Below are some of the
most popular questions asked about traffic signals and signal
Q: What sizes do
traffic signals come in?
A: Typically there are
two. They are measured by their approximate lens diameter...
8" and 12".
Q: How much does a typical traffic
A: Signals are generally made out of two different kinds
of material; cast aluminum, and polycarbonate. Aluminum
signals weigh about 30 lbs. for an 8 inch head and about 50
lbs. for a twelve inch head. Polycarbonate signals weigh quite
a bit less, especially if they are using plastic lenses and
aluminum reflectors (or LEDS!). Typical weights are 15lbs.
for the 8 inch signals and 30lbs. for the 12 inch heads.
Q: What are the measurements of a typical traffic
A: The 8" traffic signals are approx. 30" tall X
9-1/2" wide X 6" deep. The 12" are 42" tall X 13-1/2" wide X 8" deep.
These are approximate measurements of the main body of the signal without
Q: What kinds of lamps do traffic
A: Lamps for American and Canadian
traffic signals are of the medium screw base type. They are
similar in shape and size to a household light bulb, except
that they are clear and have a "U' shaped filament. Most
8" signals use 67W and 69W lamps with 8000 hrs. of life.
12" signals use 150W and 167W lamps that are usually
rated by their lumens.
Q: What are those "shade
things" over the lenses called? What are they for?
A: They are called
"visors" and they help keep the sun from reflecting
light into the lens and reflector and causing "sun
phantom" or false display of an opposing color. There
are three kinds of visors. the "cut away" type is
the oldest, but covers the least amount of lens. The
"tunnel" type is the most common. It looks like an
inverted "U" and covers all but the most bottom of
the lens. This visor is more practical because it is not
"bird nest friendly" and it keeps snow from
accumulating onto the lens. The "full circle" type
is primarily used in California, and is best at keeping out
reflective light. It also is useful in the mounting of
louver-type devices in controlling traffic in a specific
lane. This visor has the drawback of accumulation of snow (or
cups, shoes, and birds) blocking the lens.
Q: What are those black border
things around the signal?
A: Those are called
"back plates". Back plates are used to increase the
target value of the signal, or in other words, make the
signal more visible by blocking out the background around it.
This way when you are approaching the signal, especially at
night, other colored or flashing lights (from buildings or
neon signs) that normally would be a distraction behind the
signal, will be decreased.
Q: What is the typical time that
the yellow light is displayed before the red comes on?
A: The timing of the yellow varies with the particular
jurisdictions philosophy. Many use a formula that figures in
stopping distance and driver reaction times. Other cities use
the prevailing speed limit as their guide (i.e. 35 mph = 3.5
sec, of yellow, 40 mph = 4.0 sec. and so on). The M.U.T.C.D.
states that all yellow times should be at least a minimum of
3 sec. but not greater than
Q: Why were the colors red, yellow,
and green chosen for traffic signals?
A: Basically the colors came from the ones used by the British railway
semaphore signals. The railways would always use red (danger) in the
lanterns to warn of danger ahead. Green was originally white to signal the
"all clear" meaning. However, railroads later discovered that a broken red
lens in a signal could give a false clear indication...with disastrous
consequences. Green then was chosen because of it's cooler contrast to the
other colors. .Yellow came a bit later in
America as a cautionary indication in two light signals by combining the red
and green lamps. From a distance, the mixture of colors produced this new
color. Later, this color was given it's own spot between the red and green.
Oh, and a side note, traffic signals have red on the top...completely
opposite railroad signals, which have green on top! Not sure exactly why
this is, but it was established that way from the birth of the first 3-color
More info can be found