Phase: Each signal has a red, yellow, and green period. These are "phases." It may also have a phase for a turning movement, an all-red phase for pedestrians, or a phase for bicycles.

Cycle Length: The time it takes for the signal to go through all the phases (green, yellow, red, and back to green) is the cycle length.
Green time: The length of the green phase.


For each "simple" intersection of two roads crossing at 90 degree angles, there is a "major" street and a "minor" street. The major street carries more and probably heavier traffic, and is usually wider. A major street can become a minor street if it crosses a more significant intersection. The engineer studies the volume and congestion levels to decide the amount of green time to give each approach. If a heavily traveled bus and truck route crosses a local street, the engineer is likely to set the more significant street's green phase to 70% or more of the cycle length, but if there isn't much of a difference, a 55/45 split is more likely.

So, if you're traveling a long distance, you're better off taking a major street. Each time you approach a more major cross street you'll have less green time.

"Gridlock" Sam

An update to Hai-Etlik's description of British traffic lights.

First a quick Point of Information: over here, the yellow light is termed amber.

At standard lights for controlling traffic, the sort of thing you might find at a road junction. These have four phases:

  • Red -- do not go
  • Red and Amber -- prepare to move off
  • Green -- go (if it is safe to do so)
  • Amber -- slow down and stop unless it is not safe to do so

At pedestrian crossings which are controlled by traffic lights, there is an additional phase, flashing amber. This means that traffic may proceed if no pedestrians are on the crossing but otherwise the vehicles must wait, and it is accompanied by a flashing green man on the pedestrian crossing signals.

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