How traffic lights know when to change in Australia

They change using a mix of on-the-ground detectors and network-wide adaptive control. Most Australian signals are vehicle- and pedestrian‑actuated, and many are coordinated by the SCATS system, which monitors traffic in real time and adjusts green times, order of phases, and corridor coordination; priority is also granted to trams, buses, and emergency vehicles when conditions allow.

The brains behind the lights: local controllers and SCATS

Each signalised intersection has a roadside controller that runs safety‑critical timing for the lights, enforces minimum greens and clearance times, and decides when a phase can end. In most cities, these controllers are connected to the Sydney Coordinated Adaptive Traffic System (SCATS), an Australian system used nationwide (and exported globally) to adapt timings continuously based on detector data such as flow and lane occupancy.

SCATS and the local controller work together. The controller manages second‑by‑second safety and detection, while SCATS updates cycle length, green splits and offsets along corridors to form smoother “green waves,” retiming the network every few minutes as traffic ebbs and flows. Where communications are unavailable or during overnight periods, some signals run locally on traffic-actuated or time‑of‑day plans.

What tells a signal that someone is waiting

Signals rely on a range of sensors and inputs to detect vehicles, pedestrians and public transport. The list below describes the most common detection methods used around Australia and what they do.

• Inductive loop detectors: Wire loops cut into the road near the stop line sense the metal mass of vehicles, calling and extending green for busy approaches.
• Radar/microwave sensors: Overhead units detect moving and stopped vehicles without cutting the road surface, useful on bus lanes and multi‑lane approaches.
• Video analytics cameras: Vision-based detectors classify vehicles, bikes and sometimes pedestrians; used where loops are impractical.
• Magnetometers and in-ground sensors: Compact detectors embedded in the pavement identify vehicles or bicycles in constrained locations.
• Pedestrian push-buttons (audio‑tactile): Call a walk signal and provide a vibrating arrow and audible cues; some sites also use infrared or radar to detect pedestrians still crossing and adjust clearance time.
• Bicycle detection: Bike‑sensitive loops, push-buttons at handlebar height, or camera/radar detection can trigger bike lanterns or early-start phases.
• Public transport detection: Trams and buses are detected by transponders, GPS and schedule data; the system can shorten conflicting greens or bring an earlier green for a modest priority.
• Emergency vehicle priority: Connected systems use GPS/4G location and ETA to request preemption or priority, turning lights to help ambulances and fire appliances pass through safely.

Together, these inputs tell the controller where demand exists, how saturated each approach is, and whether special priority should be applied, allowing the system to change lights responsively rather than on a fixed schedule.

How a typical signal cycle decides to change

While the exact logic varies by site, most Australian signals follow a predictable sequence of checks and rules each cycle. The steps below outline how decisions are made about when to end one phase and start the next.

1. Minimum service and demand: A phase runs at least its minimum green time once there is a valid demand (vehicle detection or a pedestrian call).
2. Extension by gaps: If detectors keep registering vehicles, the controller extends green in small increments until a preset maximum or until traffic “gaps out.”
3. Pedestrian timing: When a button is pressed, the system inserts a walk interval and then a flashing clearance based on crossing width; at some sites, sensors extend clearance if people are still on the crossing.
4. Coordination with the corridor: If coordinated, the controller aims to change in sync with the corridor’s offset so platoons get a green wave, as directed by SCATS.
5. Priority requests: Tram/bus priority or emergency preemption can truncate opposing phases or call an earlier green, within safety and policy limits.
6. Safety intergreens: Amber and all‑red times ensure the intersection clears before conflicting movements receive green.
7. Fallback rules: If a detector fails, the controller may run a recall (serving a phase each cycle) or revert to time‑of‑day plans to keep traffic moving safely.

The result is a balance between responsiveness to local demand, smooth flow along key routes, and strict adherence to safety timings that prevent conflicts.

Special cases across Australia

Different cities and contexts apply the same principles with local features and policies. The items below highlight common variations and priority treatments.

• Melbourne tram priority: Trams receive specialised T‑lights and priority via loops and GPS, often with head starts or extended greens to keep services moving.
• Sydney and other cities’ bus priority: B‑lights give buses an exclusive signal, early starts, or extended greens at key corridors while limiting impacts on cross traffic.
• Emergency vehicle priority: Jurisdictions including NSW and Queensland use GPS/4G‑based systems integrated with SCATS to grant dynamic priority when safe.
• Smart pedestrian crossings: Selected sites in NSW and Victoria use radar/thermal/AI sensors to adjust walk and clearance times to actual pedestrian presence.
• Bicycle treatments: Early start for bikes, bike lanterns, and detector markings help riders trigger phases and clear before general traffic moves.
• Rail level crossing preemption: Nearby traffic signals are forced to specific safe states when trains approach to clear queues away from tracks.
• School zones and time‑of‑day plans: Signals and flashing signs align with lower speed limits and predictable peaks around drop‑off and pick‑up.

These targeted policies keep public transport reliable, protect vulnerable road users, and handle special constraints such as rail interfaces, all within the overarching adaptive framework.

Why lights sometimes change with no one there

Occasional “empty greens” can occur when coordination serves a platoon that has already passed, when detectors mis‑detect or fail and the controller runs recall, or when a pedestrian call was made but the person didn’t cross. Maintenance teams monitor and retune problem sites, and adaptive control typically reduces these occurrences compared with fixed‑time operation.

Who runs the system

Traffic signals are managed by state and territory road agencies and local councils, often in partnership. Key operators include Transport for NSW; the Department of Transport and Planning in Victoria; Queensland’s Department of Transport and Main Roads (with Brisbane City Council operating many city signals); Main Roads Western Australia; South Australia’s Department for Infrastructure and Transport; Transport Canberra and City Services; Tasmania’s Department of State Growth; and the Northern Territory’s Department of Infrastructure, Planning and Logistics. They use SCATS and related tools to monitor performance, apply incident strategies, and update timing plans.

Practical tips for road users

Simple actions can help you get a timely green and keep intersections efficient. The tips below reflect how detection works at most Australian signals.

• Stop over the stop‑line loops or within the marked detection zone so the controller knows you’re there.
• Cyclists: Position your bike over the bicycle symbol or near the loop cut to be detected, or use the bike push‑button if provided.
• Press pedestrian buttons firmly; look for the LED ring or hear the locator tone to confirm your call is registered.
• Public transport users: Obey T‑lights and B‑lights; these run separately from general traffic signals.
• Avoid creeping past the stop line: you may leave the detection zone and delay your own green.
• Report persistently faulty detectors or signals to the local road authority; they can adjust detection sensitivity or repair equipment.

Following these practices helps the detection work as designed, improving responsiveness for everyone.

Summary

Australian traffic lights change primarily because detectors register demand and adaptive control systems like SCATS optimise timings in real time. Local controllers enforce safety and serve calls from vehicles, pedestrians, bikes and public transport, while network coordination smooths flow along corridors and grants priority when appropriate. The combination delivers safer, faster, and more reliable operation than fixed schedules, especially in complex urban networks.

How do the traffic lights know when to change?

Traffic lights know when to change using two primary systems: fixed-time cycles and sensor-activated systems. Fixed-time systems operate on a pre-programmed timer, switching lights according to historical data. Sensor-activated systems use detectors, such as inductive loops under the road or radar, to sense the presence and volume of vehicles. This information is sent to a traffic signal controller, a computer that analyzes real-time traffic conditions and adjusts the light timing to improve flow and reduce congestion.
 
This video explains how traffic lights work, including the use of sensors and controllers: 1mPractical EngineeringYouTube · May 14, 2019
Types of Systems

• Fixed-Time Signals: Opens in new tabThese signals follow a predetermined schedule, changing lights based on a timer rather than real-time conditions. This system relies on historical traffic data to set the timing. 
• Traffic-Actuated Signals: Opens in new tabThese are smarter systems that respond to actual traffic demand. They use sensors to detect when vehicles and pedestrians are present and send this data to the central controller. 

This video explains how traffic lights detect cars using inductive loops: 58sRoad Guy RobYouTube · Jul 27, 2020
Sensors and Detectors

• Inductive Loops: Opens in new tabA common type of sensor, inductive loops are wires embedded in the road surface. When a vehicle’s metal body passes over the loop, it changes the magnetic field, which the controller detects to signal the presence of a car. 
• Infrared Sensors: Opens in new tabThese sensors work by detecting infrared light or heat emitted by vehicles. 
• Microwave Radar: Opens in new tabRadar sensors use radio waves to detect moving objects, making them effective in various weather conditions. 
• Video Detection: Opens in new tabCameras use advanced image processing to monitor traffic and pedestrian movements, providing real-time data to the system. 

This video shows how traffic signals use sensors like cameras: 59sInteresting EngineeringYouTube · Nov 6, 2020
The Role of the Controller

• The traffic signal controller is the “brain” of the system, a special computer that receives data from sensors. 
• It is programmed with information on minimum/maximum green times, pedestrian times, and clearance periods. 
• Based on the sensor data, the controller analyzes real-time traffic patterns and adjusts the signal timings to optimize traffic flow and reduce congestion. 

Adaptive Control and Benefits

• Adaptive algorithms: in modern systems allow controllers to make real-time adjustments to signal timing based on current traffic volumes. 
• This dynamic timing helps minimize waiting times, smooth vehicle flow, and reduce stop-and-go traffic. 
• Systems can also integrate emergency vehicle preemption, giving emergency vehicles immediate right-of-way through an intersection. 

How do traffic light sensors work in Australia?

Traffic light loops
These are embedded in the road surface close to the stop line at a signalised intersection. Loops operate through a magnetic wave. When a car disrupts the wave, the signal detects that a car is at the lights.

How do traffic lights know to change color?

“When a large metal object is positioned over the loop (for example, a car), it affects the loop’s magnetic field.” Once the traffic signal detects your vehicle, it knows it has to change the light. Sometimes setting off these sensors can be difficult since vehicles need to pass over or stop within its detection area.

How do traffic lights know an ambulance is coming?

And the traffic light returns to its normal rotation. It’s a seamless process designed to ensure that emergency. Responders can navigate through traffic safely.