Reuters

How cities can overcome some of the disadvantages of LEDs

Cities all over the world are converting streetlights to low-energy Light Emitting Diode (LED) fixtures, and with good reason. LEDs use 70 percent less energy and last four times as long, according to the Clinton Climate Initiative. Retrofits of conventional streetlights pay for themselves in three to seven years. 

In addition to energy savings and longer life, LEDs have other advantages. The brightness and color of individual LED fixtures can be adjusted after installation to meet the needs of business districts, residential neighborhoods and parks, even from season to season. A central maintenance facility can monitor when lights go out via wireless technology. LEDs can be programmed to flash during emergencies or to operate as way-finding devices during special events.

But despite the raft of benefits, there are issues. Although the purchase price of LED lamps has been dropping, they are more expensive than traditional bulbs. And there are problems of glare, light trespass and the impact on pedestrians, drivers and bicyclists. It can also be difficult to retrofit existing streetlight fixtures with new LED lamps.

Pittsburgh is planning to convert its 40,000 streetlights to LEDs over the next five years. The city commissioned Carnegie Mellon's Remaking Cities Institute (which I lead) to analyze the possible negative effects. We benchmarked best practices internationally, researched the newest LED technology, conducted field tests, interviewed stakeholders and ultimately made recommendations.

Glare from LEDs turned out to be particularly problematic for people with visual impairments or age-related vision loss. Police also report difficulty in observing activities half a block away. Light trespass to adjacent buildings and the impact on the dark sky are also worsened by the powerful LED lamps. However, these issues can be resolved with proper design guidelines and specifications.

The RCI team developed design criteria for two predominant types of streetlights: high pole fixtures that illuminate the right-of-way for drivers and bicyclists and low pole fixtures that augment the light on the sidewalk for pedestrians. We recommended that the city do away with globe fixtures, acorn fixtures and other pedestrian-scale fixtures that emit light in all directions because the glare is too overwhelming. In their place, we recommended down-firing tear-drop fixtures with cut-offs or up-firing LEDs that bounce light off horizontal white disks creating diffused light on the sidewalk. 

Furthermore, fixtures should feature cut-off devices and lenses that hide the LED light except if you're standing directly under the fixture. Color temperature should be adjustable from 2,500 Kelvin (warm light) to 5,000 Kelvin (white light) to accommodate the different needs of business districts and residential neighborhoods. 

The first 3,000 streetlight conversions to LED fixtures will be completed in Pittsburgh's business districts in the first quarter of 2012.

Photo credit: Lucas Jackson/Reuters

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