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What's Wrong With The Light? A IMSA Brochure to help the public understand traffic control systems.

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 minutes 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.

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 the installation of a traffic signal. However, other types of incidents (such as rear end collisions) may increase with the 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 appropriate. The impact on overall traffic flow upstream and downstream of the signal is also of concern.

Signal Costs

When properly applied, traffic signals represent a sound public investment in the transportation system. A new traffic signal installation may cost $75,000 to $125,000. Such expenditures should be made only when justified and with consideration of other needed projects.

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.

Traffic Signal Misconceptions

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.
This Page Last Updated Friday, May 18, 2007