How Stop Lights Know When to Change

Traffic lights change using a mix of timers, sensors that detect vehicles and pedestrians, and—at many intersections—centralized coordination or adaptive software that responds to real-time traffic. In practice, a signal turns green, stays on for a minimum time, and then looks for gaps in traffic or competing demands before switching phases, all while obeying safety rules for yellow and all-red intervals.

The Core Logic Inside a Traffic Signal

Modern signal controllers run a set of rules that decide when to display green, yellow, and red. They operate through “phases” (e.g., north–south through, left turns, pedestrian crossings) and move between them when both timing requirements and demand are satisfied.

These are the key timing concepts that determine how and when lights change:

• Cycle length: Total time to serve a complete sequence of phases before repeating.
• Split: Portion of the cycle assigned to each phase.
• Offset: The timing relationship between nearby intersections to create progression (“green waves”).
• Minimum green: The shortest green time a phase must serve once it starts.
• Passage time (gap): The maximum allowed time between detected vehicles before a phase can end (“gap-out”).
• Maximum green: The longest a phase is allowed to stay green before it must change (“max-out”).
• Yellow change interval: The warning period before red, calculated using speed and deceleration guidelines.
• All-red interval: A brief clearance period with red in all directions to empty the intersection.

Together, these parameters let a controller balance safety with efficiency, ensuring each movement gets served while maintaining flow and predictable timing.

Where the “Demand” Comes From: Detection Technologies

Signals “know” someone is waiting via detectors. These devices sense vehicles, bikes, or pedestrians and place a request—or “call”—for service to the controller.

• Inductive loop detectors: Wire loops in the pavement that sense metal mass; the most common in North America.
• Video analytics cameras: Overhead cameras identify presence and movement using computer vision; can classify and count traffic.
• Radar and microwave sensors: Detect moving and stopped vehicles reliably in rain, fog, or darkness.
• Thermal/infrared sensors: Useful in low light or glare; can detect pedestrians and cyclists.
• Magnetometers: Small in-pavement sensors that register changes in the Earth’s magnetic field as vehicles pass.
• Acoustic sensors: Use sound signatures to detect traffic in some applications.
• Pedestrian push buttons: Send a walk request; often paired with audible/tactile features for accessibility.
• Connected-vehicle inputs: Pilot systems use vehicle-to-infrastructure (V2X) signals or crowdsourced app data to anticipate arrivals.

Agencies often mix technologies to reduce blind spots—loops for accuracy, radar for queue detection, and video for classification—so the light changes only when and where it’s needed.

Types of Signal Operation

How a signal decides to change largely depends on its operating mode and local policy. Different modes match different street contexts, from quiet side streets to busy arterials.

• Fixed-time: Changes on a set schedule with no detection; common in dense downtown grids where predictability aids progression.
• Semi-actuated: Major street runs on a schedule, while side streets and pedestrians are served only when detectors call.
• Fully actuated: All movements are detector-driven; green ends when demand drops or a max time is reached.
• Coordinated (time-of-day plans): Intersections follow shared cycle lengths and offsets to form green waves during peak periods.
• Adaptive/AI-based: Systems like SCOOT, SCATS, InSync, and Surtrac adjust splits, cycle lengths, and offsets in real time based on measured or predicted demand.

Most cities use a blend: coordination for corridors at rush hour, actuated control off-peak, and adaptive control in high-variability areas to smooth flow and reduce delay.

Coordination and Central Control

Many intersections are networked to a traffic management center. Engineers push time-of-day plans, monitor performance, and adjust timing after incidents or special events. Along corridors, offsets create progression so platoons of vehicles hit successive greens at a target speed. Increasingly, agencies use cloud analytics, probe data, and V2X pilots to broadcast and optimize SPaT (Signal Phase and Timing) information, helping connected vehicles anticipate greens and cut idling.

Special Priorities and Safety Overrides

Beyond day-to-day traffic flow, signals also respond to priority requests and enforce safety-critical timing to prevent conflicts.

• Emergency vehicle preemption: Optical, radio/GPS, or acoustic systems request an immediate or near-immediate green for responding units.
• Transit signal priority (TSP): Buses and streetcars may get extended greens or shortened reds to stay on schedule.
• Railroad and drawbridge preemption: Signals clear the tracks and hold red to prevent vehicles entering conflict zones.
• Pedestrian safety features: Leading Pedestrian Intervals (LPIs), countdown timers, and accessible pedestrian signals aid safe crossings.
• Bicycle detection and phasing: Marked loop zones, radar, or camera detection and sometimes bike-specific signals improve responsiveness.
• Fail-safe and power backup: Conflict monitors force a safe flash if hardware fails; battery backups keep signals operating during outages.

These layers ensure that, when necessary, the normal schedule yields to life-safety needs and vulnerable road users.

Common Misconceptions

A few popular beliefs about traffic signals don’t match how systems actually work.

• “They use weight sensors.” Vehicle weight is not what triggers most signals; loops sense metal mass, not heaviness.
• “Cameras are for tickets.” Detection cameras typically feed the signal controller; enforcement systems are separate and clearly signed where used.
• “Ped buttons don’t do anything.” In many places they place a necessary call; in others (downtowns), pedestrian recall is automatic, but the button may add features like audible cues.
• “Yellow times are arbitrary.” Change intervals follow engineering formulas based on speed, grade, and perception–reaction time.

Understanding these points helps explain why signals behave as they do—and why a press of the button or rolling up to the stop bar often makes a difference.

What You Can Observe at the Curb

Clues around an intersection reveal how it decides to change.

• Saw-cut rectangles in pavement suggest inductive loops; stop near them to be detected.
• Small radar units or cameras on mast arms indicate presence detection and possibly adaptive control.
• Pedestrian button lights or tones confirm your request was received.
• Consistent green waves along a corridor signal coordination; variable greens hint at actuated or adaptive operation.

These cues can help you position your vehicle or bike for detection and anticipate how the next phase will play out.

Summary

Stop lights change through a combination of programmed timing, real-time detection of users, corridor coordination, and, increasingly, adaptive algorithms that react to demand. Controllers enforce safety intervals and respond to priority requests from emergency and transit vehicles while accommodating pedestrians and cyclists. The result is a carefully managed sequence that balances safety, efficiency, and predictability across the road network.

How do stop lights know when an emergency vehicle is coming?

And switching the traffic signals to give those emergency vehicles. The right of way this signal is a part of a very clever system designed to ensure safe passage for all emergency vehicles.

How do stop lights know when you’re there?

There is a loop of wire embedded in the pavement with an electrical charge running through it. When a large piece of metal, like a car, passes over the loop it modifies the electric field in the wire. The traffic light detects the change in the field and knows a car is there.

How do you trigger the sensor at a stoplight?

Vehicle positioning: When approaching a stoplight, pull up to the designated stop line or, if there isn’t one, wait behind the crosswalk. This lets the stoplight’s sensors or cameras recognize your vehicle and respond accordingly. But if you’re too far away, the sensors might not detect you at all.

How do stop lights know when to change color?

Traffic lights change color by using a computer-like controller that receives data from various sensors to detect vehicles and pedestrians, then adjusts its pre-programmed timing sequences to optimize traffic flow. Common sensors include embedded inductive loops that detect metal vehicles, radar, infrared beams, and cameras that use computer vision or detect heat. The system also accounts for factors like time of day and traffic volume, with some advanced systems connecting to a central network for real-time adjustments.
 
This video explains how traffic lights use sensors to detect vehicles: 58sRoad Guy RobYouTube · Jul 27, 2020
How the System Works

1. Detection: Sensors, such as inductive loops, cameras, radar, or infrared beams, are strategically placed at intersections to detect the presence of vehicles or pedestrians. 
2. Data Transmission: When a vehicle or pedestrian is detected, the sensor sends a signal to a traffic signal controller, which is a computer in a nearby cabinet. 
3. Processing: The controller uses pre-programmed logic to process the incoming data. 
4. Timing and Sequence: The controller then determines the optimal timing for the lights to change, considering: 
   • Traffic volume: The amount of traffic detected at each approach. 
   • Time of day: Base timing plans are established for different periods, like rush hour versus off-hours. 
   • Traffic patterns: Some systems are connected to a central network that allows for real-time adjustments to manage traffic flow and respond to events. 
   • Pedestrians: Signals are also timed to allow pedestrians to safely cross. 
   • Emergency vehicles: Specialized systems can detect signals from emergency vehicles to grant them priority. 

Types of Sensors

• Inductive Loops: Opens in new tabThese are electrical circuits embedded in the road that create a magnetic field. A metal vehicle passing over the loop disrupts this field, sending a signal to the controller. 
• Cameras: Opens in new tabModern systems use cameras with computer vision to detect vehicles and track their movement within designated zones. 
• Radar: Opens in new tabRadar sensors emit microwave signals to detect the presence and speed of vehicles. 
• Infrared Sensors: Opens in new tabThese sensors use beams of infrared light or detect heat from a vehicle’s engine to register its presence. 

This video shows how different sensors, like cameras, are used in traffic lights: 59sWCCO – CBS MinnesotaYouTube · Oct 18, 2023
Examples of Operation

• Busy Intersections: Opens in new tabAt busy intersections, a controller might extend the green light time if it detects a continuous flow of vehicles over several seconds. 
• Low-Traffic Conditions: Opens in new tabIf no vehicles are detected on a particular approach, the controller can skip that phase to give a green light to the other direction sooner, preventing wasted time.