How do Traffic Lights Know When to Change?

*IMPORTANT: Some of the facts shown in this post are only accurate in Missouri and/or the city of Springfield, MO*

Last week I waited on my bicycle as the light before me stayed red though no cars passed before me.  I waited what felt to be an unreasonable amount of time (about 2 0r 3 minutes) before running through the light (its legal).  This lead to an obvious question in my mind; how do traffic lights know when to change?

First I decided to learn about the electric traffic light.  It was invented by William Potts, a police officer in Detroit Michigan.  His light worked with three lights (red, amber, green) and less then 40 dollars worth in wiring.  This device was set up in 1920 at one cross traffic location.  At the end of its first year in use there were 14 more around Detroit.  I am unaware by what system these lights were controlled by, but in the 1970’s traffic lights were controlled by a timing mechanism and heavy taffic flow areas timed traffic lights are still used.

This leads to an obvious question;  if timers are used at some intersections, what device is used at the rest?

The answer can be found in the cracks of the road.  Before you come to a stop at a red light you will drive over large patches of asphalt which show a black square.*  In the grooves of this square is laid a metal wire.  This wire is designed as a metal detector which senses when car drives over it.  If you are on a cycle (of the bi- or motor- variety) then you will probably not have enough metal in your vehicle to set off the sensor and will have to weight the “unreasonable amount” of time until you are allowed to legally go through a red light.

In short, the only way to trip the sensor for a traffic light is to have a large amount of metal on your vehicle and to have gone over to have actually gone over the sensory.

-Warning, about to step on soap box-

Drivers who pull forward in traffic to set off the laser sensor on traffic light when you have waited a while, stop doing that.  The light is either set off by the metal detector (which is behind you and which you have already set off) or a timer system.  Pulling forward in traffic will not help and will make it more likely that you will be in a side collision (even if only slightly)

Cyclist; stop running red lights when vehicles are right next to you and have already set off the sensor.  The light will change and there is no reason to go proceed through the red.

-Steps down from soap box-

So the technology for the traffic lights is relatively simple.  Large amount of metal; change.  Small or no amount of metal; don’t change.  But, even in its simplicity, it is always fascinating to see how our urban world functions.


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*Sometimes the black square will be lightly paved over and while the detector still works, it will not be visible.


One response to “How do Traffic Lights Know When to Change?

  1. Very interesting! I’ve always wondered about that…

    Thank you!

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