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Patterns in LED Traffic Lights in Austin TX

Discussion in 'Electronic Design' started by [email protected], Feb 8, 2007.

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  1. Guest

    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

    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.

  2. Guest

    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.
  3. Guest

    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.
  4. John

    John Guest

    You made some good guesses!

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

    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.

  5. 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
    - Don Klipstein ()
  6. Guest

    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
    It seems that various other countries have traffic lights like this.
    However, the shape difference here in Austin is so slight that only
    you are very close can you see the difference. So while my
    friends and I discussed this as a theory, it was pretty easy to
    It does make me curious what light frequencies they pick for the
    since staying a way from true red and green is supposed to make
    traffic lights friendly to the color blind. But that is another

  7. default

    default Guest

    Good observation - I hadn't noticed it but it is the same here South
  8. Guest

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

  9. default

    default Guest

    Coastal North Carolina
  10. Guest

    So here are pictures of the traffic lights.[email protected]/

    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.
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