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Concept
Signal timing
Summary
Signal timing is the technique which traffic engineers use to distribute right-of-way at a signalized intersection. The process includes selecting appropriate values for timing, which are implemented in specialized traffic signal controllers. Signal timing involves deciding how much green time the traffic signal provides an intersection by movement or approach (depending on the lane configuration), how long the pedestrian WALK signal should be, whether trains or buses should be prioritized, and numerous other factors.
To understand basic signal timing fundamentals, one must also understand the different modes of operation of the signal controller which controls the signal. Traffic signals may be placed into two broad groups by their method of operation. They can be either pre-timed or actuated. Pre-timed signals provide each intersection approach a fixed amount of time on a predetermined basis, serving each approach consecutively, and repeating the pattern. In normal operation, no movements are skipped. An actuated traffic signal relies on some mechanism for detecting vehicles as they approach the intersection. Where detection has occurred, green time is provided to that approach. Approaches with no detection are skipped. These two schemes are also referred to as interval-based and phase-based signal timing.
The National Electrical Manufacturers Association (NEMA) has defined a standard scheme by which each intersection movement may be serviced without allowing conflicting movements to enter the intersection simultaneously. This scheme is commonly referred to as the NEMA Phasing Diagram.
One of the most frequently used methods of detection is induction loops. Other methods include magnetometers, video, infrared, radar and microwave detection. A typical loop detector installation could be approximately six feet square or six feet wide by thirty feet long. Other shapes may also be used including circular and hexagonal loops. These are cut into, or buried below the surface of the roadway.
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