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Drivesmart column: Triggering traffic signals

Problems occur when the vehicle does not stop over the loop
Tim Schewe

By Tim Schewe

“How about an article about the detection loops in the pavement at many traffic lights? Most drivers don’t know what they are for and quite often stop too far ahead or behind 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. Advancements in technology have improved this, first with the inductive loop and now with the video camera. Each system has its use.

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 you 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.

Cyclists can make the traffic lights cycle by pushing the pedestrian signal button if one is present. For the motorcyclist, there are devices that attach to their vehicles that are designed to trigger the loop and cycle the signal.

Traffic signals may be controlled by video cameras instead of inductive loops. The cameras are able to accurately detect the presence of motorcycles, cyclists and pedestrians and change the traffic lights appropriately.

Tim Schewe is a retired constable with many years of traffic law enforcement. To comment or learn more, please visit