How Traffic Lights Work With Sensors

Traffic lights use sensors—such as inductive loops, cameras with computer vision, radar, and pedestrian pushbuttons—to detect people and vehicles, then a controller allocates green time where and when it’s needed, extending or shortening phases to keep traffic moving and improve safety. In practice, detection tells the signal which approaches are waiting, how long queues are, and sometimes how fast traffic is moving, enabling everything from basic actuation at a single intersection to network-wide adaptive coordination and priority for emergency and transit vehicles.

The building blocks: controllers, phases, and detection

Every modern signal is run by a controller cabinet—the roadside computer that executes timing plans, interprets sensor inputs, and displays greens, yellows, and reds through “phases” (movements like north–south through, east–west left turns, and pedestrian crossings). Detection hardware supplies real-time data to the controller, which applies rules such as minimum green time, gap-out (ending a phase when cars stop arriving), and coordination offsets to manage progression along a corridor.

Types of sensors used at intersections

Agencies mix and match sensor technologies based on climate, cost, roadway geometry, and maintenance needs. Below are the most common detectors and what they do well.

• Inductive loop detectors: Wires embedded in the pavement sense a vehicle’s metal mass. They are accurate for presence and queue detection but can fail with pavement wear or utility cuts.
• Video detection with computer vision: Pole-mounted cameras identify vehicles, bicycles, and pedestrians within virtual zones; modern systems add AI to classify users and handle occlusion. Performance can degrade with glare, heavy rain, or snow unless thermal cameras are used.
• Radar/microwave sensors: Side-fire or overhead units detect moving and stopped vehicles, estimate speed, and work day or night in most weather. Increasingly popular for dilemma-zone protection and stop-bar detection.
• Magnetometers/磁 sensors: Small, in-pavement or wireless units detect changes in the Earth’s magnetic field as vehicles pass; useful where loops are impractical.
• Infrared/thermal sensors: Passive thermal cameras detect pedestrians and vehicles by heat signature; less sensitive to lighting conditions and useful for passive pedestrian detection.
• Acoustic sensors: Microphone arrays detect approaches by sound; niche use due to environmental noise variability.
• LiDAR: Scans create precise 3D detection zones, improving vulnerable road user detection and occlusion handling; still higher cost but growing in pilots.
• Bluetooth/Wi‑Fi MAC readers: Not for control, but for estimating travel times and origin–destination patterns by sampling devices; used to tune signal timing.
• Connected-vehicle (V2X) radios: DSRC or C‑V2X units exchange standardized messages (SAE J2735) to support priority, red-light warnings, and future sensor redundancy.
• Pedestrian pushbuttons: The most widespread pedestrian “sensor,” often with audible/vibrotactile feedback and countdowns; some corridors add passive ped detection to call walk automatically.
• Bicycle-specific detection: Marked loop quadrants or video/radar zones tuned to bicycles, with pavement symbols indicating where riders should stop to be seen.

Together, these sensors provide presence, passage, speed, and classification data. Agencies often combine two or more types to increase reliability across seasons and lighting conditions.

How the signal uses sensor data in real time

Actuation and timing logic

Controllers apply well-defined parameters to turn raw detections into green time. Here are the key concepts used in most North American and European signal controllers.

• Call: A detector “requests” service for a phase when a vehicle, bike, or pedestrian is present.
• Presence vs. passage detection: Presence holds a call while something is waiting; passage is a brief pulse when a vehicle crosses a zone.
• Minimum green: The smallest guaranteed green time so drivers can react and start moving.
• Gap/vehicle extension: The green continues as long as arrivals keep the measured gaps below a threshold; if a gap exceeds the setting, the phase “gaps out.”
• Maximum green: A cap to prevent one movement from monopolizing green time.
• Yellow change and all-red clearance: Safety intervals for stopping and clearing the intersection.
• Pedestrian timing: Walk, flashing don’t walk, and clearance intervals calculated from crosswalk length and assumed walking speeds; may include Leading Pedestrian Intervals (LPIs).
• Detector failure handling: If a detector fails, the controller can place a “recall” (serve the phase every cycle) or run a fixed-time plan to remain safe and predictable.
• Locked calls and extension for queues: When queues are long, some systems hold a call to ensure full service even if gaps momentarily open.

This logic lets signals respond to real demand, trimming wasted green time on empty approaches and extending service when arrivals continue.

Levels of actuation

Not all signals use sensors on every approach. Agencies choose a level of actuation based on traffic volumes and coordination needs.

• Fixed-time: No sensors; phases run on pre-set times. Useful in dense grids and during major events or comms outages.
• Semi-actuated: Major street runs on coordinated timing; minor street has detectors that call for green when needed.
• Fully actuated: All approaches have detection; the controller sequences phases dynamically within limits.

Fully actuated control saves time at isolated intersections, while semi-actuated helps maintain “green waves” on the main corridor without starving side streets.

A typical sensor-controlled cycle, step by step

At a single intersection, the controller follows a repeatable sequence influenced by detections on each approach. These steps illustrate how sensor data shapes a cycle.

1. Rest state: The signal “rests” green on a main movement or holds a coordinated plan.
2. Detection: A vehicle, cyclist, or pedestrian triggers a call on a side street or crosswalk.
3. Queue assessment: Upstream detectors or classification zones measure arrivals and sometimes queue length.
4. Phase change: When rules allow, the controller ends the current phase (gap-out or max-out), runs yellow and all-red, then serves the called phase.
5. Green extension: As arrivals continue within the configured gap, the controller extends green up to the maximum.
6. End of service: When arrivals stop or the maximum green is reached, the phase ends and the next call in the sequence is served.
7. Coordination check: If in a corridor plan, the controller adjusts start times to maintain progression with neighboring signals.

This loop repeats continuously, adapting to demand within safety and coordination constraints.

Adaptive signal control and coordination

Beyond single intersections, agencies coordinate signals along corridors, setting offsets so platoons of vehicles ride a “green wave.” Adaptive Signal Control Technologies (ASCT) go further, using detectors across multiple intersections to update splits, offsets, and cycle lengths in real time as traffic changes.

Several adaptive systems are widely deployed; each optimizes timing with different algorithms and data requirements.

• SCOOT (UK-origin): Continuously adjusts splits/offsets/cycle on 5–15 second intervals using detectors and performance measures.
• SCATS (Australia-origin): Selects plans and updates splits and cycle length based on volume/occupancy and historical patterns.
• InSync and Surtrac (U.S.): AI-based scheduling to minimize delay and stops, often with camera and radar inputs.
• OPAC/RHODES/MaxAdaptive/SynchroGreen: Optimization frameworks that reallocate green based on predicted arrivals and queues.

Agencies choose systems based on goals, staffing, communications bandwidth, and the reliability of detection across seasons and peak periods.

Special priority and safety features

Emergency vehicle preemption and transit priority

Signals can grant special treatment to reduce response times and improve schedule reliability. The methods below are common in current deployments.

• Emergency preemption (e.g., Opticom IR/GPS, radio-based, or V2X): Temporarily interrupts normal operation to provide a green path for fire/EMS, with safe transition back to coordination.
• Transit Signal Priority (TSP): Grants early green or green extension to buses and streetcars when late or carrying many passengers; often integrated with AVL data.
• Freight/port priority: Extends green for heavy trucks on designated routes to improve efficiency and reduce emissions.
• V2X priority via SAE J2735 messages: Connected buses and emergency vehicles request priority digitally, with precise ETA and approach lane information.

Priority strategies balance user benefits against impacts on cross traffic, using conditional rules to avoid gridlock and maintain safety.

Dilemma-zone protection and speed detection

On high-speed approaches, radar or advance loops detect vehicles near the “dilemma zone”—too close to stop safely, too far to clear—during the end of green. The controller can delay the yellow onset or extend green briefly to reduce red-light running and angle crashes, while enforcing a maximum so downstream coordination is preserved.

Pedestrian and cyclist detection

Modern practice increasingly uses passive detection and safer phasing for people walking and biking. The tools below are becoming standard in new projects.

• Accessible pushbuttons with audible/vibrotactile feedback: Let pedestrians call a walk phase, request extra time, or activate LPIs.
• Passive pedestrian detection: Video, thermal, or LiDAR automatically calls walk and extends clearance if someone is still crossing.
• Leading Pedestrian Intervals (LPIs): Give walkers a head start before turning traffic gets green, often triggered regardless of vehicle demand.
• Bicycle detection and markings: Loops, radar, or video zones tuned for bikes, with pavement symbols showing where to stop for detection.
• Bicycle-friendly timing: Lower speeds and “green waves” for cyclists on key corridors to reduce unnecessary stops.

These measures reduce conflicts, make detection more reliable for lighter vehicles, and improve accessibility for people with disabilities.

Reliability, maintenance, and privacy

Detectors operate outdoors in harsh conditions. Agencies plan for failures, monitor assets remotely, and address data privacy when collecting road-user information.

• Common failures: Broken loops from pavement cuts, camera occlusion from snow or glare, radar misalignment, and seasonal foliage changes.
• Monitoring: Controllers and detectors report status over NTCIP to traffic management centers for proactive maintenance.
• Fallback modes: If detection fails, signals shift to fixed-time or place recalls on affected phases to maintain service safely.
• Resilience: Battery backups and cellular/fiber communications reduce downtime during storms or outages.
• Privacy: Many agencies process video at the edge without recording, anonymize or hash MAC addresses, and follow retention limits aligned with local law.

Well-chosen sensor mixes and clear maintenance policies keep intersections functioning accurately year-round while minimizing privacy risks.

What drivers and cyclists should know at a sensor-equipped intersection

Understanding how detection works helps people get seen and reduces frustration. The tips below cover common issues users can control.

• Stop at the stop bar or bike detector symbol; many sensors are right at the line.
• Motorcycles and bicycles: Position over the loop cut or detection symbol; on video/radar intersections, keep within the marked lane and avoid far edges.
• Don’t creep into the crosswalk; you may sit beyond the stop-bar detector and never trigger a call.
• Use the pedestrian button when present; some signals won’t serve a walk without it unless passive detection is active.
• In snow or heavy rain, allow extra time; detection may be conservative to maintain safety.
• Report obvious malfunctions (e.g., never-serving side street) to your city’s traffic operations via 311 or posted cabinet labels.

Following these practices improves your chance of being detected and keeps the signal operating as intended for everyone.

The bottom line

Traffic lights with sensors turn real-world demand into smart timing: they call phases when people arrive, extend green while vehicles keep coming, coordinate corridors, and prioritize buses and emergency vehicles. With modern radar, computer vision, and emerging V2X, signals are becoming more reliable and responsive, guided by standards like the 2023 MUTCD update and NEMA/ATC controller specs. When designed and maintained well, sensor-enabled signals reduce delay, improve safety, and make streets work better for all users.

Summary: Sensors—loops, cameras, radar, magnetometers, and more—feed data to signal controllers, which apply timing rules to serve and extend phases, coordinate corridors, and provide priority. Adaptive systems optimize across networks, while pedestrian and cyclist detection and dilemma-zone protection enhance safety. Robust maintenance and privacy practices keep these systems effective and trusted.

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 traffic lights detect emergency vehicles?

Most fire engines and ambulances have a coded infrared strobe mounted on top of the vehicle. When the strobe is activated, it is detected by a sensor at the signal that turns the signal green for the approaching emergency vehicle.

How does an automatic traffic signal know that your car is there?

The wire creates an electrical field in the air above the pavement. When a large object interrupts the electric field, the signal knows that a vehicle is present and will provide a green light at the proper time in the established traffic signal cycle.

How do the sensors at traffic lights work?

Traffic light sensors work by detecting vehicles or pedestrians and signaling the traffic light controller to adjust the signal timing. Common sensor types include inductive loop detectors, which are wire coils under the road that disrupt a magnetic field when a vehicle passes over them, and video detection systems, which use cameras to monitor traffic and pedestrian movements. Other technologies like infrared sensors use beams of light or detect heat, while microwave sensors use radar. This real-time data allows for dynamic signal timing, improving traffic flow and safety.
 
How they work:

1. Vehicle Detection: When a vehicle or pedestrian is detected, the sensor sends a signal to the traffic controller, which is the “brain” of the traffic signal system. 
2. Signal Request: This signal registers a call for that specific traffic phase (e.g., the northbound lane). 
3. Dynamic Timing: The controller uses this information to adjust the length of the green light, potentially extending it if there’s still a queue of vehicles or skipping a phase entirely if the lane is empty. 

Types of Sensors:

• Inductive Loops: These are wire coils installed beneath the road surface. When a vehicle (which contains metal) drives over a loop, it disrupts the magnetic field, sending a pulse to the controller. 
• Video Detection: Mounted cameras monitor designated zones at the stop bar or intersection. Advanced image processing identifies vehicles and pedestrians, highlighting the detection zones within the controller’s view. 
• Infrared Sensors: These either emit a beam of infrared light and detect when it’s broken by a vehicle or detect the heat signature of an engine, according to CarParts.com, where the sensor notes the heat of an engine when the car nears the stoplight. 
• Microwave Sensors: These use radar technology to detect moving objects and are effective in various weather conditions. 

Benefits of Sensor Technology:

• Efficient Traffic Flow: By adapting signal timing to real-time conditions, sensors reduce stop-and-go traffic and congestion. 
• Improved Safety: Sensors can help prevent collisions by ensuring signals are provided to waiting vehicles and pedestrians in a timely manner. 
• Emergency Preemption: Many systems can be integrated with emergency vehicle preemption, which gives responding vehicles an immediate green light through an intersection.