traffic lights

[fin]

what are them dark black box things on top of them.

[Darren]

They are radar type detectors that sense the presence of a vehicle. They're used to activate the lights in rural areas or when the programmed pattern is not active. So if they're on Red and they detect a car is approaching they will turn to Green in there are no cars detected on the other pair.
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Darren Griffin

[MarkHewitt]

Very often in the middle of the night the roads are completely empty. So cycling the lights to deal with rush hour traffic would be silly and cause unnecessary delays.

[sgould]

I believe that they also have a radar "override" so that they change at least every 2 minutes. Not sure if it's a law, but it used to be said that if you had waited at a red light for more than 2 min you could assume that the lights had failed.

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