If there are no cars at an intersection the light should be red in every direction. Then when a car approaches it can quickly change to green in that direction. If it's green in one direction and someone approaches from the perpendicular, the light has to go through yellow (or until the WALK sign is finished) and the car has to slow down before the light changes. This signal arrangement saves drivers gasoline.
On a related note, signal coordination in urban areas (timing the green lights to ensure the best traffic flow) can lead to significant reductions in greenhouse gas emissions. This solution is not very sexy or expensive, and the results aren't obvious, which is why more cities don't do it.
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