I would suspect that the lights are mounted on a circuit board.

Since the late 80s it’s been rare to lay out the traces on a circuit board by hand. This is now normally done automatically by the EDA software from the schematics (which in this case would be very simple — just a Christmas-tree-like ladder between power and ground).

The process the EDA software uses to lay the traces out is called routing and it works a bit like an annealing process (actually there are several different methods, see the “Routing (EDA)” article in Wikipedia). In any case you wouldn’t expect to get an obvious result, such as all LEDs laid out in horizontal lines or small groups. You would get something a little bit more random and organic as shown in the pictures.

Perhaps because you’re less likely to notice one LED is out, and thus don’t realize there was a failure at all?

Though I think the Christmas light explanation is more likely.

In your video, you can see a string of LEDs turn off last; they have a symmetrical pattern with the failed ones in the first photo.

As for the ZR-200 camera: Thanks; Googled, and on a mental list to try and buy :-)

About the video problem reported by Paul DeLong: This is my first-ever venture into the stinking morass known as the HTML5 <video> tag. I knew I would regret it. I’ve tweaked the code a little more, but I don’t have great faith that my changes will solve everybody’s problems. All I can report with any certainty is that the video plays for me in Chrome, Firefox, Safari, Opera, NetNewsWire and Google Reader.

I should add that the video is not all that exciting, so don’t be heartbroken if you can’t see it. I’m just agog at the idea of living in a world where cheap cameras (it’s a Casio ZR-200) can record video at one frame per millisecond.

Unfortunately for us, AlGaAs LEDs simply cannot emit green light — in order to get green and blue LEDs we need to use materials in the Gallium Nitride (GaN) family, which are less well-behaved. GaN crystals are very difficult to grow, requiring much higher temperatures to achieve reasonable material quality. Modern manufacturing techniques can make decent GaN LEDs, but there are a lot more defects in the crystals. Over time the crystal defects grow, until eventually the device fails. Defect growth accelerates with temperature, so I’d expect to see more LEDs failing during the summer than the winter (although maybe not! Unlike the old bulbs, LED lights don’t get hot enough to melt the snow that piles up on them, so some municipalities have had to add heaters to the street light assemblies to keep the street lights visible in the winter. If the heater was set too high it could certainly contribute to LED failure). I could babble on about semiconductors for way too long (maybe I already have!), so let’s move on…

As to the patterns of failing bulbs, I think your explanation is probably correct, but I’m not positive. I’d bet that the different failure patterns are just due to different designs sold by different street light manufacturers. For instance, the first picture looks like there’s a ground ring around the circumference of the LED board, and the positive terminal of the LED string is at the center, giving you a ray-shaped LED string. Some of the others look like they just have clumps of LEDs on the same driver, and that final one has the characteristic “dead string” christmas light look.

Oddly, I don’t see the video link while viewing this blog entry directly (at http://bit-player.org/2012/stop-lights), but it comes through fine in Google Reader. I’m using Chrome in both cases. Though in the latter case, it looks like it was supposed to be embedded, but didn’t quite make it (I only see the “Popout” link, which opens a blank window, and another for “Original video source (greenlight_slowmo.ogv)”, linking to “http://bit-player.org/wp-content/uploads/2012/09/greenlight_slowmo.ogv”.) I was able to view it, but I thought you might like to know about the weirdness.