What Are the Sensors at Traffic Lights?

They are detection devices that tell the signal controller when vehicles, bicycles, or pedestrians are present or moving, most commonly using inductive loops in the pavement, video cameras, radar, or magnetometers. These sensors help allocate green time efficiently, trigger pedestrian crossings, give priority to emergency and transit vehicles, and feed data to adaptive traffic systems.

Why Traffic Signals Use Sensors

Modern traffic signals don’t just run on fixed timers; many are “actuated,” meaning they respond to real-time demand. Sensors provide the data that lets controllers decide when to change or extend a green, when to serve a pedestrian crossing, and how to adapt to congestion or incidents. As cities pilot connected-vehicle technologies, these detectors also support priority for buses and emergency vehicles and help manage corridors more smoothly.

Main Types of Vehicle Detection Sensors

Agencies use a mix of in-pavement and above-ground sensors, chosen for local climate, maintenance capacity, geometry, and budget. Here are the most common categories you’ll see at intersections.

• Inductive loop detectors: Wire loops cut into the pavement detect changes in inductance when a metal mass (a vehicle—or correctly positioned bicycle) is over the loop. They are the workhorse of detection in many regions.
• Video detection cameras: Pole- or mast-arm-mounted units analyze live images to detect presence, movement, and sometimes classify vehicles; thermal video variants work better at night and in glare or fog.
• Microwave/radar sensors: Side-fired or overhead radar detects moving and stopped vehicles using Doppler and range; relatively weather-robust and common where loops are hard to maintain.
• Magnetometers/in-road “pucks”: Battery-powered or wired sensors embedded in the pavement (often small discs) sense disturbances in Earth’s magnetic field from nearby vehicles; convenient for spot detection.
• Infrared (active/passive): Infrared beams or thermal sensors detect vehicles and vulnerable road users; passive thermal helps in low light and adverse weather.
• Ultrasonic/Acoustic: Less common today; detect presence or movement using sound waves or audio signatures, sometimes used in specialized applications.
• LiDAR: Scans create 3D point clouds for precise detection and tracking; still emerging but used in complex or high-precision scenarios.

Together, these technologies cover a range of traffic and weather conditions; agencies often deploy hybrids—e.g., radar plus video—to improve reliability and redundancy.

Pedestrian and Bicycle Detection

Intersections also rely on sensors tailored for people on foot and on bikes, aiming for safety and accessibility without unnecessary delay.

• Pedestrian push buttons and Accessible Pedestrian Signals (APS): The most widespread method; buttons register a crossing request and APS provides audible/tactile feedback and locator tones.
• Computer vision/thermal pedestrian detection: Cameras (visible or thermal) automatically call the walk phase when someone is in the detection zone; useful where compliance with push-buttons is low or for “automatic pedestrian recall.”
• Bicycle-specific loops/magnetometers: Sensitized loops marked with bike symbols detect smaller metal profiles; cyclists should stop over the saw-cut or the bike symbol for best detection.
• Overhead radar/LiDAR for bikes: Detect moving cyclists in bike lanes to call or extend a phase.
• Passive infrared beams at crosswalks: Trigger or extend pedestrian phases as people approach or are still crossing.

These tools reduce missed calls, improve pedestrian safety, and help cities serve micromobility users more equitably.

Special-Purpose and Connected Systems

Beyond basic detection, many intersections include equipment that prioritizes critical vehicles or collects performance data.

• Emergency vehicle preemption: Optical infrared strobe receivers (e.g., Opticom), GPS/radio systems, or acoustic sensors grant rapid green to ambulances, fire engines, and police responding to emergencies.
• Transit Signal Priority (TSP): Bus- or tram-mounted radios (DSRC, cellular/C-V2X, GPS) request modest green extensions or early greens to keep transit on schedule.
• Bluetooth/Wi‑Fi probe readers: Anonymized device detections estimate travel times and congestion; typically for monitoring, not signal actuation.
• V2I/C-V2X radios: Emerging deployments exchange Signal Phase and Timing (SPaT) and MAP messages and can receive priority requests from connected vehicles.

These systems help improve safety response times and transit reliability while providing richer data for corridor management and planning.

How Sensors Change the Light

At an actuated or adaptive intersection, detections translate into timing decisions. Here’s the typical sequence from arrival to green.

1. Detection: A sensor notes a vehicle, cyclist, or pedestrian—either presence (stopped) or approach (moving).
2. Call and queuing: The controller logs a “call” for that movement and adds it to the queue of phases to serve.
3. Gap-out or max-out logic: If a phase is green, detections can extend it as long as vehicles keep arriving within set time “gaps,” up to a maximum.
4. Serve the phase: When safe and scheduled, the controller switches to the called phase, including yellow and all-clear intervals.
5. Adaptation: In adaptive systems, performance data (queues, arrivals, occupancy) tunes cycle length, splits, and offsets in near real-time.

This logic allows signals to respond to real demand, reducing unnecessary delay and smoothing traffic flow compared with fixed-time operation.

Common Myths and Tips

Some persistent misconceptions can cause frustration at intersections. Here’s what to know—and how to improve your odds of being detected.

• No, there aren’t “pressure plates”: Modern detection is rarely based on weight; loops sense metal via inductance, not mass.
• That “camera” isn’t always recording video: Many mast-arm devices are detection sensors, not surveillance cameras; processing often happens on-device without storing identifiable footage.
• Motorcycles and bicycles can trigger loops: Stop over the saw-cut lines or the bike symbol; using the kickstand over the loop can help, but extra magnets are usually unnecessary.
• Push the button—and wait: Pedestrian signals are timed with vehicle phases; it can take a full cycle before WALK appears.
• Weather matters: Fog, glare, snow, or pooled water can degrade video; radar and thermal are more resilient in adverse conditions.

Understanding how detection works helps road users position themselves for reliable calls and reduces false expectations at signals.

Privacy and Data Handling

Detection systems are increasingly designed with privacy in mind. Many video detectors process images at the edge and output only presence data, discarding raw video unless explicitly configured for recording. Bluetooth/Wi‑Fi readers typically hash or randomize device identifiers and use aggregated statistics. Policies vary by jurisdiction; agencies often publish data governance practices for transparency.

Maintenance and Trade-offs

Each sensor type has pros and cons that shape agency choices and maintenance strategies.

• Loops: Accurate and inexpensive but require pavement cuts and are prone to failure from road wear and utility work.
• Video: Flexible zones and classification but sensitive to lighting, weather, and lens cleanliness.
• Radar/microwave: All-weather reliability and low upkeep but can struggle with very slow or stationary detection at long range without proper configuration.
• Magnetometers: Easy spot installs with minimal trenching but require battery maintenance if wireless.
• Thermal/IR/LiDAR: Strong in complex scenes or low light but costlier and require careful calibration.

Blended deployments—mixing, for example, radar and video—improve resilience and detection accuracy across seasons and traffic conditions.

Bottom Line

The “sensors at traffic lights” are a toolbox of detection technologies—inductive loops, cameras, radar, magnetometers, infrared, and more—supplemented by pedestrian and bicycle systems and specialized priority hardware. Together, they let signals respond to real-world demand, prioritize safety-critical movements, and feed data to smarter, more adaptive traffic operations.

Summary

Traffic-light sensors detect vehicles, cyclists, and pedestrians to inform when and how a signal changes. The most common are in-pavement inductive loops, video and thermal cameras, microwave/radar units, and magnetometers, with additional systems for pedestrian push-buttons, bike detection, emergency preemption, and transit priority. Controllers use these inputs to call and extend phases, reduce delay, and enhance safety, while privacy-aware designs and mixed technologies address environmental and maintenance challenges.

What are those sensors on top of traffic lights?

Sensors on traffic lights include radar detectors, video cameras, and infrared or microwave sensors to detect vehicles and pedestrians and adjust light timing, while other devices are for emergency vehicle preemption. These technologies use different methods, such as radar waves, infrared or microwave energy, or video analysis, to monitor traffic flow, detect waiting vehicles or pedestrians, and manage the intersection efficiently.
 
Types of sensors:

• Radar Detectors: Opens in new tabThese are often seen as white boxes on the top of traffic lights and use radar technology to detect the presence and movement of vehicles. 
• Video Detection Systems: Opens in new tabThese cameras monitor the entire intersection for vehicle and pedestrian movements, allowing for more comprehensive data collection on traffic flow. 
• Infrared Sensors: Opens in new tabThese sensors emit and detect beams of infrared light, sensing vehicles by interruptions in the beam. 
• Microwave Sensors: Opens in new tabSimilar to infrared sensors, these emit and detect electromagnetic waves, sensing vehicles by detecting reflections. 
• Optical Sensors: Opens in new tabThese can be used for preemption by detecting strobes from emergency vehicles to grant a green light. 
• Emergency Vehicle Preemption Devices: Opens in new tabThese systems, which may look like small black devices or antennas, are designed to detect emergency vehicles and change traffic lights to a green light. 

Their purpose:

• Traffic Management: Opens in new tabThe primary goal is to optimize traffic flow by adjusting light timing based on detected traffic volume. 
• Vehicle & Pedestrian Detection: Opens in new tabSensors can detect the presence of vehicles in lanes and identify pedestrians waiting at intersections, especially in areas with inconsistent traffic. 
• Emergency Vehicle Response: Opens in new tabDedicated systems allow emergency vehicles to trigger a preemption sequence, ensuring a clear path through intersections. 
• Data Collection: Opens in new tabSome systems collect comprehensive data on traffic flow, which traffic engineers can use to understand and manage complex intersections. 

Are there sensors under the ground at traffic lights?

Yes, many traffic lights have sensors, most commonly in the form of buried inductive loops (wire coils) that detect vehicles by disrupting a magnetic field. These sensors communicate with the traffic signal controller to adjust signal timing, which is particularly useful at less busy intersections to ensure cross-traffic gets a green light. You can often see evidence of inductive loops as a rectangular pattern of lines cut into the pavement before the stop line.
 
How they work

1. 1. Wire coils: Inductive loops, which are essentially wire coils, are embedded under the pavement at varying distances from the stop line. 
2. 2. Magnetic field: An electrical current runs through these coils, creating a magnetic field. 
3. 3. Vehicle detection: When a vehicle’s metal frame passes over or stops above the loop, it disrupts the magnetic field and changes the inductance. 
4. 4. Signal to the controller: This disruption is detected by the traffic signal controller, which acts as the “brain” of the system. 
5. 5. Adaptive timing: The controller uses this information to adjust signal timings, such as extending a green light or turning a light green for a waiting vehicle. 

Why you should pull up to the stop line

• Ensures detection: Pulling up to the limit line ensures your car is directly over the sensor, making it easier for the sensor to detect your vehicle. 
• Adjusts for gaps: The sensors extend the duration of a green light for each car that drives over them, and a longer gap in traffic can shorten the green light. 
• Other detection methods: While inductive loops are common, some intersections use overhead infrared or microwave sensors or even cameras to detect vehicles. 

What are the sensors on traffic lights?

Inductive Loop Sensors: Embedded beneath the roadway, these sensors detect vehicles by measuring changes in magnetic fields. Infrared Sensors: Use beams of infrared light to detect vehicles by measuring interruptions in the beam.

Are there weight sensors at traffic lights?

No, traffic lights do not use weight sensors; they use other technologies like inductive loops embedded in the road that detect the presence of a vehicle by sensing changes in a magnetic field, not by its weight. Other sensors used include radar, ultrasonic, laser, and vision systems that detect metal or vehicles directly, rather than relying on weight. 
How typical traffic light sensors work

• Inductive Loops: Opens in new tabThese are the most common type of sensor, consisting of a wire loop buried under the pavement. When a vehicle with sufficient iron (like a car) drives over the loop, it disrupts the loop’s magnetic field. The change in the field is sent to the traffic controller, which registers a vehicle’s presence and can adjust the signal. 
• Other Sensors: Opens in new tabNewer systems may use radar, ultrasonic, laser, or camera-based vision systems to detect vehicles and trigger light changes. 

What the sensors do

• Vehicle Detection: Opens in new tabThe primary function is to detect when a vehicle is waiting at an intersection. 
• Traffic Adjustment: Opens in new tabBy detecting vehicles, these sensors help traffic signal systems to dynamically adjust signal timings, improving traffic flow and reducing delays. 
• No Weight Sensitivity: Opens in new tabIt’s a myth that weight is a factor. The presence of a vehicle is key, not its mass.