Signal timing

Signal timing is the technique which traffic engineers use to distribute right-of-way at a signalized intersection. Signal timing involves deciding how much green time the traffic signal provides to an intersection approach, how long the pedestrian WALK signal should be, 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 round robin basis, serving each approach consecutively, and repeating the pattern. 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 method of detection is induction loops. Other methods include magnetometers, video, infrared, radar and microwave detection. A typical loop detector installation could be approximately six foot 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. The preferred wire is stranded copper with an insulating cover which is then loosely surrounded by a protection jacket. Electrically this is a 'flat' coil in the pavement structure that detects vehicles by changes in the magnetic inductance field of the coil when ferrous metal from a vehicle passes through the field. The electronic sensor in the controller cabinet senses the change in the magnetic field. The output from the sensor electronics is a 'switch' closure. This can be an electro-mechincial relay or solid-state. The 'switch' is normally closed (NC) in the de-engerized state but held open when power is applied to the circuit. This is called 'Fail-Call' so that if there is a failure in the sensor electronics the output will place a 'Call' to the controller as if a vehicle is present on the loop detector.

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