Traffic light control and coordination
The first automated system for controlling traffic signals was developed by inventors Leonard Casciato and Josef Kates and was used in Toronto in 1954.
A "phase" is a period of time during which a set of traffic movements receive a green signal - equivalent to the concept of a "stage" in UK and US.
In the UK, a filter phase allows non-conflicting traffic to make particular turns (normally left or ahead) through a junction.
Normally, conflicting movements are not permitted, except for opposed right or left turns (depending on driving side) or, in some jurisdictions, pedestrians and vehicles moving in parallel directions.
[12] Solid state controllers are required to have an independent conflict monitor unit (CMU), which ensures fail-safe operation.
The CMU is programmed with the allowable combinations of lights, and will detect if the controller gives conflicting directions, for instance, green signals facing both northbound and eastbound traffic at a cross intersection.
Conflict monitors are susceptible to false activation during thunderstorms due to power surges and noise induced by nearby lightning strikes.
In the areas that are prone to power interruptions, adding battery backups to the traffic controller systems can enhance the safety of the motorists and pedestrians.
In the past, a larger capacity of uninterruptible power supply would be required to continue the full operations of the traffic signals using incandescent lights.
In 2004, California Energy Commission recommended to have local governments to convert their traffic lights to LEDs with battery backups.
A graphical representation of phase state on a two-axis plane of distance versus time clearly shows a "green band" that has been established based on signalized intersection spacing and expected vehicle speeds.
[15] In some countries (e.g. Germany, France and the Netherlands), this "green band" system is used to limit speeds in certain areas.
Lights are timed in such a way that motorists can drive through without stopping if their speed is lower than a given limit, mostly 50 km/h (30 mph) in urban areas.
This system is known as "grüne Welle" in German, "vague verte" in French, or "groene golf" in Dutch (English: "green wave").
[15][16] In modern coordinated signal systems, it is possible for drivers to travel long distances without encountering a red light.
On the other hand, some traffic signals are coordinated to prevent drivers from encountering a long string of green lights.
Non-coordinated sensors occasionally impede traffic by detecting a lull and turning red just as cars arrive from the previous light.
The most high-end systems use dozens of sensors and cost hundreds of thousands of dollars per intersection, but can very finely control traffic levels.
Benefits include:[18][19] Examples: Traffic light systems are designed using software such as LINSIG, TRANSYT, CORSIM/TRANSYT-7F or VISSIM.
