LED traffic signal modules as an incandescent lamp alternative

Institute of Transportation Engineers. ITE Journal, Apr 2003 by Iwasaki, Randell H

As the initial phases of the test ended, the AlGaAs technology was performing better and became the LED of choice. At first, meeting intensity targets was difficult but experimentation with different configurations of LEDs and power supplies generated sufficient output almost to equal that of their incandescent counterparts.

As with the development of any new technology, laboratory research was needed to look into the aspects of adopting LED signal performance. Caltrans contracted with the University of California-Berkeley to conduct human factors studies to compare the perception of light from LED sources with light from incandescent sources. The research concluded that red light from an LED light source was just as visible to the human eye as filtered red light from an incandescent traffic signal. (It may seem intuitively obvious that the human eye should be able to see red LED light, but there was concern that the human eye might react differently to the monochromatic LED light compared to the wide-spectrum incandescent red light from a spectrum-restrictive red lens.)

In 1995, Caltrans was awarded the California Energy Commission (CEC) State Energy Award for the Fresno LED field test project. In the same year, the U.S. Department of Energy awarded Caltrans its National Energy Award for the same project and for demonstrating low power solutions for traffic signals. These awards put California's LED efforts in the national spotlight.

BROADER SCOPE FOR LEDS

With this national recognition, other agencies got on the LED bandwagon, including the states of New York, Oregon, Vermont and Minnesota and the cities of Denver, Philadelphia, San Jose, Sacramento and San Diego. Many of these governments started red module deployments in 1997 and 1998 that were larger than Caltrans' initial test bed.

At about the same time, California utilities like the Sacramento Municipal Utilities District and Pacific Gas and Electric Company (PG&E) as well as CEC began to recognize the value of LED traffic signals as a viable load management and conservation retrofit. They began to provide co-funding resources for both field testing and/or monitoring of products as well as product rebate incentives. The cost effectiveness of LED signal upgrades started to look "do-able". In the end, Caltrans leveraged $4 million in rebate programs from PG&E, CEC and others to help offset the cost of the LED modules.

After the conclusion of the University of California study, Caltrans was in a position to start assembling a purchase specification. Starting with information from ITE-sponsored efforts, Caltrans put together an aggressive specification that included not only intensity and power requirements but also requirements for allowable total harmonic distortion, power factor and a wide operating temperature range. By fall 1997, this specification was ready for "prime time"; in winter 1998, Caltrans went out for the purchase of 4,000 red ball and arrow modules. From that point on, the specification for red has remained essentially unchanged. This also is the specification that has served as a basis for other colors and configurations of LED modules.