TRAFFIC SIGNALS
A traffic signal is an electrically powered traffic control device
by which traffic is warned or directed to take some specific action.
Under favorable conditions, traffic signals can be coordinated to
ensure a continuous or nearly continuous flow of traffic at a defined
speed along a given route.
A thorough study
is always made by the City Traffic Engineer before a traffic signal
is installed as they exert a profound influence on traffic flow
and safety. Experience shows that right angle collisions may decrease
after the installation of a signal, but the number of rear-end collisions
may increase. If a traffic signal is installed when not warranted,
it may also increase the overall delay, increase the adverse effects
of collisions, and reduce intersection capacity.
In evaluating
the need for traffic signals, the City Traffic Engineer will compare
the existing conditions against nationally accepted minimum guidelines
or "warrants." These warrants are briefly explained in the following:
Warrant
1 - Minimum Vehicular Volume
The number of vehicles on the street for each of any eight hours
of any average day are compared with the national traffic signal
warrant criteria to determine if there is a need for the signal
to relieve the congestion.
Warrant 2 -
Interruption of Continuous Traffic
The number of
vehicles on the major street for the same eight hours are analyzed
to determine if congestion on the major street causes cars on the
minor street to wait excessively to enter or cross the major street.
Warrant
3 - Minimum Pedestrian Volume
The pedestrian
volume crossing the major street is analyzed for the same eight
hours to determine the frequency of vehicle and pedestrian conflicts.
Warrant
4 - School Crossings
The volume of vehicles and school children crossing are evaluated
to determine if special controls are needed for the protection of
school children.
Warrant
5 - Progressive Movement
This warrant is satisfied if the installation of a traffic signal
will facilitate a continuous flow of traffic along the roadway.
Warrant
6 - Collision Experience
This warrant is satisfied if there are at least three reportable
collisions in a twelve month period at the intersection that are
of the type susceptible to correction by a traffic signal.
Warrant
7 - Systems Warrant
A traffic signal
installation may be warranted to encourage concentration and organization
of traffic flow on the arterial network.
Warrant 8 - Combination of Warrants
In exceptional
cases, signals may be justified where no single warrant is satisfied,
but where any two of Warrants 1, 2 or 3 are satisfied to the extent
of 80 percent or more of the stated numerical values.
Warrant
9 - Four Hour Volume Warrant
This warrant
is satisfied when, for each of any four hours of an average day,
the vehicles per hour on the major street and the corresponding
vehicles per hour on the higher volume minor street approach are
above the Four Hour Volume Warrant values published in the Traffic
Manual.
Warrant
10 - Peak Hour Delay Warrant
This warrant
is intended for application where traffic conditions are such that
for one hour of the day, the peak hour, the minor street traffic
suffers undue delay in entering or crossing the major street.
Warrant
11 - Peak Hour Volume Warrant
This warrant
is satisfied when the vehicles per hour on the major street and
the corresponding vehicles per hour on the higher volume minor street
approach are above the Peak Hour Volume Warrant Values which are
published in the Traffic Manual.
It is important
to point out that the satisfaction of one signal warrant does not
necessarily justify the installation of a traffic signal. Other
factors that must be considered are delay, congestion, driver confusion
and other evidence of the need for right-of-way assignment.
If a traffic
signal is installed prior to meeting established warrants, or even
when meeting just one out of the eleven warrants, it could create
more problems than it was intended to solve, including the addition
of excessive vehicle delays. Traffic studies have shown that excessive
vehicle delay results in driver frustration and may cause disobedience
to traffic signal indications which can lead to a higher accident
rate. Additionally, an improperly installed or unwarranted traffic
signal can create increased traffic volumes on adjacent routes,
which may not have been designed to handle the increased flow of
vehicles which are trying to bypass the unaccepted traffic signal.
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