Why Street Lights Across America Are Turning Purple - The Autopian (2024)

Streetlights tend to serve two main purposes. They exist to illuminate the streets for drivers to see better, and to make public spaces more welcoming and less scary for pedestrians at night. Traditionally, they’re some variant of orange or warm yellowish light, but in recent years, many have mysteriously turned purple. The change has confused motorists, excited Prince fans, and highlighted issues with municipal procurement.

The issue is currently a hot topic in Milwaukee, where significant stretches of highway are bathed in a lovely purplish glow.The city has pledged to rectify the issue in March, though the city’s purple patch has been in the news on and off for years now.

It’s not just affecting the Cheese State, either. Reports of purple street lights have come in from Tennessee, North Carolina, Florida, Texas, Vancouver, and more. Even better, they’ve been spotted in Minnesota, in what must feel like a rather fitting tribute to Prince. Indeed, one YouTuber by the nameGod, Guns and Truckstook this loose connection as proof that the purple lights were a celebration of some kind of royalist New World Order taking over the United States.

Sadly, though, you’ll need to put your conspiracy theories aside. This isn’t some new cool hip trend in Departments of Transport across the country, and it’s not some attempt at widespread UV sterilization. Instead, it’s down to the switchover to LED streetlights that many municipalities have made in order to try and save energy. LEDs use a fraction of the power of most other lighting technologies, so they’re very attractive for government organizations trying to save money and cut carbon emissions.

The unintended consequence was street lights in a powerfully purple hue. As it turns out, our resident RV lover Mercedes also loves these purple lights, so she’s taken pictures of the groovy scenes created by these lights in Lake Villa, Illinois, and St. Petersburg, Florida, and you can really see just how intense the coloring really is.

So why the purple? Well, in virtually every case, these bulbs were supposed to be white, or something close to it. Now, if you think back to your high school science lessons, you’ll remember that white light is actually made up of a whole bunch of different wavelengths. In contrast, a conventional single-chip LED only puts out a single wavelength of light. So, if you want white LEDs, you have to get tricky.

You could combine a red, blue, and green LED into a single package, and some white LEDs have done that, but it’s expensive. Instead, a lot of white LEDs are just blue, purplish, or UV LEDs at heart. Then, the outer casing of the LED is sprayed with a phosphor material. This phosphor captures some (but not all) of the blue light, and converts it to other wavelengths, typically yellow, but sometimes red and green as well. The combination of this converted light with the blue light passing through the phosphor provides a white or off-white output. By varying the composition of the phosphors on the bulb, it’s possible to fine-tune the exact shade of white light that comes out of the LED.

The reason these streetlights end up looking purple is because the phosphor coating has degraded over time. As it flakes off and there’s less phosphor to convert light, more of the core LED’s base color is visible. There are no other wavelengths being generated to sum up to a white or off-white color, so the bulb just projects blueish-purple light by itself.

It’s a strange defect, as noted by experts speaking to Scientific American. White LEDs are manufactured in the millions, if not billions every year, using the same phosphor-coating techniques. It seems that the lights chosen by many municipalities are failing due to bulbs with poorly manufactured phosphor coatings.

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Purple street lights have become popular photography spots for some automotive enthusiasts.

Most municipalities have elected to fix the problem by replacing the purple lights. As reported by the Milwaukee Journal Sentinel, the city first installed the defective lights in 2018, before realizing in mid-2021 that there was a problem with bulbs sourced from American Electric Lighting. An early estimate at the time suggested replacing all the affected lights would take 10 weeks or so across the winter period.

The problem didn’t go away, however. The Wisconsin Department of Transportation is preparing to replace more lights next month after they took a purple turn last summer. Even though the problem has been well-known and understood for years, it seems there are still bulbs out there failing.The Autopian has contacted the Wisconsin DOT to determine whether bulbs replaced in earlier work are now failing as well.

Meanwhile, in Texas, the defective lights, numbering 1,100 or so, were apparently sourced from Acuity Brands Lighting according to the city of Fort Worth. The same brand was reportedly responsible for a batch in Florida, too, while Maryland energy company BGE Now has replaced over 14,000 of defective Acuity-sourced bulbs under warranty in Baltimore alone.

The Autopian has contacted American Electric Lighting and Acuity Brand Lighting for comment on the matter, particularly regarding the factors at play behind the degradation of the bulbs. Notably, American Electric Lighting is a subsidiary of Acuity, as its website lives on the latter’s domain name. Acuity appears to be the prime brand behind the problem in most cases where a manufacturer is cited by public officials.

You might be wondering—is this really a problem? Isn’t purple a lovely color? I myself would agree, it’s quite fetching. And yet, it’s not really appropriate for street lighting. Like just about any other part of the road network, street lights are chosen carefully to offer the utmost in safety. They need to provide a clear, even light to help drivers spot hazards and other vehicles, without dazzling or otherwise distracting motorists. Studies have been undertaken that suggest cool white provides better visibility, with greater contrast and easier color perception, too.

When they’re working, modern white LEDs are a big step up from the old monochrome orange sodium lamps.It’s intuitive, too, that white light would be an improvement over monochromatic blue or purple light from these failing LED bulbs. Imagine trying to read a book at night solely under a blue lamp—you’d reach for a white light every time.

If you’re living somewhere with purple street lights, go out and enjoy them while you can. Your municipality is probably working to replace them, and if they’re not, they should be. As cool as they look, they’re really not the right choice when it comes to illuminating a roadway for safety reasons.

Image credits: WFMY News 2 via YouTube Screenshot, Department of Energy