A 15
watt LED stop light lasts 20,000 hours (about 10 times as long as the
incandescent stop lights). The reason these lights are so
efficient compared to normal incandescent (traffic lights typically use between a 60 watt and 150 watt bulb) lights is because almost all the power put into these lights comes out as
visible light energy
of a narrow frequency band (about 20 to 50 nm) rather than some visible light, some heat and some
other frequencies which is then filtered. This advantage becomes especially prevalent in the arrows where a
stencil blocks a large portion of the light. Fewer
LEDs are required to handle these lights. Overall, an average incandescent traffic signal uses about 1000 kWh/year while the LED traffic light uses 100 kWh/year. Within California, this is expected to save Caltrans (which operates 7% of the traffic lights in California) $3 million per year in reduced energy costs.
Each light is composed of a honeycomb of about 300 LEDs. From a distance, this appears to be a solid light, however up close it is possible to see distinct points of light. Each green LED costs about $0.65 while the
red and amber LEDs cost $0.20. Furthermore more green and amber LEDs are necessary to get the same light power as the red LEDs - it requires a higher luminescence density to get the required "no missed signals" for yellow and green compared to red. In most cases, the first lights to get this new replacement are the red lights.
Initially, the red LED stop lights cost on the order of $750 however, they have since come down in price to about $100 (yellow lights are about $150). While this is more expensive per head for materials compared to a $3 replacement bulb for an incandescent lamp the advantages of lower power consumption and longer life do come into play. A cost that is hard to compute is that of human life - that of drivers when a stoplight burns out and the repair crews (often in the middle of the intersection on a ladder of some type) fixing emergency situations with the hazard of impaired drivers.
One disadvantage is related to the effect of temperature upon the light - each half degree above 70 degrees F results in a 1% loss of energy output. In the hot areas of California this can cause a problem. However, even with 85% degradation of output this is still within acceptable brightness range. Furthermore, a energy regulator boosts the power as the temperature degrades the performance. In cold weather, the LEDs used in traffic lights become more efficient.
Another disadvantage is with color blind people. Because of the narrow band of light from the LED rather than the broader spectrum of the incandescent, individuals who are color blind will have a harder time seeing the lights.
http://spectacle.berkeley.edu/news/97/news70811.html
http://www.lrc.rpi.edu/Ltgtrans/LED/
http://ledmuseum.home.att.net/