What Is It?

• Traffic signals control vehicle movements at intersections.
• Traffic Control Systems are interconnected electronic systems that control a network of traffic signals. They typically offer several coordination plans, which define the cycle length, phase splits and offset for each intersection. See our Telecommunications Diagrams on Adaptive Signal Control and Fixed Signal Control for more information.
• Traffic Control Systems utilize control logic which can be divided into three categories: pre-timed, where the signal phases and cycle length are predetermined using historical data; actuated, where the signal phase lengths are adjusted in response to traffic flow as registered by the actuation of vehicle and/or pedestrian detectors; and traffic responsive, where the system responds to inputs-- such as vehicle actuation, future traffic prediction, and pattern matching-- which reflect current traffic conditions.

Key Results

Where properly implemented, traffic signal control can provide significant decreases in travel time, fuel consumption, and emissions, as well as some increases in safety. The effectiveness of the control system depends heavily on the quality of the prediction model. Surprisingly, a 1980's study of UTCS systems in the United States found that simpler, pre-timed systems often performed better than their more complex, traffic-responsive counterparts. The study proposed several reasons for the limited success of adaptive control:

• Inherent inaccuracies in the measurement prediction cycle prevented the strategies from responding fast enough.
• The frequent transition in signal timing can incur considerable delays.
• Insufficient time was allocated for models to calculate a good optimum.

• Reduced travel time
• Reduced delay and number of stops
• Increased fuel efficiency and reduced emissions
• Increased safety for vehicle movements at intersections

Costs

Costs include the installation of physical components (lights, wiring, computer systems) and the development of signal control logic. The cost of a control system ranges widely, depending on the level of complexity and the system supplier: in 1994, systems cost anywhere from $8,000- $350,000. In addition, traffic signals use a large amount of electricity, which must be factored into their long-term cost. However, advances in LED lighting technology are working to significantly reduce the electricity demands of traffic signals. Traffic signals are not a panacea for surface-street congestion, as they do have some disadvantages:

• Increased vehicle speeds at intersections, which significantly increases the fatality rate in accidents with bicyclists, pedestrians, and other vehicles
• Higher incident rates when compared with roundabouts or traffic circles, due to a higher number of conflict points

Implementation Challenges

Traffic signals are used in almost every city in the developed world. Therefore, their benefits are usually widely recognized. However, traffic signals are still sometimes opposed by the public for various reasons. Often, people feel that a traffic signal will detract from the aesthetics of their neighborhood, or that it will be more inconvenient for drivers. Proper research and public involvement are key to successful installations.

Author: Rebecca Pearson