Patterns in LED Traffic Lights in Austin TX

Nearly all of the traffic lights in Austin TX are now using LEDs. That said,

All the Green lights are arranged in a circle. There is clearly a center LED, and all the other LEDs are arranged in circles around the center.

All the Red lights are arranged in a honeycomb pattern. Thus along the edges, you can see these little black notches where the honeycomb pattern doesn't quite fit the circle.

All the Yellow lights are arranged differently. I can't remember exactly how at the moment, because I so rarely close enough to the light when it is yellow to examine the pattern.

The "burning" question is why? Why would all the LED lights in Austin follow this rule? Does it have something to do with how many LEDs they are trying to pack in to get an even brightness when green is compared to red is compared to yellow? (One Guess) Or is it just a design thing (another Guess). I have done some web searches, as have some other engineers I know, and we have all come up empty.

Thanks!

Reply to
snow.paul
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Are they actually a "honeycomb pattern" which is a hexagon or are they octagonal like a "stop" sign?

This could be for the benefit of those that are color blind so that they could distinguish exactly what the light means like they used to have the lights arranged in a particular pattern if they were in the horizontal rather in the usual vertical.

Reply to
jfma

LED voltages vary with efficiency. I don't recall if the voltage varies with color. Anyway, they may change the number of LEDs in a string depending on operating voltage.

I recall the high efficieny LEDS are higher voltage.

Reply to
miso

You made some good guesses!

There are specifications for traffic signals. You can buy the specifications at

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Among other things, the intensity, color, and angular emission characteristics are specified. It is a very competitive business, so it behooves the manufacturer to use as few LEDs as possible while still meeting the specifications. Different manufacturers arrange the LEDs differently. In addition, different colors using different quantities of LEDs may give rise to different arrangements.

Signals are submitted to the state's department of transportation for examination and, once approved, the manufacturer is put on a list of approved suppliers. When a city is interested in changing from incandescent to LED, they get the approved manufacturer list from the state. This gives them a starting point. I don't know if the cities are required to use the approved ones, but wouldn't you? As a result, you will see a certain uniformity from city to city within the state.

You didn't ask about the rest of this, but I'm on a roll.

It might interest you to know that another specification for LED traffic signals is power factor. Yep! They have power factor correction built into each one. Night time dimming is optional. Maximum power is specified. Power savings over incandescent is significant. As I recall, incandescent was about 100 to 150 watts while the LED signals were on the order of 30 watts. Imagine how much money that saves the taxpayer in electricity.

As I recall, cities replaced incandescents about once a year. The LED signals usually have a 5-year warranty (but may last longer). That is a big savings in bucket-truck trips. Also, when an incandescent goes out, it's out. LED signals may loose an LED in a string causing a group of LEDs to go out, but the signal is still functional. The current that is no longer used in the dead string may be diverted to the active strings maintaining pretty much the same overall intensity. The higher current may shorten the life of the remaining LEDs, but at least the signal will still function and can be replaced when convenient.

That's a few of the things I remember from working in the industry about 7 years ago. I'm sure technological advances have dated some of my comments, but, there you are.

Cheers, John

Reply to
John

In article , John wrote in part:

They got the power consumption for red and green ones down to 11-15 watts, and around 15 watts or a little more for yellow, a good 3 years ago, maybe more.

The really efficient red and green LEDs have 2-3 times the luminous efficiency of superlonglife vibration resistant incandescents, and roughly

2/3 of the light from incandescents is blocked by the red and green filters.

- Don Klipstein ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

There are descriptions of lights on the web where the shape provides a clear indication of the meaning of the light (i.e. Octagonal for the red). It seems that various other countries have traffic lights like this. However, the shape difference here in Austin is so slight that only when you are very close can you see the difference. So while my engineering friends and I discussed this as a theory, it was pretty easy to reject. It does make me curious what light frequencies they pick for the lights, since staying a way from true red and green is supposed to make traffic lights friendly to the color blind. But that is another question...

Paul

Reply to
snow.paul

Good observation - I hadn't noticed it but it is the same here South East

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Errr.... South East .... What context are we talking here? The U.S.? or Texas? A

Paul

Reply to
snow.paul

Coastal North Carolina

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So here are pictures of the traffic lights.

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Nearly all the traffic lights in Austin use the pattern you see for the green for green, the yellow pattern for yellow, and the red pattern for red.

I say "nearly". A few days ago I noticed at the north end of town a red light which used the green pattern.

So perhaps Austin is using three different vendors? Or perhaps the intensity of the LEDs varies, and the pattern is picked to accommodate different numbers of LEDs (I Haven't actually counted the LEDs, but to my eye, the Yellow has more LEDs than the Green, which has more than the Red.

Reply to
snow.paul

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