How Traffic Lights Sense a Waiting Car

Most modern traffic lights detect a waiting car using inductive loop sensors embedded in the pavement; increasingly, agencies also use video analytics cameras, microwave/radar, and in-pavement magnetometer sensors. These devices register a “call” to the signal controller, which then allocates or extends a green light for the detected approach.

The core ways intersections detect vehicles

Engineers deploy several sensor types to determine when a vehicle is present at a stop line or approaching an intersection. Each technology has strengths and trade-offs in cost, reliability, weather performance, and maintenance.

• Inductive loop detectors: Wire loops cut into the pavement form an electromagnetic field that changes when a vehicle’s metal mass is above them, signaling presence.
• Video detection cameras: Pole-mounted cameras with computer vision detect vehicles in user-defined zones, count queues, and sometimes classify vehicles and bikes.
• Microwave/radar sensors: Side-fire or overhead radar detects moving and stopped vehicles in multiple lanes and works well in rain, fog, and darkness.
• In-pavement magnetometers/wireless “puck” sensors: Battery-powered sensors embedded in the roadway detect disturbances in Earth’s magnetic field from nearby vehicles.
• Infrared or ultrasonic sensors: Above-ground detectors that sense presence or motion; used less widely due to weather sensitivity or range limits.
• Legacy pressure plates or pneumatic tubes: Historically used for counts or temporary work zones; rarely control permanent signals today due to durability issues.

In practice, inductive loops remain common because they’re accurate and inexpensive per lane, while above-ground sensors are favored where cutting pavement is difficult or where agencies want richer data like counts, speed, and queue length.

Inside an inductive loop: what it actually measures

An inductive loop is a few turns of wire in a sawcut rectangle sealed into the asphalt and connected to an electronics unit in the signal cabinet. The loop is part of an oscillator; when a large conductive object (your car) enters the loop’s magnetic field, the loop’s inductance drops slightly, shifting the oscillator frequency. The detector interprets this shift as “vehicle present” or “vehicle passage.” Contrary to myth, it’s not about weight, and you don’t need a magnet—metal mass and position over the loop matter most.

Here’s how a typical loop-based call flows through the signal controller.

1. A vehicle stops over the loop (ideally near the loop’s edges), changing the loop’s inductance and triggering a presence call.
2. The controller adds or holds the green for that approach, subject to timing rules like minimum green, extension (gap) time, and maximum green.
3. If advance loops upstream detect approaching traffic, the controller can anticipate arrivals and extend green to flush the queue.
4. When the vehicle clears and the loop returns to normal, the call drops; if no vehicles are present and timers expire, the signal “gaps out” and serves the next phase.

Stop-line loops detect vehicles waiting at the line, while advance loops detect arrivals and speed. Proper placement, sensitivity tuning, and maintenance are essential to reliable operation.

Above-ground sensing: vision, radar, and lidar

Where pavement cuts are impractical or richer data are desired, agencies use pole-mounted sensors. These systems draw virtual detection zones in software and can cover multiple lanes with one device.

These are the common options and their field performance considerations.

• Video analytics: Flexible and multi-lane, with features like queue length and bicycle detection. Performance can degrade with glare, heavy rain/snow, low sun angles, or dirty lenses, so alignment and maintenance matter.
• Microwave/radar: Robust in all weather and darkness; can detect both moving and stopped vehicles and estimate speed. Requires careful aiming to avoid occlusion at the stop line.
• Lidar: High spatial resolution for precise stop-line presence and queue profiling, but higher cost and less common outside specialty or pilot deployments.

Many modern signals combine video and radar for redundancy, using logic that cross-checks presence to reduce missed or false calls.

Bikes, motorcycles, and small vehicles

Smaller vehicles have less metal and may sit outside a loop’s most sensitive area. Many agencies mark bicycle detection points or install higher-sensitivity loops, magnetometers, or video zones tuned for two-wheelers.

If you ride, these practical tips improve your chance of being detected.

• Stop over the sawcut rectangle (or bicycle symbol) near its edge, where the loop’s magnetic field is strongest.
• Position the heaviest metal part of the bike (crankset or frame) directly above the loop edge rather than the center.
• Look for bicycle-specific pushbuttons or call boxes at the curb if loops are unreliable.
• If a phase never serves after a full cycle, local laws vary: using a pedestrian button or repositioning is preferred; some jurisdictions allow proceeding cautiously after waiting, but rules differ—check local regulations.

Agencies increasingly use video or magnetometer detection for bikes, which reduces missed calls and improves safety and compliance.

What drivers can notice at the intersection

Clues around the stop line can tell you how the signal senses vehicles and why a phase might not change if you stop too far back.

• Rectangular or circular sawcuts in the pavement near the stop bar indicate inductive loops.
• Small box-like cameras on mast arms or poles (distinct from red-light enforcement cameras) suggest video detection zones.
• Flat, round “puck” covers embedded in the lane mark magnetometer sensors.
• Side-mounted radar units on poles point across the lanes and cover multiple detection zones.

Stopping within the marked detection area ensures your presence call reaches the controller and helps the signal serve your movement efficiently.

Reliability, failure modes, and maintenance

Detectors are exposed to traffic, weather, and construction, so agencies tune and maintain them to avoid “stuck on” calls or missed detections. When a detector fails, controllers may temporarily switch to fixed timing or set a phase to always serve (“recall”) to keep traffic moving.

Here are common issues and how they’re managed in the field.

• Broken loop wires from pavement cracking or resurfacing can cause permanent calls or no detection; crews test and repair, and timing is adjusted until fixed.
• Mis-aimed or dirty cameras, sun glare, and night headlight bloom can reduce video performance; regular cleaning and reconfiguration mitigate this.
• Construction milling often severs loops; temporary above-ground sensors or fixed-time plans bridge the gap until final paving.
• Snow, ice, or debris can obscure lenses and covers; winter maintenance includes inspections and recalibration.

Many controllers log detector health and performance, helping technicians spot problems quickly and minimize delays or unsafe conditions.

Emergency vehicles and transit priority work differently

Emergency preemption doesn’t rely on “waiting” detection. Fire and EMS often use coded optical emitters, acoustic sensors, or GPS/radio systems to request immediate green. Transit signal priority uses GPS/AVL data, radio, or connected-vehicle messages to extend or advance greens for buses and trams without fully preempting other traffic.

Emerging approaches: connected vehicles and AI

Some signals now communicate with equipped fleets using cellular V2X or dedicated short-range radios to receive priority requests and broadcast signal timing (SPaT) messages. Adoption in everyday passenger cars remains limited, but pilots are expanding. Meanwhile, AI-enhanced video and radar at the edge can better distinguish bikes, pedestrians, and turning queues, optimizing greens while preserving privacy through on-device processing and metadata-only storage.

Why detection matters for timing

Detection feeds the control strategy that determines who gets green and for how long. The more accurately an intersection senses demand, the more efficiently it can reduce delay, fuel use, and emissions.

These are the common signal control modes you’ll encounter.

• Fixed-time: Runs the same cycle regardless of traffic; no vehicle detection.
• Semi-actuated: Main street is green by default; side streets and left turns are served on detection.
• Fully actuated: All approaches are detected; greens vary continuously based on demand and gaps.
• Adaptive/traffic-responsive: Uses real-time detector data across corridors (e.g., SCOOT, SCATS) to adjust cycle length, splits, and offsets dynamically.

Well-placed, well-maintained detectors are the backbone of actuated and adaptive systems, enabling signals to serve real demand rather than a fixed schedule.

Summary

Traffic lights most often sense a waiting car through inductive loops in the pavement, with video, radar, and magnetometer systems increasingly common. These detectors place a call to the controller, which allocates or extends green time based on demand and timing rules. Proper placement, tuning, and maintenance ensure reliable detection for cars, bikes, and buses, while emerging connected-vehicle and AI tools promise even more responsive intersections.

What traffic light tells you to wait?

Yellow Traffic Light
A yellow light means that the green light is about to end, and you must stop unless unable to do so safely. A yellow light also tells you that a red light is coming and gives you time to adjust your movement decisions.

How could a traffic light sense that a car is waiting?

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 traffic lights detect cars?

Traffic lights detect cars using various sensors, most commonly inductive loops embedded in the road, which create an electromagnetic field that a vehicle’s metal body disrupts, or video cameras mounted on poles that use machine vision to identify vehicles. Other methods include radar and infrared sensors, which detect a vehicle’s presence by changes in magnetic fields, reflected waves, or emitted heat. When a sensor detects a vehicle, it sends a signal to a traffic light controller, which then adjusts the signal timing to allow the vehicle to proceed.
 
This video explains how inductive loops, video cameras, and radar are used to detect vehicles: 55sRoad Guy RobYouTube · Jul 27, 2020
Common Detection Methods

• Inductive Loops: Opens in new tabThese are wire coils placed under the pavement that generate an electromagnetic field. When a large metal object, like a car, drives over the loop, it disrupts this field, which the system detects as a vehicle present. 
• Video Detection Systems: Opens in new tabCameras mounted at the intersection monitor specific areas, like the stop bar. Special software uses machine vision to “see” a car in these zones and sends a call to the controller to change the light. 
• Radar and Microwave Sensors: Opens in new tabThese sensors emit microwave or radar waves, which reflect off vehicles. The sensor detects the returning waves or disruptions in its magnetic field to identify a waiting car. 
• Infrared Sensors: Opens in new tabThese sensors emit an invisible beam of infrared light across the road. When a vehicle interrupts this beam, the sensor registers its presence. 

How the System Works

1. 1. Sensing: A vehicle enters the detection zone, activating the specific sensor (loop, camera, radar, or infrared). 
2. 2. Signal Transmission: The activated sensor sends an electrical signal to a traffic signal controller, which is a small computer at the intersection. 
3. 3. Controller Logic: The controller uses the signal to request a green light for the waiting vehicle at the appropriate time in the traffic cycle. 
4. 4. Light Change: Once the controller determines it’s time for the detected vehicle’s phase, it changes the light to green, allowing traffic to move. 

This video shows how a camera system detects vehicles at an intersection: 1mTraffic Light DoctorYouTube · Nov 17, 2024

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.