City Home Page
The Old Days
Years ago, traffic
signals were controlled by a simple electric clock. These clocks allocated a
specific amount of time to each traffic direction, in a specific pattern. The
problems with this arrangement are obvious under today's heavy traffic
conditions. The clocks left no room to adjust for peak traffic periods, or
unusual conditions.
The next step was to create a clock that operated differently at different times of the day. Still largely mechanical, these devices can still be found in use in some locations. They may have several different patterns for different times of day.
As electronics have influenced everything, so have they changed traffic control. Today, the traffic signal controller is a device that monitors traffic in several directions, and adjusts the signal timing and sequence based on that traffic.
Phases and Control
Before going further, the term "phase" must be defined. When used in traffic control terminology, a phase is a direction of traffic flow that is controlled differently from all other directions. At a simple intersection, with no left turn signals, there might only be two phases: one for Main Street and one for Elm.
As the intersection progresses, more phases are added. If Main Street has left turns, a phase may be allocated to the turning movements. The total number of phases is now 3. If, however, the designer wishes the left turn signals to be able to operate independently, that is North bound traffic on Main could get a turn arrow, while South bound did not, a separate phase is required for each turn movement. This would bring the number of phases to 4.
In a fully actuated and complicated intersection, the number of phases can reach 16. The intersection shown uses 12 phases. These phases include the walk signals. Walk signals must, of course, also be coordinated with the traffic signals.
The situation can become even more complicated if several signals are coordinated. A brief explanation of coordination is provided for you.
It is the responsibility of the traffic signal controller to keep track of the different phases, to make sure that all get the green time they need, and that no conflicting movements get a green signal at the same time.
Safety Measures and Conflicts
Equipment can break down and programming errors do occur. Because of the safety issues involved, signals are equipped with a "conflict monitor." A conflict monitor is a simple device, completely independent of the controller, that watches the signal operate. It does this by monitoring a number of conditions, including the voltage to the individual bulbs in the heads.
If a condition occurs which is not normal (for example opposing greens) the conflict monitor detects the condition and shuts down the intersection. Normally, it places the signal on "flash mode." The main street is given a flashing yellow, to indicate that the situation is not normal and caution is needed. The secondary street is given a flashing red light that should be treated like a stop sign. For safety reasons, the signal will not normally reset itself. A technician must visit the intersection, determine the problem and reset the controller.