LED traffic lights have a matrix of dozens of small LEDs filling up the circle, so when looking at the light from relatively close you can see the individual LEDs (disclaimer: do not stop at a green light to look at the LEDs :eek: ). Also the color is more brilliant than conventional lights. Another interesting characteristic is that they have virtually no start-up and shut-down time. When you turn on or off an incandescent light, it takes a brief time to warm up or cool down on an exponential curve. The perception of LEDs is that they are instantly on or off. This is particularly noticable if you are behind a new Cadillac with LED taillights, and you see the turn signal blinking. It’s kind of a strange effect when your brain is wired to see incandescent lights blinking.
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At a simple intersection of two streets with one light facing in each of four directions, the green lights and red lights would get exactly the same time on (disregard the yellow lights for the moment).
But at a busier intersection, with left turn signals in each direction (UK and Japan please substitute “right” for “left”), let’s say you have one signal for direct traffic each way plus one signal for left turns each way, for a total of 8 signals. At any given moment (again disregarding yellow), only 2 of them can be green (direct traffic in both directions on one street gets green; left turns both directions on one street gets green; or direct traffic plus left turns coming from the same direction on one street gets green) and the other 6 are red . Therefore red is on three times as much as green.
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You might also have a selection effect, in the lights that you see. Consider a simple intersection (no turn signals) between a major street and a side street. The signal is probably set up to be mostly green for the major street, and may even be a demand light. Therefore, one would expect the first lights to need replacing to be the green on the major street, and the red on the side street. But you’re on the major street more often than on the side street, so you’re more likely to see a recently-replaced green light than a recently-replaced red light.
In UK they are all LED; Red, amber and green.
Update: I’ve seen several red LED signals, sometimes with green LEDs, sometimes without. And the yellow LED signal I thought I saw was a regular one with a glass cover that had a checkerboard pattern etched on it.
As CookingWithGas mentioned, there is little or no ‘warm up time’ - I have noiced that the LED traffic signals in the UK seem to ‘flash’ brightly when they first illuminate, then settle back to their regular brightness, but I still haven’t decided if this effect is simply a trick of perception.
I’ve come across two all-LED signals: One in Beltsville, MD (at an industrial-area intersection), and one at Joyceston Dr. and Campus Way South in Largo, MD.
Boise is finally getting some LED traffic signals, too. Two recently reconstructed intersections that I often pass through have them now. I must say they are much brighter than the old incadescent lights.