to work effectively, an entire network of detectors is needed to collect the data that the system will then use to make its decisions on how to manage the traffic flow. Where these new and dynamic traffic systems have been installed throughout the world, they have been found to improve traffic efficiency, reduce delays in traffic, and enhance overall safety and performance in the area.

RHODES (Real-Time Hierarchical Optimized Distributed Effective System)

A RHODES is meant to analyze a traffic control problem and break it down into smaller problems, then organize these problems into a hierarchy to make them easier to manage. It allows for optimization algorithms to be used to solve problems quickly and efficiently to ensure safe travel through traffic. RHODES collects its data by using computerized communication and sensors. RHODES has been successfully used in many places, such as Tempe, Arizona where it worked well within the traffic system and was able to perform just as well as the previous semi-actuated control system.

SCATS (Sydney Coordinated Adaptive Traffic System)

SCATS was developed in Sydney, Australia, and is now used worldwide to optimize traffic signal timings and improve efficiency. The main goal of SCATS is to adjust signal times to reflect real-time traffic conditions, optimizing traffic movements to reduce delays, increase safety, and improve travel times. SCATS uses a network of detectors installed at intersections that feed real-time information about travel conditions to a central control system that then feeds that data into algorithms and adapts signal timing in response. One of the most successful uses of SCATS in the United States is in Metro Atlanta, where it worked so well that Atlanta chose to expand the number of intersections using the technology.  

Just like other control systems, SCATS coordinates with other intersections and traffic signals to keep traffic progressing without interruption. Unlike other systems, SCATS uses traffic management strategies to optimize signals during peak traffic hours, can prioritize public transportation or emergency vehicles, and can be adjusted for special events in the area.  

SCATS has been implemented worldwide and is known for its adaptability, responsiveness, and excellence at accommodating traffic demands in high-volume areas.

Smart Traffic Signals

Smart traffic signals are a new approach to traffic control that utilizes developing technologies to collect and analyze data to optimize the timing and coordination of traffic signals in real-time. There are many sensory technologies that smart traffic signals use, depending on what works best in the intersection. They might use above-ground sensors like radars or video cameras, or embedded sensors like loop detectors or pressure plates. The data collected by these sensors is fed into advanced algorithms to assess traffic patterns and optimize traffic flow continuously throughout the day.  

Apart from the main benefit of optimizing traffic to maintain safety and efficiency, smart traffic signals can also prioritize different traffic like emergency vehicles, public transportation, and pedestrians. They can even lengthen their traffic signal times to ensure longer vehicles like buses or trams can make it through an intersection safely.

The two largest challenges for smart traffic signals are costs and maintenance. Between the sensors and control systems, the upfront cost is substantially higher than more basic traffic systems, and maintenance is necessary to keep everything working properly over time. The benefits tend to outweigh the costs, however.

Connected Traffic Signals

Vehicle-to-Infrastructure (V2I) communication is a technology that enables vehicles to communicate with the surrounding infrastructure, such as traffic signals, roadside sensors, toll booths, parking facilities, and transportation management systems. By enabling vehicles and infrastructure to work together, V2I communication allows for a safer and more efficient traffic area.

Pedestrian-Activated Traffic Signals

Pedestrian-activated traffic signals are traffic control devices designed to prioritize pedestrian safety and provide pedestrians with a dedicated signal phase to cross the road. These signals are typically located at intersections or mid-block crossings where pedestrians need assistance in safely crossing the road. They can be push buttons, which are activated by the pedestrian wishing to cross, or sensors like infrared or microwave detectors that register the presence of the pedestrian without any extra effort on the part of the pedestrian. The type of sensor and signal depends on the type of intersection and the types traffic signals that already exist there. These enhance pedestrian safety by giving the pedestrian a timetable for when they can cross, instead of leaving them to try to decide when to cross on their own.

Eco-Friendly Traffic Signals

Eco-friendly traffic signals are traffic control devices designed to reduce energy consumption and their impact on the environment. Some of these types of traffic signals use solar technology to power them while others use different types of lights that are more energy-efficient like LED bulbs. Traffic signals can also be designed to minimize how much power they consume during their use, which often includes optimizing signal times to reduce usage when traffic is light. One of the easiest ways to promote sustainability in traffic signal design is to use recyclable materials when building.

Summary & Conclusion

There are many options out there for traffic signals, from fixed-time signals that don’t change regardless of the time of day or traffic flow to actuated signals that use detectors to adjust based on traffic conditions in real-time, and any options in between. It is important that a traffic designer considers the amount of traffic in the area, the budget for a project, and the types of traffic signals nearby when deciding what type of signals to build. As technology advances, signals will only continue to get smarter, continuing to improve the safety and flow of traffic.