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As is probably known a traffic light may be turned off in which case normal rules applies to the intersection.

While the traffic light is active then red & red+yellow means that drivers are required to stop, while green means that the drivers are allowed to drive. Yellow also means that you're allowed to drive, but cautions the driver that it will/may turn red on which the driver are required to stop.

Now by what sequence do one start/stop the traffic lights? Stopping seem quite obvious, just make sure that all are showing red (or only those that will have the right of way have green) and then just turn off the light.

But how to turn the lights on again. I think that just reversing the sequence may not be fair - if the light all the suddenly turns red you're required to stop (but may or may not have the time to stop your vehicle). On the other hand entering with green light might be dangerous.

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  • $\begingroup$ You are assuming all the lights have to switch on (or off) at the same time. They don't. Each light can switch on/off when it is green without causing any issues for drivers. Switching on, when that light has cycled through to red another light can switch on as green, etc. $\endgroup$ – alephzero Aug 31 '17 at 11:17
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    $\begingroup$ I'm voting to close this question as off-topic because it's asking for design rules encoded in government standards. Not engineering. $\endgroup$ – Carl Witthoft Aug 31 '17 at 15:43
  • $\begingroup$ Traffic lights may be turned off, but they don't have to be. In the part of the UK where I live there are many lights systems which include vehicle sensors some distance away from the lights. When road usage is light (e.g. overnight) the switching logic changes from a set pattern to "on-demand", so nobody who stays within the speed limit has to wait unless there really is more than one vehicle approaching the lights on roads with different traffic priorities. $\endgroup$ – alephzero Aug 31 '17 at 15:44
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    $\begingroup$ @CarlWitthoft Those standards and any changes to them are based on principles of traffic engineering and flow analysis. $\endgroup$ – DLS3141 Aug 31 '17 at 16:40
  • $\begingroup$ @CarlWitthoft Frankly I didn't know where this question would belong, but since I saw there were question about traffic-lights here and tags for that I gave it a shot. If you have a suggestion where the question is better asked I'm all ears. $\endgroup$ – skyking Sep 1 '17 at 5:11
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What country? This is different between different countries.

I can give the sequence in Poland, which is standard in most of EU.

    • 180 seconds of Blinking Yellow. (Groups [**] with Yellow blink yellow, the rest remains off. If the controller was already in this mode, past duration is counted towards the 180 seconds.)
    • All Yellow. Whichever groups don't have the yellow signal, display red.
    • All Red. Duration of the shortest Intergreen[**] time, no less than 5 seconds.
  1. (optionally) - Startup program. A short custom sequence that opens most prioritized lanes (through normal sequence, like 1s of Red+Yellow, followed by Green).
    • Standard traffic program. Normally side lanes are opened first, main direction later.

Ending just requires minimum intergreen of red signal after stopping all groups normally, before transition to YB - the controller can transition to dark (off) 180 seconds later.

[*] Group - a set of signal lights (often just one signal light) that display the same signal. May be a set of lanes that allow transition to the same exit, or a signal light hanging above the lane, repeated by another on the side of the lane - or two signal lights on two sides of the same zebra crossing.

[**] Intergreen time - time between stopping one group and activating another, necessary for vehicles/pedestrians to vacate the crossing without risk of collision with traffic from the newly opened lane. The intergreen times are assembled into a matrix, each starting group to each ending, with non-colliding entries empty, the remaining containing time that needs to count down from the end of the ending group) before the starting group is allowed to start.

Below is a standard stop and start sequence of a controller; diagram states of each group. The numbers above the diagram are the second of given phase, and phase number of given program. The names on the left are the group names. The program is made for development purposes, so it doesn't retain the 180 seconds restriction on yellow blinker (just to save the time of the developer); actual traffic program would require a 180 second gap between the start and end.

enter image description here

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There is another state the lights can be in: blinking yellow. Which means that the intersection is not being controlled and normal priority rules are in force. Going from off to all blinking does not change how you should behave on the intersection.

Going from blinking yellow to solid yellow (longer than the normal 3 seconds) to red gives drivers enough warning that the light will turn red.

If you have a major and minor road on the intersection where if the lights are off the minor yields to major, you can make the minor road turn red and the major turn green. Possibly showing the minor road an extended solid yellow first.

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  • $\begingroup$ Solid Green = Go, Solid Red = Stop, Solid Yellow = Stop sign with forgiveness, Blinking Red = Stop Sign, Blinking Yellow = Yield Sign...at least here in the US $\endgroup$ – DLS3141 Aug 31 '17 at 16:47
  • $\begingroup$ Overheard from child in rear of car: "Red means stop. Green means go. Amber means Mammy stop, Daddy go." $\endgroup$ – Transistor Aug 31 '17 at 22:29

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