# LEDs on traffic lights--why just green? **URL:** \ **Category:** Factual Questions\ **Created:** [May 23, 2002, 9:31pm UTC](https://boards.straightdope.com/t/leds-on-traffic-lights-why-just-green/110558 "2002-05-23T21:31:36Z")\ **Posts on this page:** 8\ **Page:** 2
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [May 28, 2002, 1:14pm UTC](https://boards.straightdope.com/t/leds-on-traffic-lights-why-just-green/110558/21 "2002-05-28T13:14:02Z")
> [@](#): > > \*Originally posted by k2dave \* > \*\*How can you tell the difference? \*\* 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. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [May 28, 2002, 1:25pm UTC](https://boards.straightdope.com/t/leds-on-traffic-lights-why-just-green/110558/22 "2002-05-28T13:25:12Z")
[tangent] > [@](#): > > \*Originally posted by chula \* > \*\*Red and green are on more than yellow, but you don’t mean to imply that green is on more than red, do you? That would lead to a lot of traffic accidents. \*\* 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. [/tangent] ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 28, 2002, 7:01pm UTC](https://boards.straightdope.com/t/leds-on-traffic-lights-why-just-green/110558/23 "2002-05-28T19:01:37Z")
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. ---
**Author:** ![The\_Griffin](https://avatars.discourse-cdn.com/v4/letter/t/a87d85/32.png) [@The\_Griffin](https://boards.straightdope.com/u/The_Griffin)\ **Post date:** [May 28, 2002, 7:33pm UTC](https://boards.straightdope.com/t/leds-on-traffic-lights-why-just-green/110558/24 "2002-05-28T19:33:25Z")
In UK they are all LED; Red, amber and green. ---
**Author:** ![AWB](https://avatars.discourse-cdn.com/v4/letter/a/bb73d2/32.png) [@AWB](https://boards.straightdope.com/u/AWB)\ **Post date:** [May 30, 2002, 6:23am UTC](https://boards.straightdope.com/t/leds-on-traffic-lights-why-just-green/110558/25 "2002-05-30T06:23:18Z")
> [@](#): > > \*Originally posted by AWB \* > \*\*In the DC area, I’ve noticed the LED lights only in Fairfax, Loudoun, and Prince William Counties, Virginia. Only greens have been installed, except for one yellow, which I cannot recall its location. \*\* 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. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [May 30, 2002, 8:25am UTC](https://boards.straightdope.com/t/leds-on-traffic-lights-why-just-green/110558/26 "2002-05-30T08:25:49Z")
> [@](#): > > \*Originally posted by k2dave \* > \*\*How can you tell the difference? \*\* 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. ---
**Author:** ![AWB](https://avatars.discourse-cdn.com/v4/letter/a/bb73d2/32.png) [@AWB](https://boards.straightdope.com/u/AWB)\ **Post date:** [July 23, 2002, 3:39am UTC](https://boards.straightdope.com/t/leds-on-traffic-lights-why-just-green/110558/27 "2002-07-23T03:39:10Z")
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. ---
**Author:** ![dwc1970](https://avatars.discourse-cdn.com/v4/letter/d/a183cd/32.png) [@dwc1970](https://boards.straightdope.com/u/dwc1970)\ **Post date:** [July 23, 2002, 5:48am UTC](https://boards.straightdope.com/t/leds-on-traffic-lights-why-just-green/110558/28 "2002-07-23T05:48:26Z")
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. [Previous page](https://boards.straightdope.com/t/leds-on-traffic-lights-why-just-green/110558.md?page=1)