Advancing the effective use of light for society and the environment.

Friday, January 16, 2004

LED traffic signal study evaluates short-term performance, energy consumption

All across the United States, municipalities are replacing the incandescent lamps in their traffic signals with newer, energy-efficient LEDs that promise longer life and less maintenance. Proposed legislation in New York would require all new installations or retrofits of existing traffic signals to use LEDs for the red and green balls. But are LED signals living up to expectations?

“Typically, signals are not tested after installation, which means we don’t know how well they are performing initially or over time” said Yutao Zhou, LRC research specialist. Zhou, along with LRC Transportation Lighting head John Van Derlofske and a team of graduate students, recently evaluated the short-term performance and energy consumption of LED traffic signals currently on the market.

“From this study, we wanted to know whether LED traffic signals are meeting their performance requirements. Failure to meet the requirements may make it difficult for drivers to see the signals,” Zhou said.

LRC researcher Yutao Zhou sets up a red traffic signal ball for evaluation.

Experiment evaluates whether traffic signals meet standards

Traffic signals are required to meet certain color, brightness and visibility guidelines. These guidelines are set by the Institute of Transportation Engineers (ITE), a standards-setting organization for transportation systems. For LED traffic signals, ITE specifies a minimum and maximum luminous intensity distribution, a metric that describes the intensity of a light source. ITE also specifies a 3-year timeframe that a signal must maintain this intensity.

LRC researchers tested a series of 8-inch and 12-inch red and green LED traffic signal balls from eight manufacturers and compared their luminous intensities and energy consumptions. Tests were conducted at two ambient temperatures and four directional positions. The results showed that performance can vary dramatically across manufacturers, and that not all LED traffic signals are ITE-compliant. For example, at 25°C three signals failed to meet ITE specifications because their luminous intensities were either insufficient or too great.

“LED signals that are too bright or too dim should be avoided for safety reasons,” said Zhou. “A low intensity level could result in a driver not seeing a signal, while a higher level could cause glare and be uncomfortable.”

The energy evaluations showed a dramatic reduction in energy consumption; the tested LED signals demonstrated a 83% to 94% energy savings at 25°C compared with the average incandescent signal. Under elevated temperatures, however, the energy consumption for some signals increased and their light output dropped. “This indicates that the luminous efficacy for these signals dropped significantly, meaning that some manufacturers’ signals might not be as energy-efficient in hotter climates.” However, Zhou adds that these signals are still considerably more efficient than their incandescent counterparts.

More studies needed on long-term performance

An overwhelming majority of the signals passed the test initially, although there is no guarantee that they will continue to perform well, said Zhou. Previous LRC studies have shown that LED light output, depreciation rate and life can vary depending on numerous factors, including system component interactions and operating temperature. LED signals are also subject to electronic and mechanical failures that can affect performance. “Since the long-term performance of an LED traffic signal relies on many variables, it’s not certain whether all products will meet the ITE requirements over time,” said Zhou. He added that meeting requirements at one temperature does not necessarily mean the signal will meet requirements at other temperatures.

Zhou and Van Derlofske presented their findings to ITE last August and at the Transportation Research Board’s annual meeting this month. They noted that the next step will be to gather long-term performance data.

Related Stories

LRC evaluates LEDs for use in outdoor signage (October 2003)

LRC develops new LED system evaluation method (July 2003)

LEDs rapidly advancing to meet lighting needs (July 2003)

About the LRC

The Lighting Research Center (LRC) is part of Rensselaer Polytechnic Institute and is the leading university-based research center devoted to lighting. Founded in 1988, the Lighting Research Center has built an international reputation as a trusted and reliable source for objective information about lighting technologies, applications, and products. Its mission is to advance the effective use of light and create a positive legacy of change for society and the environment

2004 Rensselaer Polytechnic Institute, Troy, NY 12180 USA.

Rennselear Polytechnic Institute