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.



Taking leave of your sensors ...

Some good information here on how pavement sensors - and pedestrian pushbuttons - work to trigger a green light for cars, and/or a 'Walk' symbol for pedestrians. This technology makes our roadways much more efficient, moving all traffic in accordance with demand.

Probably most pedestrians don't think about it much, but 'Walk' / 'Don't Walk' signals weren't ever created to help them cross the road; all they ever needed for this was to be facing a green light. So in fact, they were created to prevent them crossing the road, in order to allow some vehicles to turn whilst the light is still green. Incidentally, if you're wondering whether a given traffic light is controlled on a timer system or a demand system, and haven't definitely identified the loops in the road surface, the presence of pedestrian pushbuttons is the clue.

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.

In fact, traffic engineers will sometimes design sensors (such as for a protected left turn arrow) so that this cycle will only be triggered after two or three cars have driven into the left turn chute (achieved by placing the sensors ten or fifteen meters prior to the stop line, ignoring the initial vehicles that just pass over them - they can wait for the solid green phase to end when oncoming traffic is constant). Plus which, sensors can also detect when traffic has ceased to cross the sensors, thus triggering the next phase of traffic movement at that light.

Whether it's magnetic sensors in the roadway, or pushbuttons, any device like this can become defective over time; and it's not always practicable to send out a repair crew right away. But that big grey box on the corner that controls all this stuff can be opened up and reprogrammed to deliberately create a constant false demand for a light or signal in the interim; this only takes an engineer a few moments, and keeps everything functional, if less efficient. We've seen this in action recently during the Covid19 outbreak in many municipalities, where all of the pedestrian pushbuttons are activated 24/7; it's like there are a bunch of people on every corner, punching those buttons all the time. Typically, they will hang little signs on the posts advising pedestrians not to bother pushing the button activating the 'Walk' signal they want; it's gonna happen as soon as practicable.

Here's a thought, from a guy who has seen all of this technical growth during my driving lifetime (and endured the stupidity of timer controlled traffic lights changing to green for the cross-traffic in the middle of the night, even when there is no cross-traffic). I reckon that with the traffic light technology we use these days, it has become redundant to allow traffic to turn right (or left, into a one-way) on the solid red light. And the world (or at least, our part of it) would be a safer place if they changed the law to reflect modern times.

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