Are There Weight Sensors at Traffic Lights?

No—most traffic lights do not use weight sensors. Instead, they detect vehicles with technologies like inductive loop detectors (which sense metal), video analytics, radar, and in-pavement magnetometers. While pressure-based or “weigh-in-motion” sensors exist for traffic studies and truck enforcement, they are rarely used to control everyday signal timing.

How Traffic Lights Actually Detect Vehicles

Modern traffic signals are either fixed-time (running a set cycle regardless of traffic) or actuated (changing timing based on detected demand). At actuated intersections, detectors tell the signal when a vehicle is present so green time can be given or extended. The most common method isn’t weight; it’s electromagnetic or optical detection of presence and movement.

Here are the main detection technologies engineers deploy today, and what they do best:

• Inductive loop detectors: Wire loops cut into the pavement sense changes in inductance caused by a vehicle’s metal mass. They’re the workhorse of U.S. traffic detection, especially at stop bars and turn lanes.
• Video detection with computer vision: Cameras mounted on poles classify and track vehicles, bikes, and pedestrians. Newer systems use AI to improve detection in complex scenes and provide richer data for signal timing.
• Radar/microwave sensors: Overhead or roadside units detect moving and stopped vehicles in most weather conditions, useful for multi-lane approaches and higher-speed roads.
• Magnetometers/magneto-resistive sensors: Small, in-pavement or wireless “puck” sensors (often battery-powered) detect disturbances in Earth’s magnetic field caused by vehicles. Common where cutting loops is difficult.
• Infrared and ultrasonic sensors: Used in some deployments for presence and counting, though less common than video or radar for stop-bar detection.

Together, these tools allow signals to respond to real-time demand, extend greens when queues persist, and skip phases when no vehicles are present, improving flow and reducing delay.

Why Not Weight Sensors?

Weight-based systems are poorly suited to daily signal control. Pavement-embedded pressure plates are hard to maintain, sensitive to weather and wear, and can miss lighter vehicles while being overbuilt for routine detection. Weigh-in-motion (WIM) sensors—such as piezoelectric strips used to estimate axle loads—are designed for data gathering and enforcement, not reliably actuating a green light. By contrast, inductive loops, radar, and video provide vehicle “presence” quickly and at lower lifecycle cost.

Common Exceptions and Misconceptions

Several situations can make it seem like signals are triggered by weight, but they aren’t.

Consider these frequently confused cases:

• Pneumatic road tubes: Those rubber hoses across lanes record counts and speeds for temporary surveys. They don’t control the signal.
• Weigh-in-motion sites: Installed near freight corridors or enforcement zones to estimate truck weights on the fly; generally independent of signal actuation.
• Fixed-time signals: Especially downtown or in coordinated corridors, signals run on schedules, so stopping “triggers” nothing—they were going to change anyway.
• Emergency preemption: Optic, acoustic, or radio/GPS systems grant green to responding emergency vehicles—again, not weight-based.

These systems may operate near traffic lights and affect timing indirectly, but they do not rely on vehicle weight to detect demand.

Advice for Cyclists and Motorcyclists

Lighter vehicles sometimes struggle to be detected, especially by poorly tuned or aging inductive loops. Placement and approach can help.

Use these tips to improve your chances of being detected:

• Stop over the loop’s saw-cut lines: Look for rectangular or circular cuts in the pavement. Position the bike frame over the edge of the loop (where the field is strongest).
• Choose steel over aluminum/carbon when possible: Steel frames and wheels create a stronger inductive signature than carbon fiber and some alloys.
• Find the bicycle detector marking: Some cities stencil a bike symbol where detection works best.
• Use push buttons when available: Many intersections provide pedestrian or bike call buttons that place a reliable signal request.
• Know the law: In parts of the U.S., “dead red” or malfunction laws allow motorcyclists and cyclists to proceed after a full stop when a signal fails to detect them, but only where legal and when safe.

If detection still fails, report the location to your city’s traffic operations; agencies can boost sensitivity or adjust detection zones for micromobility.

How Signals Decide to Change

At actuated intersections, detectors call phases and extend green time when vehicles keep arriving. Key parameters include minimum green (the shortest green after a valid call), passage time (how long to wait for the next vehicle before ending green), and max green (the cap to maintain coordination and fairness). Left-turn lanes often have dedicated detectors to grant protected turns only when needed. Pedestrian push buttons register crossing demand and can add WALK intervals into the cycle.

What’s New in 2024–2025

Agencies are accelerating the use of AI-enabled video and radar for multimodal detection—counting and prioritizing buses, bikes, and pedestrians—while improving privacy with on-device processing. Wireless in-pavement sensors are expanding where trenching loops is costly. Early deployments of vehicle-to-everything (V2X) communications support transit signal priority and safety messages, but widespread signal actuation via connected vehicles is still emerging. None of these trends point toward weight-based actuation for general traffic; presence and classification sensors remain the standard.

Key Takeaways

The fundamentals of traffic detection are often misunderstood. Here are the essentials to remember:

• Traffic lights typically use inductive loops, video, radar, or magnetometers—not weight—to detect vehicles.
• Weight sensors are used for data and enforcement (like truck weighing), not routine signal control.
• Some signals are fixed-time and don’t detect vehicles at all.
• Cyclists and motorcyclists can improve detection by positioning over loop edges or using provided buttons.
• New tech emphasizes AI video, radar, and connected systems, but still focuses on presence, not weight.

Understanding what’s under the pavement—and above it—makes it easier to navigate signals and know what to expect when you pull up to the stop line.

Summary

Traffic lights almost never rely on weight sensors. Instead, they detect vehicle presence with inductive loops, video analytics, radar, and magnetometers, or they run fixed-time schedules. Weight-based systems are reserved for temporary traffic counts and truck weighing—not for changing a light from red to green.

Are there sensors in 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. Wire coils: Inductive loops, which are essentially wire coils, are embedded under the pavement at varying distances from the stop line. 
2. Magnetic field: An electrical current runs through these coils, creating a magnetic field. 
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. Signal to the controller: This disruption is detected by the traffic signal controller, which acts as the “brain” of the system. 
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. 

Does every traffic light have a sensor?

No, not all traffic lights have sensors; some operate on a fixed-time schedule, while others use detectors like inductive loops, infrared sensors, or microwave radar to sense the presence of vehicles. The use of sensors versus timers often depends on the location, with fixed-time systems being more common in busy cities and sensor-based systems preferred for managing inconsistent traffic in suburbs and on rural roads.
 
Types of Traffic Light Systems

• Fixed-Time Traffic Lights: Opens in new tabThese lights follow a predetermined schedule, changing at set intervals regardless of vehicle presence. They are often used in areas with high, consistent traffic volumes, such as major urban intersections. 
• Sensor-Activated Traffic Lights (Actuated Traffic Lights): Opens in new tabThese systems use various sensors to detect vehicles and pedestrians and adjust the light cycle accordingly. 

Common Sensor Types

• Inductive Loops: Opens in new tabBuried under the road surface, these loops create an electromagnetic field that is disrupted by the metal of a passing vehicle, signaling its presence to the controller. 
• Infrared Sensors: Opens in new tabThese sensors can detect heat and are often used to trigger changes, sometimes even for detecting emergency vehicles. 
• Microwave Radar: Opens in new tabThese sensors can efficiently detect both stationary and moving vehicles and are common in suburban areas. 
• Video Analytics & LiDAR: Opens in new tabEmerging technologies that use cameras and laser sensors to analyze traffic flow and presence. 

Why the Difference? 

• Traffic Volume and Inconsistency: Fixed-time systems work well where traffic is predictable, but sensors are better for managing fluctuating traffic patterns.
• Cost and Efficiency: For areas with less traffic, sensors offer a more efficient and cost-effective way to manage the light cycle compared to a constant timer.

Are there weight sensors in roads?

Weigh-in-Motion (WIM) systems are an array of sensors used to measure various features of vehicles in motion. WIM systems are comprised of not only the electronics and sensors but include just as importantly flat, smooth, and straight roadways.

Are there weight plates under traffic lights?

There aren’t “pressure plates” as such, but magnetic loop sensor wires embedded in the concrete. You can see them as a square line where the wire is embedded. It senses large metal objects such as a motor vehicle.