Did you ever wonder why our street stoplights designate red as “stop” and green as “go”?
Well, in the 1840s the
British railroads adopted a flag, lamp and semaphore signal system where red
meant danger, white meant safety and green indicated proceed with
caution. They took their inspiration from early industrialization
where factory machines used red to indicate when equipment was off and green
when turned on. But one time the red
glass lens on a signal lamp dropped out of its socket, showing a white light,
which then caused a rail collision.
Traditionally red has
evoked danger and green, a more calming influence. But it was
optical science that reinforced the choice.
Red has the longest wavelength in the visible spectrum and is less
likely to be interfered with by other light sources in what’s known as “light scattering”. Think of fog or dust in the air. Red light
penetrates best.
By the 1860s traffic
conditions in London prompted officials to seek a way of controlling
horse-drawn carriages with a signal system and opted for the railroad scheme of
color-coded semaphores and lights controlled by a policeman, often perched on a
raised kiosk in the middle of the intersection.
You can credit American
police officer William Potts for the invention of the first traffic lights in
Detroit in 1920. But back then they were still sequenced by an
officer, making traffic control expensive. Eventually, a timer
system was introduced to sequence the flow. But there was also a system activated by
sound.
A microphone was
installed on the light pole and when a car approached it would honk its horn
and the light would turn green… but just for ten seconds to allow that one car
to get through. You can imagine the problem that was going to
create.
Today we use not only
timers but some sophisticated measuring devices to sequence traffic lights,
including inductive loops. You’ve probably seen signs of them,
buried in the pavement, as you pull up to an intersection,. They
measure the metal in cars as they drive over them, allowing the system to know
that a car is there waiting for a green signal.
Even the traffic lights
themselves have improved. They now measure either eight or twelve
inches in diameter and must be visible in every lighting
condition. The older incandescent bulbs that illuminated them used
to burn at 175 watts and needed constant replacement. Now they’re
being replaced with high endurance LED lamps which give as much light but only
require 10 – 25 watts of electricity.
To help the 13 million Americans who are color blind, stoplights are always arranged with red on top and green
on the bottom.
Given the sophisticated
technology and engineering time spent on designing a stoplight system for an
intersection, they’re not cheap. A fully equipped setup can cost
between $250,000 and a half-million dollars with an annual maintenance cost of $8000. That’s why towns and CDOT are so reluctant to
add new lights, despite requests.