LED research lights the path for U.S. roadway design guidelines
With significant contributions from the Virginia Tech Transportation Institute (VTTI), the National Academies of Science, Engineering, and Medicine have published the first national guidelines for using light emitting diode (LED) lamps to light U.S. roadways. VTTI conducted the research in partnership with engineering firm WSP.
Gradually, light emitting diode lamps are replacing the familiar orange-hued sodium vapor lamps that have traditionally been used on U.S. streets and highways. No traditional lamps have been installed in the past five or six years and about 10-15 percent of highway lighting has already been converted over to LED lamps. VTTI lighting engineers expect that percentage to increase significantly in the next five years, because of the energy-efficiency of LED technology.
“By installing LED, energy usage is reduced by 50 percent right away,” explained Ron Gibbons, director of VTTI’s Center for Infrastructure-Based Safety Systems and associate professor in the School of Architecture + Design. “With LED lamps, there are also these benefits of the tighter optical control. A traditional light takes about 10 minutes to warm up and turn on, but LEDs come on right away. Unlike vapor lamps, LEDs can be dimmed when there is low traffic volume and no pedestrians. There is a lot of potential for energy savings.”
Gibbons pointed to an example of a night football game at Virginia Tech to explain how light dimming could work in practice. On football nights, lighting designers could program the lights to shine at full intensity. In contrast, on calmer nights, the lighting could sit at 50 percent power and be dimmed down to a quarter power in the early morning or other low-traffic hours. The city of Cambridge and University of California – Davis have already implemented this dimming based on demand and have reported energy savings of over 80 percent compared to the traditional sodium vapor lamps.
However, the precise optics of LEDs can sometimes work a little too well. One disadvantage that the researchers explored in their study was the lack of surround ratio, which is lighting that spills over onto sidewalks.
“A sodium vapor lamp works like a butcher knife — it casts light very broadly, meaning that it also helps illuminate roads and sidewalks that pedestrians are using. But an LED is more like a surgical scalpel because it only shines on one spot. This is great for precision and efficiency but in some cases, can also eliminate some of the surround lighting benefits of the old system,” explained Rajaram Bhagavathula, senior research associate for the Center for Infrastructure-Based Safety Systems.
To correct for this lack of lighting for pedestrians, Bhagavathula and Gibbons suggest that lighting designers account for at least one lane width outside of the roadway when installing LED lamps.
“What we are recommending from our study is that for at least one lane width outside of the roadway, you need 80 percent of the amount of light you’ve got in the roadway. This will help ensure visibility for pedestrians walking on a nearby sidewalk or getting ready to cross the street,” said Gibbons.
“There was no formal guidance on LEDs for roadway lighting design until we did this project,” said Bhagavathula. “This gives us a blueprint for how to handle this new technology so that everybody can benefit.”
· NCHRP Research Report 940: Solid-State Roadway Lighting Design Guide: Volume 1: Guidance
· NCHRP Research Report 940: Solid-State Roadway Lighting Design Guide: Volume 2: Research Overview.