Installation of Traffic Signals
When Are Traffic Signals Installed?
Traffic signals have been used for many decades to
control traffic flow. This has provided engineers
opportunities to study their positive and negative
impacts, to identify when signals should be installed
and to determine how they should be operated. National
and state standards have been developed in order to
provide uniformity and maximum benefit to the public.
When used under the right conditions, traffic signals
may improve traffic flow and safety. In considering when
and where traffic signals should be installed, engineers
gather information on traffic flows. This may include
the amount of vehicle traffic for each fifteen minute
period of the day, the direction of traffic, turning
vehicles and pedestrians and bicyclists (especially near
schools). Traffic speeds and the development in the area
are also considered along with the number of travel
lanes available at the location.
Traffic flow information assists in determining the
potential impacts of a traffic signal on travel delays
and how many vehicles can be served by the intersection
(capacity). All signals have the potential to increase
delays or decrease capacity to some traffic movement and
decrease delays or increase capacity to other traffic
movements. Both total delay and capacity impacts and
individual movements are considered.
The history of incidents (collisions) at the location are also reviewed. This is of particular importance since some types of incidents (such as right angle or broadside) may be reduced in frequency with installation of a traffic signal. However, other types of incidents (such as rear end collisions) may increase with installation of a signal. When one street serves a much greater number of vehicles than the crossing street, the potential for an increase in the frequency of traffic incidents is higher.
Other considerations include growth trends, potential traffic diversions, where other future controls in the area may be likely and whether other changes may be more appropriate. The impact on overall traffic flow upstream and downstream of the signal is also of concern.
When properly applied, traffic signals represent a sound
public investment in the transportation system. A new
traffic signal installation may cost $150,000 to
$200,000. Such expenditures should be made only when
justified and with consideration of other needed
When not properly justified, the cost of a traffic signal is much greater than the installation cost. Other costs to the public may include time lost to unnecessary delays, increased frequency of incidents, increased fuel consumption and pollution and increased inconveniences and frustration. These wasteful public costs may easily exceed the installation cost of the signal in a single year.
New Signal Caution
Whenever a new traffic control is installed, the public may need several months to adjust to the new control. Many drivers need time to break old driving habits. Although drivers should always drive with caution, they should be especially careful the first few months after installation of a new signal.
People may request traffic signals based on
misconceptions of traffic signal impacts. For example,
persons may hear of one incident, especially if a
fatality occurs, and conclude that a traffic signal
should be installed. As previously stated, traffic
signals do not necessarily reduce the frequency of
incidents. In any case, making significant conclusions
from a single or few incidents, especially without
knowledge of the actual incident causes, will typically
result in incorrect actions.
People may also request a traffic signal because it is
difficult to cross or enter a busy street. If this alone
were the reason for installing a traffic signal, busy
arterials would have signals at almost all intersections
and driveways, making travel extremely difficult.
Sufficient traffic on cross streets is necessary to
justify stopping major street traffic. The majority of
miles traveled is on arterial streets. If cross streets
were signalized at low traffic volumes, the few moments
saved entering or crossing the major street would be
lost many times over in travel along the arterial. In
short, drivers are in the major street flow far more
often than they are in the cross street flow. Even when
the cross street volumes are sufficient to justify
stopping the major street flow, the total delay to
traffic may be increased.
Closely spaced traffic signals are of concern since they
typically become more difficult to operate together and
minimize stops and delays along major streets. Close
spacing may result in gridlock when the line of vehicles
from a signal backs up into other intersections.
Diversion of traffic may also be a concern, especially
if traffic is encouraged to use residential streets
either to reach or avoid a signal.