Basic Signal Timing Operation
To understand basic signal timing fundamentals, one must also understand the different modes of operation for the traffic signal controller. Many intersections have some sort of mechanism for detecting vehicles as they approach the intersection. Most common are induction loops. These are buried in the roadway and detect vehicles by changes in their magnetic field by the metal in passing vehicles. Other common methods are video detection which uses pixelation, microwave detection, and infrared detection among others. An intersection equipped with detection is said to be actuated. An intersection without detection is said to be fixed.
There are different categories of actuated signals. To save money on maintenance, some agencies opt to design an intersection as semi-actuated. Semi-actuated means the intersection has detection on the minor street approaches and major street left turns only. The major street is then programmed to operate a fixed time every cycle, but the controller will service the other movements only when there is demand. In signal coordination, most signals operate in a semi-actuated mode.
In fixed operation, a controller has a set programmed time to service all movements every cycle. The controller will service all movements whether or not there is vehicle demand. When a detector at an actuated signal breaks, that movement will then have to operate as fixed until the detector is repaired.
There are three general ways for a traffic signal to operate, FREE, COORD, and FLASH operation. In FREE operation, the signal is running based on its own demand and timing parameters based on the information provided by its detectors. It is not operating under any background cycle length. In COORD operation, short for coordination, the signal is running a background cycle length. Non-major street movements are usually still actuated, and the controller will rest on the major street until the background cycle length is fulfilled. The final mode is FLASH operation. When the volume of vehicles at an intersection no longer warrants the signal to be active, the signal can switch to FLASH mode. When volume picks up again the signal switches back into either FREE or COORD operation. The daily operation of a signal may involve it in FLASH early in the morning, COORD during the day, FREE in the evening, and back to FLASH late at night.
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