- Associated Press - Sunday, March 30, 2014

FLAGSTAFF, Ariz. (AP) - Since the passage in the 1980s of Flagstaff’s comprehensive lighting ordinance, the golden hues of low-pressure sodium lights have come to prevail in Flagstaff skies.

They are a response to one of the three key components of the lighting law: minimizing the light spectrum.

Some find the glow romantic, others think it dingy.

The low-pressure sodium lights are about as energy efficient as the modern technology of LEDs, but they have just a tiny fraction of the impact on night skies. Whereas a white LED emits light across almost the entire light spectrum, from red to blue, low-pressure sodium lights throw out yellow light in just a tiny bandwidth.

But as the city looks to celebrate its dark skies, new challenges continue to arise.

Until recently, the city had a policy of replacing its old high-pressure sodium lights with the more efficient and dark-sky friendly low-pressure sodium. About 60 percent of the city’s street lights are now low-pressure sodium, with the remainder being high-pressure sodium.

But in 2012, the city converted streetlights to LEDs on a one-mile stretch of Highway 89 just past the Flagstaff Mall.

That caught astronomers’ attention.

Flagstaff had agreed to take possession of the stretch of highway from the Arizona Department of Transportation under the premise of providing dark-sky friendly road lights.

“That lighting was designed right from the get-go using low-pressure sodium,” said Chris Luginbuhl, an astronomer at the U.S. Naval Observatory and leading dark skies expert. “The city changed about half in October 2012 to LED. That got our attention and we started a conversation with the city.”

The light poles often fail because they were designed for the smaller, high-pressure sodium lights and no engineering went into adding much larger fixtures.

Steven Hill, of Flagstaff’s Public Works section, told the Flagstaff City Council on Tuesday night that 12 of the light poles had failed on that stretch of Highway 89 the previous winter. Eight more of the poles have failed in the last year, plummeting 40 feet to the ground under wind stress.

“To date, none of these assemblies have fallen onto pedestrians or vehicles,” Hill said. “However, the failure of streetlight mast arms creates the potential for injury.”

The mast arms are about the length of a car.

Out of the city’s 3,500 streetlights, he estimated that about 3,000 poles might be at risk of failure. Because of that, the city expects a steep rise in maintenance costs in coming years.

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