In the UK the order of the lights are:
RED        Stop
RED+AMBER  Prepare to move away
GREEN      Free to go provided your exit is clear
AMBER      Stop at the light provided it is safe to do so
RED

although the order and law may vary in other countries. In the US the order is:

RED
GREEN
AMBER
RED
A "traffic light" is also the name of a cocktail, consisting of three layers. Making them is not really easy, since you have to pour three different drinks into a glass, without mixing them.

First, fill 30% of the glass with red berry liquor. Here in Holland they sell it, but I don't know about other countries. Of course you can try it with other red liquors.
Then carefully pour orangejuice on top, so that it won't mix. This can be accomplished by holding a spoon in the glass and slowly pouring the juice on it. Fill the glass with orangejuice until about 60% of the glass is full.
Finally, add Pisang Ambon, a green liquor with banana-taste. Pour this in the same way you did with the orangejuice: very carefully. Don't fill it to the top of the glass, or else you can't add ice.
If you do it right, you can see three layers in the glass.
Put in a straw, and enjoy.

A traffic light is an object, usually found hanging in the middle of intersections of roads, and usually not found on highways, except in intersections in small towns. The purpose of the traffic light is to control the flow of traffic at that intersection so everyone can drive through safely. Most traffic lights have 3 lights on them, Red, Green, and Yellow. When the light is green, one can safely pass through, if no one is running a red light. Yellow means caution, because the light is about to turn Red. Red means stop, don't go. While you "have the red light" another road "has the green" and they can go.

Some traffic lights have more than those three lights. These extra lights are usually green, with an arrow shaped gobo infront of the light. These lights are used to tell when it's safe for a lane to turn, or not. If the arrow is green, it is safe. If the extra light is not green and the traffic light displays green for your road then turn when it is safe too.

A brief history

The very first traffic light was actually installed in Parliament Square, Westminster, London on 9th December 1868, a long time before the invention of the motor car. Consisting of a gas lamp on a revolving pillar, it had only two colours, red for 'Stop' and green for 'Caution'. It was operated by a policeman who turned it with a lever, so that the appropriate light faced the traffic. Sadly, on 2nd January 1869, it exploded, injuring the constable operating it.

A Detroit, US police officer, William Potts, devised a method of controlling traffic using electric lights. Using the same light colours (red, amber and green) used in railway signals, he installed his device on the corner of Woodward and Michigan in 1920. A year later, Detroit had fifteen such manual traffic lights.

In 1923, Garrett Morgan (reportedly the first African American to own a car in Cleveland, Ohio), designed and built the automatic traffic light. His semaphore design was superceded by the familiar three-light system still in use today, but the principles of operation remain unchanged.

Wolverhampton was the first city in the UK to use electric traffic signals. Installed in November 1927 in Prince's Square, they paved the way for similar signals throughout the UK.

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/

When I was living in Beijing in 1998, I happened upon a smart little innovation: some traffic lights were equipped with a device that showed the time left until the light changed. It was just a bar of lighted LEDs, getting shorter with time. When it was gone, the light would change.

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