The new traffic lights you are seeing are made out of arrays of light emitting diodes (LEDs). These are tiny, purely electronic lights that are extremely energy efficient and have a very long life. Each LED is about the size of a pencil eraser, so hundreds of them are used together in an array. The LEDs are replacing the old-style incandescent halogen bulbs rated at between 50 and 150 watts. Most cities are in the process of replacing their incandescent traffic lights with LED units because of three big advantages:
- LEDs are brighter. The LED arrays fill the entire "hole" and have equal brightness across the entire surface, making them brighter overall.
- LED bulbs last for years, while halogen bulbs last for months. Replacing bulbs costs money for the trucks and people who do the work, and it also ties up traffic. Increasing the replacement interval can save a city big dollars.
- LED bulbs save a lot of energy.
The energy savings of LED lights can be huge. Assume that a traffic light uses 100-watt bulbs today. The light is on 24 hours a day, so it uses 2.4 kilowatt-hours per day. If you assume power costs 8 cents per kilowatt-hour, it means that one traffic signal costs about 20 cents a day to operate, or about $73 per year. There are perhaps eight signals per intersection, so that's almost $600 per year in power per intersection. A big city has thousands of intersections, so it can cost millions of dollars just to power all the traffic lights. LED bulbs might consume 15 or 20 watts instead of 100, so the power consumption drops by a factor of five or six. A city can easily save a million dollars a year by replacing all of the bulbs with LED units. These low-energy bulbs also open the possibility of using solar panels instead of running an electrical line, which saves money in remote areas.