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Signal timing
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Signal timing
Signal timing is the technique which traffic engineers use to distribute right-of-way at a signalized intersection. The process includes selecting appropriate timing values, 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.
In signal timing fundamentals, there are different modes of operation that the signal controller can use to control the signal. Traffic signals may be grouped into two broad categories by their method of operation. They can either be 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 a mechanism to detect 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 commonly used methods of detection is the use of induction loops. Other methods include magnetometers, video, infrared, radar, and microwave detection. A typical loop detector installation can be up to 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. The preferred wire is stranded copper with an insulating cover, loosely surrounded by a protective jacket. Electrically, this is a 'flat' coil embedded in the pavement structure that detects vehicles by changes in the coil's magnetic inductance field as ferrous metal from a vehicle passes through it. The electronic sensor in the controller cabinet detects changes in the magnetic field. The output from the sensor electronics is a 'switch' closure. This can be an electromechanical relay or a solid-state relay. The 'switch' is normally closed (NC) in the de-energized 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.
Video (both normal and infrared) uses changes in the contrast of the image detection zone to detect traffic. All detection methods, except inductive loop detectors and magnetometers, suffer from reduced accuracy due to occlusion. This limits the camera's view in certain instances.
There are different categories of actuated signals. To save on maintenance costs, some agencies opt to design an intersection as semi-actuated. Semi-actuated means the intersection has detection only on the minor-street approaches and major-street left turns. The whole intersection is then programmed to operate for a fixed time every cycle, but the controller will service the other movements only when there is a 'call' or demand. An arterial series of signals operating in a semi-actuated or fixed mode, and importantly, with the same cycle length, can be coordinated. During signal coordination, most signal systems are typically designed to operate in a semi-actuated mode.
In fixed operation, a controller has a set 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 a signal can operate: FREE, COORD, and FLASH. In FREE operation, the signal runs according to its own demand and timing parameters, as determined 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, in which all vehicle signal heads continuously display a flashing red, or the main street displays a flashing yellow while others display a flashing red. Pedestrian heads are dark.
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Signal timing
Signal timing is the technique which traffic engineers use to distribute right-of-way at a signalized intersection. The process includes selecting appropriate timing values, 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.
In signal timing fundamentals, there are different modes of operation that the signal controller can use to control the signal. Traffic signals may be grouped into two broad categories by their method of operation. They can either be 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 a mechanism to detect 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 commonly used methods of detection is the use of induction loops. Other methods include magnetometers, video, infrared, radar, and microwave detection. A typical loop detector installation can be up to 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. The preferred wire is stranded copper with an insulating cover, loosely surrounded by a protective jacket. Electrically, this is a 'flat' coil embedded in the pavement structure that detects vehicles by changes in the coil's magnetic inductance field as ferrous metal from a vehicle passes through it. The electronic sensor in the controller cabinet detects changes in the magnetic field. The output from the sensor electronics is a 'switch' closure. This can be an electromechanical relay or a solid-state relay. The 'switch' is normally closed (NC) in the de-energized 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.
Video (both normal and infrared) uses changes in the contrast of the image detection zone to detect traffic. All detection methods, except inductive loop detectors and magnetometers, suffer from reduced accuracy due to occlusion. This limits the camera's view in certain instances.
There are different categories of actuated signals. To save on maintenance costs, some agencies opt to design an intersection as semi-actuated. Semi-actuated means the intersection has detection only on the minor-street approaches and major-street left turns. The whole intersection is then programmed to operate for a fixed time every cycle, but the controller will service the other movements only when there is a 'call' or demand. An arterial series of signals operating in a semi-actuated or fixed mode, and importantly, with the same cycle length, can be coordinated. During signal coordination, most signal systems are typically designed to operate in a semi-actuated mode.
In fixed operation, a controller has a set 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 a signal can operate: FREE, COORD, and FLASH. In FREE operation, the signal runs according to its own demand and timing parameters, as determined 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, in which all vehicle signal heads continuously display a flashing red, or the main street displays a flashing yellow while others display a flashing red. Pedestrian heads are dark.