# How long would a stoplight work after the apocalypse?

The apocalypse has come! Everything is dead, and I do mean everything. People. Animals. Plants. Don't ask how, I'm not entirely sure.

Let's assume this town is in an out-of-the way area as far as natural disasters go. It's not somewhere that gets slammed by hurricanes, or is prone to earthquakes, or gets tornadoes or boatloads of snow and freezing weather every winter. I'm not sure such a place actually exists, but for the sake of the question let's say it does.

With a stoplight run by timers and solar panels mounted on the top - How long would it remain functional, and what would kill it first? I've eliminated plants, animals, and extreme weather because those can be wildly unpredictable.

Edit: Otherwise, standard rainfall. Presume no high-level electronics (No computers, microcontrollers, etc), and long-lasting LED lights for ilumination.

• Does it rain in the Fortunate Isles? I don't know about other places, but heavy rain kills traffic lights in Bucharest with grim regularity. Does it have incandescent lightbulbs? Those will die in a year or two. Does it contain electronics? Those will die in a decade. The solar panels themselves will die within a century. Apr 1, 2019 at 0:43
• @AlexP I've updated the question with more specifics on the construction. Apr 1, 2019 at 0:46
• "LED lights": A LED lamp is a marvel of modern electronics. (Hint: unlike incandescent lightbulbs which are fed a constant voltage, LEDs must be fed a constant current. LED lightbulbs contain a miniaturized current source.) The timers themselves include voltage regulators and digital components. Sorry, but modern electronics intended for non-military use are just not designed to last a very long time. The LED lamps will die within a decade. The timers will die within one or, at most and with fantastic luck, two decades. Apr 1, 2019 at 0:56
• @AlexP I think you'll need to qualify that comment a little more. I have a TI-55 calculator with an LED display that is around 40 years old and still works great. I also have a collection of 8-bit computers and devices from that same era which have voltage regulators and digital components which also work as well today as they did when they were new. Apr 1, 2019 at 1:29
• @AlexP I'm also, specifically, not looking for longevity. I want to know when they'll stop working - be it four years, a decade, two decades. It sounds like you have an answer so why not post it as one? Apr 1, 2019 at 1:31

## No more than 12 years.

To answer this question we can look at the different components' lifetimes to try and decide which one will fail first. Once one area fails, your system will break down. I have provided my own rationalization of factors, however you can find a detailed report of expected LED Traffic Light Lifetimes Here.

1. Solar Panels: 40+ years

Solar panels made after the year 2000 have a degradation rate in efficiency of about 0.5% per year. However the newest solar panels are rated even better.

… a panel manufactured today should produce 92% of its original power after 20 years

Assuming that the traffic signal was created sometime after 2000 AD, 40 years is a realistic panel lifetime. If 80% or greater efficiency is required to keep the bulbs running, because for example the government was on a budget and used as little solar panel as possible, then the prediction -0.5% / year x 40 years = -20% gives us 80% efficiency after 40 years.

Newer panels could last even longer, but this is not the shortest lived component so further investigation is not necessary.

2. Solar Panel's Battery: 5 - 15 years

The general range for a solar battery's useful lifespan is between 5 and 15 years.

3. Traffic Lights: 1 - 12 years

Typical incandescent traffics lights have been tested and are known to last about 8000 hours. This comes out to about 0.91 years if left on continuously, or double that if used only at night. So these would last somewhere between 1 and 2 years.

A typical incandescent traffic light bulb using 150 watts generally lasts 8000 hours as studies have shown.

The very best LED light are rated at around 50,000 hours. If they were left on continuously they would last about 5.7 years, or double that if used only at night. So these would last somewhere between 5 and 12 years.

Many LEDs have a rated life of up to 50,000 hours. This is approximately 50 times longer than a typical incandescent, 20-25 times longer than a typical halogen, and 8-10 times longer than a typical CFL. Used 12 hours a day, a 50,000 bulb will last more than 11 years.

4. Copper-Wiring: 20 - 50 years

The standards that cables are manufactured to do not specify a particular life expectancy. Some cable manufacturers will determine a likely life expectancy based on typical conditions. For example a household fixed wiring cable with typical electrical loading, wired using the appropriate wiring guidelines, could be expected to last 20 years. However, in some cases cables which have not been used excessively have been found in relatively good condition up to 50 years after installation.

## Conclusion:

From these facts you should see that the traffic light will likely be where this system fails first. If you are lucky your traffic light might last a decade. However there are at least two different components that are rated to fail in the 5-15 year range, so anything beyond this range is very unlikely. Finally if the traffic light is using a typical incandescent bulb, then it will last even less time and go out after only 1 year.

• The estimate of less than a year on traffic lights is somewhat shocking. I have never - ever - seen a work crew working on the lights in my town. They must be ninjas about changing the bulbs on every light every year. Apr 1, 2019 at 18:06
• It does seem pretty often. But I guess the technicians only have to show up once a year, and it could be easy to miss if they do it at night, or while we are at work. quora.com/How-often-do-traffic-lights-get-replaced Apr 1, 2019 at 18:08
• From the linked site, Photovoltaic (PV) modules typically come with 20 year warranties that guarantee that the panels will produce at least 80% of the rated power after 20 years of use. That's a great deal less than 200. That 80% degradation is important. Remote stoplights are designed for cost efficiency. The panel won't be substantially more than the stoplight needs. An 80% drop in power could shut it down completely. Apr 1, 2019 at 18:13
• Rated LED life is usually MTBF (mean time before failure) and often misunderstood. It's a statistical measure, so with a 50K hour MTBF, you can expect a single LED to last that long. But with a large collection of LEDs (such as in a typical traffic light) it's a bit different. The larger your sample size, the more likely it is that some will fail far sooner, and some will last much longer. Depending on how many LEDs you need to consider the light "functioning", you can expect the light as a whole to last significantly longer than the MTBF of a single LED.
– Gene
Apr 1, 2019 at 19:20
• @TylerS.Loeper, you misunderstand how LEDs work. They're diodes. If a minimum voltage doesn't exist, they simply don't turn on (unlike an incandescent light, which emits something at all voltage levels). And the OP is asking, "what would kill it first?" That's hardly asking for the longest period of time. Apr 1, 2019 at 20:36

Sorry to say, even long life LED fail early. I have about ten of them installed in my house, 2 have failed way earlier than the advertised lifetime.

As any device, they have a statistical distribution for their life. Some happen to die early, some happen to last longer, the majority live as much as designed.

Same would go for your led stoplight, and the driving cicrcuitry supplying it.

• stop lights are not the same as your houselights Apr 1, 2019 at 6:57
• Actually, they're not that different. Check out the Sylvania A21 -- a 116W heavy duty light bulb of standard type and construction. Even the led variety is just an array of, well, leds. Apr 1, 2019 at 11:50
• @Kilisi, citation required. Same LED tech, just different enclosure manufacturing. Apr 1, 2019 at 18:16
• @JBH No. First, stoplight LEDs are never white, which eliminates the degradation of the blue-to-yellow phosphor in house lights. This is one of the main mechanisms of lumen loss. Second, the housing is different, which makes all the difference to LEDs: semiconductors have have a lifetime highly dependent with temperature. Traffic lights are much larger and can run at lower temperatures than a tightly packed incandescent retrofit bulb. Finally, due to accident liability, stop lights are built to a much higher standard than light bulbs from a big box store. Apr 2, 2019 at 1:04

# The tall pole in the tent isn't the LED light or the solar panels - it's the battery

Solar-powered anything can't run at night unless there's a battery. That means your solar panels are both running the light and charging the battery during the day.

No traffic control system — no solar power system at all — is designed to operate for years or months without the sun.

This manufacturer claims their 24-hour flashing warning light will last 14 days without sunlight. Note that actual traffic lights don't strobe. A light is on 99% of the time vs. half the time (I'm willing to accept that the power for the control circuitry is negligible compared to the lights themselves).

Realistically, depending on too many conditions you haven't specified, you could trust your light to stay on 7–10 days after the snow falls or dirt/dust compromises solar panel efficiency.

• Agreed, 100% correct, the batteries are you're first and foremost point of failure here. You are looking at about 5-7 years before they fail.
– Rob
Apr 2, 2019 at 18:53
• @Rob, what's a pain is that there's a lot of variability. I've had commercial-grade UPS batteries fail after one year of use. The longer the battery stays drained (say, through the winter), the less likely it'll hold as much charge the next time the panels start working. Every cycle is a roll of the dice. The world is fascinated by solar panels and LEDs, but it's the battery tech that will really advance our world (when it finally happens). Apr 2, 2019 at 20:18
• Also worth noting is that the type of battery has a significant impact here, too. Lead Acid batteries are a different beast than a Lithium Polymer Apr 2, 2019 at 22:14
• @Andon true, but I suspect the average lifetime would be similar. A car battery, unmaintained, dies in 3-5 years (water boils/evaporates out of the battery, damaging it). Lithium batteries will, too, because they're not meant to be completely drained. Apr 3, 2019 at 5:33

Not very long. The trick is 'all plants dead'. That will lead to massive fires. One lightning strike and puff, the world will be ablaze. That will lead to large amounts of ash that cover up the panels, assuming the lights aren't consumed by the fires. Its certainly possible for the light to be far enough away from burnables - out in the middle of the desert away from civilization perhaps - but then, that's not a typical location for a traffic light.