“Evidence-based design” is something of a buzzword, but the idea of doing what the numbers suggest is, in fact, an old concept and one that should fall into the category of “common sense.” The Washington State Department of Transportation (WSDOT) put the old idea into action with an LED pilot project at a single highway interchange. Given the evolving technologies of LEDs for public streets and highways, WSDOT plans to collect data before committing to any particular model for the state’s 7,000 miles of roadway and 3,000 vehicular bridges.
“We’ve been looking at LEDs for five or six years now,” says Keith Calais, a signal and illumination engineer for WSDOT, “and it seems every one to two years a new generation comes out.” As with most municipal and state highway systems, WSDOT primarily uses high-pressure sodium (HPS) lights, most identified by their cobra-style fixtures. LEDs require an expensive replacement of those fixtures, but the promise of a 50 percent reduction in energy use and less maintenance (studies show that LEDs last longer than HPS, metal halide, and most other technologies) can more than make up for the cost.
But several factors make it difficult to know with certainty exactly how those savings will accrue. One is that with new technologies no one is willing to predict with certainty what will happen over the next 10 or 15 years, even though the working numbers say that an LED bulb will last 50,000 hours versus 12,000 hours for an HPS light. Second, with energy prices that vary by state, the return on investment changes dramatically from place to place. The cost of electricity in California is 13.1 cents per kilowatt-hour while in Washington it is only 6.6 cents due to a plenitude of hydroelectric power in the state. Overall, that’s a good thing, but it means WSDOT’s return on investment could take almost twice as long as California’s.
Acknowledging that the return could be as long as 12.5 years, Calais is candid and pragmatic. “We want information and knowledge about how LEDs work,” he says. “But we also need to know how to do it, how it works. There’s more to this than light bulbs and new fixtures.” That “more” part includes how LEDs handle such things as power surges and if there are ways to save additional energy.
WSDOT will employ a dimming system that powers down lights during times of lightest traffic, such as after midnight. Before starting this testing, WSDOT reviewed collision histories from illuminated versus non-illuminated areas to see if there were any concerns with testing this at night. WSDOT used a photometric calculation tool known as AGi32, which allows users to compute illuminance from a proposed lighting design before it is actually built.
In the design, each luminaire is equipped with a light sensor, and collectively, these sensors communicate with the system’s central command to allow for necessary adjustments. Such adaptive systems of fixtures, bulbs, and sensors consequently cause shifts in the supply chain, prompting manufacturers to change from being component suppliers to providing comprehensive solutions for the whole system. In WSDOT’s experience, different manufacturers under the same parent company were able to provide multicomponent products that are technologically seamless. “Without these parts being paired together, there could be functional problems such as color control,” Calais says.
The pilot project, launched in early 2013, included 88 poles at a US 101 interchange west of Olympia, Washington. The light is whiter and brighter than its predecessor, as expected, and this single installation could save the state $75,000 in maintenance and operating costs over the next 15 years. According to the Department of Energy, the widespread use of LEDs could generate $750 million in savings per year.