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Sixty Percent Less Energy With LED Traffic Lights

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Schematic figure of an LED disk and a single LED

In a conventional red traffic light the monochromatic appearance is achieved by subtracting all the non-red wavelengths. First, the lamp produces white light, i.e. full-spectrum light. The green, yellow, and blue wavelengths are absorbed by the red lens and converted into heat. In the case of arrow traffic lights, still more light is wasted by the screen that blocks out all light except for that escaping through the arrow-shaped hole. A LED, on the other hand, produces monochromatic light of a given color. Thus, an American, red, 12-inch arrow, which is equipped with a 150-watt incandescent lamp, can be replaced by a 9-watt LED array, resulting in a 94 percent savings in power! For a full ball light, the difference is smaller, but the savings potential is still impressive. A conventional red traffic light in Europe (which is normally smaller than in the US) typically uses a 70-W conventional incandescent lamp or an equivalent 30-W halogen lamp. These lights can be replaced by a 10-W LED package. (The exact figure depends somewhat on the number of diodes and how they are arranged.)

FIELD TESTS AND SAVINGS
Red LEDs are now being tested in a number of places in Canada and the United States (See Red LEDs in Philadelphia and Minnesota, IAEEL 3-4/95). In Europe, Sweden and France are leading the way. Sweden could save at least 3 megawatts (including peak capacity), or up to 2 million kWh energy, on red lights alone for its 50000 traffic lights. In the US's 260000 intersections, 300 megawatts could be saved, or $300 million in electricity costs annually. In Finland, the Finnish Center for Information on Energy, Motiva, is planning a demonstration project together with the city of Helsinki. A preliminary estimate of the theoretical savings potential is ~1,5 million kWh per year for the Finnish capital's 18000 traffic lights.

The energy and power savings alone can pay for the extra costs of the LED retrofit in many countries, even though the LED package is expensive. Today, with the market still fairly small, a red retrofit set costs some $230 according to Sven Persson of Stockholms Entrepenad, the body responsible for maintaining traffic lights in the Swedish capital. Although electricity is cheap in Sweden, a switch to LEDs is still considered very economical. Very large savings can also be realized in maintenance costs. For example, the light bulbs in the red lights are presently replaced three times per year in Stockholm. By contrast, the expected life of the LED packet is at least ten years. There might also be other benefits apart from not having to replace lamps. LED disks don't seem to be as prone to light depreciation, due to dust adhering to the lens, as incandescent signals; thus there may be less need for cleaning. This hypothesis is now being tested in the field in Solna, a town neighboring the Swedish capital Stockholm.

The technology used so far for tests in Stockholm is of US/Canadian origin but modified for use with the European 230 volt standard (It's 120V in the US). Substantial losses are associated with the transformers needed for this conversion which reduce the energy savings. However, the Swedish light-pole manufacturer ITAB in Jönköping, southern Sweden, recently obtained a license from the Canadian Ecolux Inc. and is now starting to produce packages specifically for the European voltage, thereby avoiding the current transformer losses.

GREEN:THE MISSING LINK
There are now red and yellow LED lights that meet both North American and European standards, which are fairly similar. The red lights are also the most important since they are on about 65 percent of the time. The yellow light is on only 3 percent of the time, so it makes little economic sense to change them now, given their high costs. ("Portland orange" LEDs, used in US for pedestrian signals, also meet the standards.)

As of November 1995, there still are no green LEDs that meet the standards for car traffic lights, though they are considered good enough for pedestrian and bicycle signals in some countries. They are common in France, for example.

"Developing traffic standard green LEDs is the critical factor", says Kalle Hashmi, project leader at the Department of Energy Efficiency at Nutek, the Swedish Board for Industrial and Technological Development. Nichia Chemical Industries of Anan, Tokushima, Japan, is close to meeting the specifications but they are still too much on the blue side. Hashmi hopes that even if it does not fit the bill exactly, a few yellow-green LEDs mixed in the cluster could do the trick. According to sources at HP, the company may have a green LED that meets the standards for road safety by the end of 1996.

CHANGING THE POLE DESIGN
If this can be achieved, it would be worthwhile to switch the yellow lights as well, even if hard to justify in terms of energy and maintenance savings alone. However, if all three signals were to consist of LEDs it would be possible to change the whole pole design, opting for flat-structure, light-weight signals. The power demand could then be lowered to the extent that the grid connection could be disposed of and replaced by photovoltaics. "That's what we hope to achieve once we get the green LEDs working," says Hashmi.

The power consumption of the traffic flow control equipment (mostly computers) is small compared with that of the lamps, but could be substantial at an intersection with all-LED signals. That does not worry Hashmi, who foresees the development of independent, "smart" poles connected to each other with radio links only. The traffic-light pole would then be installed and remain in service for 15 years or so, with virtually no need for maintenance.

Hypothetical examples of variable cost reductions for the city of Stockholm's Western District (92 intersections). The real investment costs, should the city decide to embark upon a large-scale retrofit project, are not exactly known. But even today's quite expensive retrofit sets would be very cost-effective, the city claims.

ADDITIONAL BENEFITS
We are not yet there. Nevertheless, LED traffic lights seem to offer a number of additional benefits:

• Conventional incandescent lamps have reflectors. With low angle, perpendicular sunlight, it is difficult to tell whether the signal is red, green, or yellow. Since drivers coming in the opposite direction can be blinded by the same low sun, dangerous situations can arise. LEDs have fewer such problems since they do not need reflectors.
  
  
• Even with a strict maintenance schedule that aims at replacing the lamps before they fail, failures do occur. The red-light failures are obviously the most dangerous. LEDs rarely fail, but even if a few of some hundreds of LEDs did break, nobody would notice. (However, to some degree this depends on the kind of series/parallel connection of the diodes. If string connected, one diode failure can cause one of maybe 25 strings to go dark.)
  
  
• Maintenance, planned or unplanned, disturbs the traffic flow. LEDs can be expected to run for at least 10 years.
  
  
• LEDs often meet stricter demands than their predecessor lamps, owing to scrupulous specifications and extra caution by the producers, for example, with regard to luminance and its angular distribution, and to performance under a varying applied voltage - down to 89 volts from a standard 230 in Europe.
  
  
• The plating of the reflectors takes a lot of energy to produce and is environmentally harmful. The less need for it, the better.
  
  
• According to the independent Colorado-based energy information service E-source (February 1994) it has been estimated that in the Canadian province of Quebec alone, 400 to 500 signals are replaced every year after having been shot at with guns. This also seems to be a problem in the state of Oregon, US, where a "shoot-out" was arranged. "After sustaining shots from a pellet gun, a high-powered pellet gun, a .22-mm long rifle cartridge, a 9-mm handgun, and a 12-gauge 3-inch shotgun shell fired from 25 feet, only 112 of the 600-odd LEDs in the disk had failed." And after sustaining this damage, reports E-Source, it was still a viable red light.

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