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auto.howstuffworks.com/car-driving-safety/safety-regulatory-devices/how-does-a...

­There is something exotic about the traffic lights that "know" you are there -- the instant you pull up, they change! How do they detect your presence? Some lights don't have any sort of detectors. For example, in a large city, the traffic lights may simply operate on timers -- no matter what time of day it is, there is going to be a lot of ...

www.wikihow.com/Trigger-Green-Traffic-Lights

Once a vehicle is detected by the loops, the traffic light system is signaled that there is someone waiting to proceed. The lights for the cross traffic will then begin to change after a safe time period before the light turns green for you. Check for signs of loop detectors in the pavement just before the stop line and crosswalk markings.

www.wonderopolis.org/wonder/how-does-a-traffic-light-work

How does a traffic light work? Do all traffic signals work in the same way? ... (large metal objects!) are stopped above them. So how do sensor-based systems maximize traffic efficiency? In a rural area, for example, the main direction of travel can remain on green to allow the majority of vehicles to pass through quickly. ... Inductive loops ...

en.wikipedia.org/wiki/Traffic_light_control_and_coordination

But the recent introduction of the flashing yellow arrow (see Traffic-light signalling and operation) makes the lead-lag signal, an aid to progression, available with protected/permissive turns. In modern coordinated signal systems, it is possible for drivers to travel long distances without encountering a red light.

www.automatesystems.co.uk/how-traffic-light-sensors-work

How Do Traffic Light Sensors Work? Every traffic light signal has either a timer or sensor which helps it direct traffic flow. In large cities where vehicles cross road intersections across the clock, traffic is usually dictated by traffic lights that use timers.

www.tech-faq.com/how-do-traffic-lights-work.html

Red color indicates to a driver to stop immediately before the crossing line and not drive ahead. A traffic signal is most commonly a timer-based system. The Green, Yellow and Red lights go On and Off at regular and synchronized intervals to start and stop the flow of traffic. The interval to change lights may range between 30 seconds to 120 ...

www.quora.com/How-does-the-light-sensor-works

Light dependent Resistor (LDR) Sensor or Infrared Sensor (IR Sensor) are beautiful example of Light sensor which works on principle of light and has transmitter and receiver installed on same module. Infrared rays are emitted by IR sensor and received by photo diode installed on module.

auto.howstuffworks.com/car-driving-safety/safety-regulatory-devices/red-light...

Red-light systems typically have two induction-loop triggers positioned under the road near the stop line (more on this later). The computer is the brains behind the operation. It is wired to the cameras, the triggers and the traffic-light circuit itself. The computer constantly monitors the traffic signal and the triggers.