Are red lights timed or sensor-based?

Both. Many signals run on preset timing plans, many use vehicle and pedestrian sensors to adjust in real time, and a growing number do both with adaptive software. What you experience at any given intersection usually depends on location, time of day, and whether detectors see demand on each approach.

How traffic signals decide who goes

Modern traffic signals are governed by controllers that cycle through “phases” (for example, main street through traffic, left turns, then cross street). Controllers can follow fixed schedules, respond to sensor inputs, or continually optimize timing based on live traffic data. Cities often switch strategies by time of day to balance smooth flow on busy corridors with fair service for side streets and pedestrians.

The main control strategies you’ll encounter

The following list outlines the common ways intersections are managed and how that affects your red light experience.

• Fixed-time (pretimed): Signals follow a repeating schedule with set green splits and cycle lengths, regardless of whether cars are present. Common in dense downtown grids where coordination and pedestrian flow are priorities.
• Semi-actuated: The major road gets green by default. Side streets and left-turn lanes are equipped with detectors and receive green only when demand is detected.
• Fully actuated: All approaches are sensor-controlled. Minimum green times can extend if vehicles keep arriving (“gap out”), then switch when demand drops.
• Adaptive/traffic-responsive: The system adjusts cycle length, green splits, and offsets continuously using live data (from loops, radar, cameras, and increasingly probe/connected-vehicle data). Examples include SCATS, SCOOT, InSync, and Surtrac.

In practice, agencies blend these modes—for example, adaptive or actuated operation during off-peak hours and time-of-day coordination during rush hours to maintain “green waves” on major corridors.

What counts as a “sensor” at a signal

“Sensored” signals rely on detectors that confirm the presence or movement of vehicles, bicycles, pedestrians, and priority vehicles. These inputs tell the controller when to serve a phase or extend a green.

• Inductive loop detectors: Wire loops cut into the pavement near the stop line or in turn pockets. They detect metal mass above the loop—still the workhorse in many regions.
• Video analytics cameras: Small cameras on mast arms or poles use computer vision to detect vehicles, bicycles, and pedestrians. They are not the same as red‑light enforcement cameras.
• Radar/microwave/ultrasonic sensors: Boxy or flat-panel units that detect moving and stopped vehicles in all weather, often mounted on the side of the road or mast arms.
• Magnetometers/pucks: Small, round sensors embedded in the pavement that detect changes in the magnetic field caused by vehicles.
• Infrared/LiDAR: Used at some intersections for precise presence and counting, including pedestrian detection in crosswalks.
• Pedestrian push buttons and passive ped detectors: Buttons register a crossing request; some systems use cameras/radar to detect people and adjust walk time automatically.
• Bicycle detection: Marked loop zones or camera/radar settings tuned to recognize bikes; some cities add bike-specific signal heads.
• Emergency vehicle preemption: Systems (e.g., IR, GPS, or radio-based) that grant early greens to ambulances, fire trucks, or police, then manage recovery.
• Transit signal priority (TSP): Extends or advances greens to help buses and streetcars maintain schedules.
• Probe and connected-vehicle data: Aggregated speeds and volumes from smartphones and vehicle fleets increasingly feed adaptive systems and retiming decisions.

Contrary to myth, most modern signals do not use weight sensors. Enforcement cameras, where deployed, are separate systems aimed at the stop bar and are not used for routine detection.

Timing, coordination, and why you still hit red

Even with detectors, timing rules shape what you see. Key parameters include cycle length (total time to serve all phases), green splits (how much of the cycle each movement gets), offsets (to create a progression or “green wave”), and minimums for safety (yellow and all‑red clearances, pedestrian walk/clearance times). During coordinated periods, side streets may have to wait for the next window, even if a sensor sees you.

Common reasons a red seems “stuck”

These are typical factors that make a red feel longer than it is or actually extend it.

• Outside the detection zone: Stopping too far back, between loops, or beyond camera view can fail to register. Motorcycles and bicycles may be harder to detect on some loops.
• Coordinated timing: During rush hours, the main corridor is held green to maintain progression; side streets wait for the coordinated phase.
• Pedestrian timing: A requested crossing adds fixed walk and clearance time, which can be substantial on wide roads or where accessibility settings add extra time.
• Priority operations: Emergency preemption or transit priority can disrupt the normal sequence; controllers then “recover” to the plan.
• Detector failure: If a sensor fails, controllers may default to fixed recall or skip phases until maintenance arrives.
• Special preemption: Rail crossings, drawbridges, or movable barriers can lock signals for safety.
• Work zones or power/communication issues: Temporary configurations or faults can alter timings.

If you repeatedly encounter excessive delays at the same location, it may be a detection or timing issue worth reporting to the responsible transportation agency.

How to tell what your intersection uses

You can often spot the control type by looking for these clues.

1. Pavement clues: Rectangular saw-cut loops near the stop line or in turn lanes indicate presence detection.
2. Hardware on poles: Small cameras aimed at lanes or flat radar panels suggest video/radar detection; round “pucks” in pavement indicate magnetometers.
3. Phase behavior: If the main street stays green until a car arrives on the side street, it’s actuated. If left-turn arrows only appear when cars are present, those lanes are detected.
4. Pedestrian features: Push buttons and countdown timers point to actuated ped phases; longer walk times hint at accessibility timing.
5. Progression feel: A consistent “green wave” at a specific speed suggests coordinated, time-of-day timing.

Don’t confuse enforcement cameras (often aimed at the stop bar, sometimes with visible flashes) with detection devices (usually aimed along the approach or mounted over lanes).

What drivers and cyclists can do

Following these practices can improve your chances of being detected and reduce unnecessary waits.

• Stop on the marks: Position your vehicle over the loop markings; cyclists should align wheels over the loop cut or bike symbol, and may lean the bike to increase metal over the coil.
• Use the button: If walking, press the pedestrian button; it won’t shorten safety times, but it ensures the crossing is served.
• Report issues: If a movement never gets served or detection seems unreliable, note the location, direction, and time, and contact your city’s 311 or transportation department.
• Know local laws: Some jurisdictions allow motorcycles/bicycles to proceed after a full stop and a long wait if the signal fails to detect them—only where explicitly legal and safe.

Small changes in positioning and reporting can help agencies keep signals responsive and safe for everyone.

Trends in 2024–2025

Agencies are expanding adaptive control, using AI-based video detection to better recognize bicycles and pedestrians, and leveraging anonymized probe data from vehicles and phones to retime corridors faster. Connected-vehicle pilots are broadcasting SPaT/MAP messages to enable in-car “green light optimal speed advisories.” Many cities are also adding leading pedestrian intervals, transit priority, and automated recovery after preemption to improve safety and reliability.

Bottom line

Red lights can be timed, sensor-controlled, or both. Most modern intersections combine detectors with time-of-day coordination, adjusting to demand while maintaining progression on busy roads. What you experience depends on the intersection’s design, the time of day, and whether the system detects you.

Summary

Traffic signals use a mix of fixed timing, actuated detection, and adaptive optimization. Sensors include loops, cameras, radar, magnetometers, and pedestrian/bicycle inputs, with special systems for emergency and transit vehicles. Coordination, safety minimums, and priority operations can make reds feel long even when sensors are present. Recognizing hardware cues and using best practices can improve detection, while emerging technologies continue to make signals more responsive.