Triggering Traffic Signals

Traffic Signal Inductive LoopHow about an article about the magnetic circles (or rhomboids) in the roadbed ahead of many traffic lights? Most drivers don't know what they are for and quite often stop way ahead of them.

Once upon a time traffic signals operated on timers and would change according to the clock and not for any other reason. This could not reliably take into account the traffic flow changes that occur at different times of day and under different conditions. Enter the inductive loop controller and now you have a signal that will not change unless traffic is detected that requires it.

The inductive loop is a coil of wire embedded in the pavement at the approach to a traffic signal. An electric current is passed through it creating a magnetic field. When a large object containing iron such as a car or truck is near the loop, the nature of the magnetic field changes and the signal controller can take notice of it. If the vehicle stays at the loop for a set period of time the controller will cycle the signal to give the waiting traffic priority.

Problems occur when the vehicle does not stop over the loop. Too far ahead or too far back and the controller decides nothing is there and does not cycle the signals. Unless the driver realizes and repositions the vehicle over the loop, they may wait a long time for a green signal. So, pay attention to the stop line when you can see it, and make your best estimation when ice and snow covers it up. This will position your properly if you cannot see the tar covered loops in the pavement surface.

Some loops may not recognize motorcycles and bicycles because they don't contain enough iron to disturb the magnetic field sufficiently. In the case of the cyclist, it is easy to make the signals cycle, simply press the pedestrian crossing button if it is present. For the motorcyclist, there are devices that attach to their vehicles that are designed to trigger the loop and cycle the signal.


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