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How Traffic Light Sensors Work

Updated April 17, 2017

Traffic signals are used to control the flow of vehicles through an intersection, which can have devices that detect the presence of vehicles in a traffic lane. Detection increases the efficiency of traffic signal operations. Three common technologies for detecting vehicles are loop detectors, video detection and radar detection.

Loop Detectors

Loop detectors are strands of wire embedded into the pavement in a rectangular or round loop shape of standardised dimensions. When energised, the loop creates a magnetic field. When a vehicle passes over the loop, the frequency of the magnetic field changes. A device in the traffic signal controller cabinet detects this change in frequency and signals the traffic signal controller to provide that vehicle with a green indication during the traffic signal cycle.

Video Detection

Video detection uses cameras mounted on poles over the travel lanes. Machine vision technology analyses the video images and sends an electronic signal to the traffic signal controller when a defined change in the imagery occurs.

Radar Detection

Radar detection uses microwave radar sensors mounted over the travel lanes. Energy is sent from the radar unit to the traffic lane and the reflected energy is measured by a sensor. A defined change in the reflected energy is used to signal the controller to serve that vehicle.

Selection Considerations

Public agencies consider a range of factors in selecting the most appropriate vehicle detection technology for a given location, including initial cost, accuracy, reliability and ongoing maintenance requirements.

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