- The most visible components of
a traffic signal installation are the various
vehicular and pedestrian signal heads and their
supporting structures. Most members of the motoring
public are unaware of the complexity of controller,
detection, and systems elements of a signal
installation, but are familiar with the appearance of
the signals themselves.
Although the three-color vehicular
signal face has been around for more than 50 years without
significant changes, new materials and manufacturing
techniques have made them easier to maintain and install.
Pedestrian signals have undergone several design changes
since there initial use, including the colors and design of
their indications. Mounting locations and techniques have
also changed over the years, although with substantial
variations by geographical area.
Design requirements for
vehicular traffic signal heads are mandated in the MUTCD
(Manual of Uniform Traffic Control Devices) and are also
addressed in standards published by ITE (Institute of
Signal head housings
can be constructed from either cast aluminum or plastic
(typically polycarbonate) materials. Each alternative
provides it's own set of advantages and disadvantages.
Aluminum signals must
be painted, both at the original time of manufacture and at
regular intervals to maintain an attractive appearance.
Plastic signal heads have the color molded into the plastic
material, and do not need to be painted during the life of
Plastic signals are
lighter than aluminum signals, an advantage when it becomes
necessary to add signal heads to an existing support
structure. However, being lighter, plastic heads will swing
in the wind more when used in a free-swinging overhead
typically don't have the strength of aluminum signals, and
have shown a tendency to break when used in either a top or
bottom-mounted rigid installation. An example is that some
states that use the span wire installation require that the
top section of the signal, where stresses are higher, be
aluminum, although lower sections may be plastic.
Visors are typically
made of the same base material as the housing. Yellow plastic
visors are usually coated with an opaque material on the
inner surface to eliminate light transmittance through the
Traffic signal lenses
are designed to add color and direct light to the approaching
motorist. The outer surface of the lens has a convex shape
and is smooth to avoid the collection of dirt. The inner
surface of the lens is concave and is patterned to best
redirect the light from the lamp and reflector to the roadway
approach. The lens is designed to be oriented in relation to
the roadway, and is usually embossed with the word
"TOP" to indicate the proper orientation.
Lenses can be made from
either glass or plastic material. As was the case with signal
housings, each material has it's own set of advantages and
disadvantages. Glass lenses are heavier than plastic lenses,
a drawback if weight of the signal head is a concern. Glass
lenses will break if hit by a stone or a bullet, whereas
plastic lenses will usually remain intact after all but the
most severe impacts.
Although both materials
can meet or exceed the specifications for light transmission,
many engineers feel that the glass lens provides a better
light transmission, and thus a better indication. Some
plastic lenses tend to discolor, and even melt, when larger
wattage bulbs are used in indications that are illuminated
for long periods of time; a prime example is the arterial
green indication at very low-volume semi-actuated
Arrow lenses can either
have the arrow pattern embossed, with uniform prismatic
diffusion surface and opaque masking on the inner surface of
the lens, or masked, with an opaque coating, on the outer
surface of a standard lens. This second option requires care
in selecting the proper orientation of the lens, because an
upside-down lens will provide significantly less intensity. A
third option for providing an arrow indication is the use of
a metal mask, the same size as the lens and with a cutout
arrow symbol. The mask is placed in front of a circular
indication lens, and can be rotated in any direction without
concern about the intensity of the indication.
Signal lamps for
traffic signal applications are specifically designed to
provide a long life in harsh environmental conditions,
including the vibrations from wind or passing heavy vehicles.
In the standards for these lamps, there are four terms that
one should be familiar with:
Length- The dimension from the center of the filament
to the tip of the base of the lamp. An improper light
center length will result in the uneven illumination
of the signal lens.
"Initial" Lumens- The average amount of
light produced by a sample of lamps operating at
rated voltage, after having been seasoned to 1/2 to 1
percent of rated life.
Rated Life- The
average of burning hours for a sample number of lamps
operated at rated voltage and defined operating
conditions. All traffic signal lamps should be
designed to provide a minimum rated life of 6000
The design operating voltage of the lamp. Operating a
lamp at higher than rated voltage will provide higher
light output, with reduced life; operating a lamp at
less than the rated voltage will extend life at the
expense of light output.
Most filaments for traffic signal lamps have
a "U" or "W" design; to maximize the life
of the bulb, the open side of the filament should point up,
so that one section of the filament is not heated by the
section below it.
The reflector directs
the light output from the lamp forward through the signal
lens. The reflector has a parabolic shape, and is designed
for the lamp filament to be located at the focal point of the
parabola- thus the need for a specific light center length of
the lamp. The reflector, working with the visor and lens, is
designed to minimize the "sun phantom" effect.
available in three materials: mirrored glass, specular
anodized aluminum, and metalized plastic. Mirrored glass
usually gives a more intense indication, but is subject to
breakage by impact and damage by water intrusion into the
head. The aluminum reflector is lighter in weight and will
not break, although corrosion is occasionally a problem,
particularly in salt spray areas. The plastic reflector has
not seen significant usage in vehicular signals, although it
has been used in some pedestrian signals.
Lamp sockets must be
made of a material capable of resisting the heat generated by
the lamp, and are designed to rotate to permit the proper
orientation of the lamp filament / focal point relationship
for maximum illumination of the lens.
(PV,or optically programmed) signals are designed to direct
their light output to user-specified sections of the roadway
approach. Typical installations include adjacent-lane
programming, in which an indication for one lane is to be
veiled from an adjacent lane; skewed-intersection
programming, in which geometric conditions would allow
motorists on one approach to see indications for the other
approach, if standard signals were used; and closely spaced
intersections, in which a far-side indication conflicts with
a near-side indication.
Typical front and side view of PV signal
Each section of the PV
signal has a flat front lens with an internal fresnel lens
pattern, two convex lenses (one clear and one frosted)
sandwiched together at the rear of the signal, followed by a
sealed beam mounted in a rear access door. Opening the door
and removing the frosted lens permits the user to view the
roadway approach, and, using a special foil-based adhesive
tape, mask out the areas of the roadway on which motorists
should not be able to see that indication. Of course, these
signals require a rigid mounting to be the most effective.
Signals for pedestrians
include a number of different legends, housings, and
illumination techniques. Two sets of legends are defined in
the MUTCD- word legends, with an orange DONT WALK and a white
WALK; and symbols, with an orange raised hand and a white
walking man. The symbols, which were first approved in 1977,
are the most popular because of their better legibility and
recognition by non-English speaking pedestrians. Two primary
types of signal housings are available, one uses a separate
section for each indication; the other combines both
indications in a single housing. The first type uses
incandescent lamps and is virtually identical to the vehicular
signal. It comes in two standard sizes, 9 inch and 12 inch.
The smaller 9 inch unit is limited to crosswalks less than
60ft in length.
The single housing
design has been adapted for incandescent, neon, fiber optic,
and LED illumination techniques. Incandescent word legend
designs use a three-line legend lens (orange DONT/ orange
WALK/ white WALK), two lamps, and a compartmentalized
reflector to illuminate the appropriate displays. Neon, LED,
and fiber optic word legend designs use a clear two-line
(DONT/ WALK) lens, with the light source providing the color
to the appropriate displays. Symbol legends can be
illuminated by any of the four techniques.
Fiber optic, LED, and
neon displays can provide a substantial energy saving over
the incandescent displays; they also reduce the potential for
the "sun phantom" effect. However, the incandescent
fixture, being electrically simpler, may be easier and less
expensive to maintain. A louvered "egg crate" visor
can also help to reduce the "sun phantom", and
eliminates the need for a longer standard visor. However,
egg crate visors are subject to accumulations of snow and ice
that can block the indications.
Audible signals for the
visually impaired have been used for a number of years with
limited success. Most of these devices consist of an
electronic "bird in a box" with
"chirping" sounds for one direction of travel and
"coo-coo" sounds for the other direction.