The first electric signal in the United States was installed
in Cleveland, Ohio in 1914. From these modest beginnings,
traffic signal technology has greatly expanded and has become
a critical element in the safe and efficient control of traffic
on our streets and highways. Traffic signals are used to assign
vehicular or pedestrian right of way. By providing alternate
right of way traffic signals exert a profound influence on
traffic flow and can operate to the advantage or disadvantage
of the vehicles or pedestrians they control.
A careful analysis of traffic operations and other factors
at a large number of signalized and unsignalized intersections,
coupled with the judgment of experienced engineers, have provided
a series of warrants that define the minimum conditions under
which signal installations may be justified.
Traffic signals can not be installed unless one of the warrants
specified by the Manual on Uniform Traffic Control Devices
(MUTCD) has been satisfied. The MUTCD is a document that is
published by the Federal Highway Administration and has been
adopted by the Kansas Department of Transportation. These
warrants are based on a number of factors including: the number
of vehicles, pedestrian activity, vehicular speeds, accident
history, population of the city, and number of traffic lanes.
The satisfaction of a warrant or warrants is not in itself
justification for a signal. A traffic engineering study must
be conducted to determine if the traffic signal should be
ADVANTAGES OF SIGNALS
Traffic signals are valuable devices for the control of vehicle
and pedestrian traffic. Warranted traffic signals, properly
located and operated, usually have one or more of the following
- They can provide for the orderly movement of traffic.
- Where proper physical layouts and control measures are
used, they can increase the traffic-handling capacity of
- Under favorable conditions, they can be coordinated to
provide for continuous or nearly continuous movement of
traffic at a definite speed along a given route.
- They can be used to interrupt heavy traffic to permit
other traffic, vehicular or pedestrian, to cross.
DISADVANTAGES OF SIGNALS
It is unfortunate that traffic signals have become regarded
by the public as a panacea or "cure-all" for any and all traffic
problems at intersections. The following factors can result
from an improper or unwarranted signal installation:
- Excessive delay may be caused. Even the best designed
and operated signals usually increase delay when compared
to unsignalized intersections. However, unnecessary delay
is a common feature of an unwarranted or an improperly designed
traffic signal. This unnecessary delay results in significant
fuel waste and higher motorist costs.
- Disobedience of the signal indications is encouraged.
Delay at unwarranted or poorly designed traffic signals
can breed gross disrespect toward signals as well as other
traffic control devices.
- The use of less adequate routes may be encouraged in
an attempt to avoid such signals.
- Accident frequency can be significantly increased at
unwarranted signals or at locations where installation was
not based on sound engineering analysis. Accidents related
to signal control usually develop during periods of comparatively
low volume and result from rear-end collisions, and drivers
either willfully or unintentionally running the red light.
COST OF SIGNALS
Traffic signals are much more costly than is commonly realized,
even though they represent a sound public investment when
justified. A modern signal can cost taxpayers between $50,000
and $100,000 to install depending on the complexity of the
intersection and the characteristics of the traffic using
On top of this, there is a perpetual cost which is almost
never considered - the cost of the electrical power consumed
in operation a signalized intersection 24 hours a day, and
the associated maintenance costs. These costs can be $1,000
to $2,000 a year.
LEFT TURN SIGNALS
There are three types of left turn phasing: permissive, protected/permissive,
and protected only.
In permissive left turn phasing, vehicles are required to
wait for an adequate gap in the opposing vehicular and pedestrian
traffic prior to making their turn.
With protected/permissive phasing the vehicles
are given a protected phase (green arrow) in which they may
turn and opposing vehicles are controlled by stop (red) indications.
This is followed by a permissive (green ball) phase. As with
the permissive left turn phasing above, drivers must choose
an adequate gap in opposing traffic. This type of control
is commonly signed "LEFT TURN YIELD ON GREEN (GREEN BALL)".
When an approach is controlled by protected
only phasing, drivers may turn only when they receive a green
arrow. This type of control is commonly signed "LEFT TURN
FLASHING TRAFFIC SIGNALS
Traffic signals may operate in a flashing operation during
emergencies, night time/low traffic volume periods, and special
events. Flashing signal indications have the following meaning:
When a red flashing lens is illuminated, drivers shall
stop at a clearly marked stop line, but if none, at the
point nearest the intersecting roadway where the driver
has a view of approaching traffic on the intersecting roadway
before entering the intersection, and the right to proceed
shall be subject to the rules applicable after making a
stop at a STOP sign.
When a yellow flashing lens is illuminated, drivers of
vehicles may proceed through the intersection or past such
signal only with caution.
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