What are the small sensors on top of traffic lights?

They’re most often vehicle-detection devices or emergency-vehicle preemption receivers that help the signal know when to change, not speed or red‑light ticket cameras. In practice, agencies mount compact cameras, radar, infrared or radio receivers above or beside the signal to detect traffic, give priority to fire trucks and buses, and optimize timing.

What those little boxes and domes actually do

Modern traffic signals are increasingly “actuated,” meaning they respond to real-time conditions instead of running a fixed schedule. The small sensors you see—usually a camera-like box, a small dome, or a narrow rectangular unit perched on the mast arm or atop the signal head—enable the controller to detect vehicles, bikes, pedestrians, and emergency vehicles and to communicate with nearby equipment. They improve safety, reduce delays and idling, and can support data-driven traffic management.

Common types you’ll see at intersections

Video detection cameras

These are compact cameras aimed at the approach lanes. They don’t record for enforcement; they process images to determine presence, queues, and sometimes classify vehicles or detect bicycles. If you see a small camera box pointing at the stop bar or lanes with no bulky strobe unit nearby, it’s likely a detection camera used to trigger or extend a green phase.

Microwave radar detectors

Often a slim rectangular box or small round/oval unit angled toward approaching traffic, radar sensors (Doppler or FMCW) measure presence, speed, and distance. They’re used for dilemma-zone protection, advance detection, and adaptive timing—especially where pavement loops aren’t feasible.

Infrared or lidar presence sensors

Short-range infrared or lidar units appear as small, lens-like pods. They detect vehicles, bikes, or pedestrians at the stop line or in a crosswalk, and are increasingly used where accuracy in all weather is important.

Emergency vehicle preemption (EVP) detectors

Typically a small cylindrical or box-shaped receiver mounted near the top of the signal or on the mast arm and aimed down the approach. Legacy systems (e.g., Opticom) detect coded infrared strobes from fire/EMS vehicles; newer versions use GPS/radio. When an equipped emergency vehicle approaches, the system requests a green in that direction. Many intersections also have a tiny white “confirmation” lamp visible to responders; that’s a light, not a camera.

Transit signal priority (TSP) receivers

Similar in appearance to EVP gear, these devices listen for requests from buses or streetcars to slightly extend or shorten phases, reducing transit delay without fully preempting the signal like emergency vehicles do.

Thermal imaging for bikes and pedestrians

Small thermal sensors (often short cylindrical housings) can detect people in crosswalks and cyclists in bike lanes, improving detection at night or in rain and snow and helping the signal hold the walk phase when someone is still crossing.

Connected-vehicle roadside units (V2X)

Boxy modules with one or more short antennas may broadcast SPaT/MAP messages (signal phase and timing) to compatible vehicles or work with pilot safety apps. In the U.S., agencies have been rolling out C‑V2X units on a corridor basis since the FCC authorized broader use of 5.9 GHz C‑V2X in recent years.

Communications antennas and radios

You might see small puck or whip antennas used for interconnecting signals over radio or cellular, letting the traffic management center monitor status, push timing updates, or coordinate corridors without trenching fiber.

How to tell them apart at the intersection

The shape, placement, and any companion hardware offer strong clues about what a device is—and what it is not. Use the quick cues below to identify common equipment you’ll see in the field.

• Vehicle detection camera: Small camera housing pointed at lanes or the stop bar; no big flash; often one per approach or lane group.
• Radar detector: Slim rectangular or oval unit angled toward incoming traffic, sometimes mounted higher or farther upstream.
• Emergency preemption receiver: Small box or tube facing down the approach, often near the signal head; may coincide with a tiny white confirmation light facing the approach.
• Red‑light enforcement camera: Separate, larger box on its own pole or cabinet with visible flash/IR illuminator; usually accompanied by warning signs as required by law.
• Blue “tattletale” light (some U.S. states): A small blue lamp on the signal pole that lights when the signal is red, aiding police observation; it’s a light, not a sensor.
• V2X/communications gear: Small radio box with short antennas; may be mounted higher on the pole and not specifically aimed at the lanes.
• Pavement loops (for context): Not visible above ground; look for sealed saw-cut rectangles in the asphalt near the stop line or upstream.

If you don’t see a dedicated flash unit or signage, the device is almost certainly for detection, priority, or communications—not for issuing tickets.

What they are not

Because many devices look like cameras, they’re often mistaken for enforcement systems. The distinctions below help dispel common myths.

• Not speed cameras by default: Those are typically separate installations with conspicuous housings and warning signage.
• Not general CCTV surveillance: City CCTV is usually mounted much higher on dedicated poles with pan‑tilt‑zoom domes.
• Not streetlight photocells: Those are small twist-lock modules on top of streetlight fixtures, not on traffic signals.
• Not 5G small cells (usually): Cellular small cells have larger radio/antenna assemblies and power equipment; they’re often on utility poles or dedicated mounts.

In short, the compact devices attached to signal hardware are operational tools for managing traffic, not covert enforcement or constant video surveillance.

Why transportation agencies use them

Above-ground sensors let agencies actuate signals without cutting the pavement, adapt timing to real-time demand, improve safety in dilemma zones, and prioritize emergency and transit vehicles. They also reduce fuel-wasting idling and can provide anonymous performance data to tune corridors. Newer deployments support connected-vehicle safety messages to prepare for future in-vehicle alerts.

Regional notes

Implementations vary by country. In the U.S. and Canada, video, radar, and Opticom-style EVP are common; some states add blue confirmation lights. The U.K. and parts of Europe often use compact AGD radar or thermal detectors with SCOOT/MOVA control, alongside loops. Australia and New Zealand deploy similar above-ground detectors within SCATS systems. Regardless of region, enforcement cameras are generally larger, separately mounted, and signed.

Summary

Those small boxes and domes on or above traffic lights are mostly sensors and receivers that detect vehicles, bikes, and pedestrians, or that grant priority to emergency and transit vehicles. They help signals run smarter and safer. Unless you see a separate, larger camera with a flash and posted warnings, they aren’t red‑light or speed enforcement devices.

What are the sensors on traffic lights?

Traffic light sensors are devices integrated into traffic signal systems that detect the presence, speed, and type of vehicles and pedestrians at intersections. Their primary function is to provide real-time data that allows traffic controllers to adjust signal timings dynamically, ensuring optimal traffic flow.

What are the little black cameras on top of traffic lights?

Traffic light cameras help keep our roads safe. They’re usually found in high-risk areas, such as busy intersections or crossings, where there’s a higher risk of collisions. You’ll also see smaller safety cameras on top of traffic lights. These cameras observe road users, helping to predict and control traffic jams.

What are the little things on top of traffic lights?

Things on top of traffic lights include sensors for traffic management, red-light cameras, and emergency vehicle preemption equipment, which may look like boxes, cylinders, or antennas. These devices detect vehicles and pedestrians, monitor traffic flow to optimize light timing, provide a blue light to confirm violations, and give emergency responders a way to change the light to green.
 
Sensors and Cameras

• Vehicle/Pedestrian Detectors: Opens in new tabSensors such as radar, microwave, or infrared devices are used to detect the presence of vehicles and pedestrians, allowing the traffic light to adjust the timing of green and red lights accordingly. 
• Red-Light Cameras: Opens in new tabSome small cameras, sometimes white or black, are installed to detect and record red-light runners, helping to enforce traffic laws. 

Emergency Vehicle Systems 

• Preemption Devices: These systems use sensors (like infrared or optocom units) to detect emergency vehicles and trigger the traffic signal to turn green in their direction.

Confirmation and Data Collection 

• Blue Confirmation Light: Opens in new tabA small, blue light is sometimes mounted on or near the traffic signal. When the light turns red, the blue light illuminates, allowing law enforcement to confirm red-light running violations. 
• Traffic Management Systems: Opens in new tabOther devices may collect data to monitor traffic flow, which is then used to improve traffic management strategies. 

What are the little cameras on top of lights?

Fixed Speed Cameras on Top of Street Light Poles
Fixed and mobile speeding cameras are positioned in strategic locations, such as major freeways, busy intersections and areas with high accident rates, to monitor vehicle speeds effectively.