Streetlight Basics

State and municipal roadway lighting accounts for roughly one third of all light pollution in the United States. That's because there are so many of them (for example, 240,000 in the city of Los Angeles alone), and because virtually every one of them stays on all night, every night. That's about 4,100 hours per year.

The most common streetlight is a cobra-headed fixture with a glass "salad bowl" enclosure for the bulb, like the one shown at lower left. These streetlights emit up to 5% of their light above horizontal and up to another 15% in the "glare zone" within the 10° angle just below horizontal (much of which also ends up in the sky).

Typically streetlights use one of three bulb types: high-pressure sodium (HPS), the most common, emits a peach-colored light; metal halide (MH) has a natural white color; mercury vapor (MV) is white with a tinge of blue-green. Mercury-vapor fixtures are disappearing rapidly because they're not very efficient, have relatively short lifetimes, and contain toxic mercury.

Another important characteristic is how much power the fixture uses, which is usually indicated by a large decal affixed to the underside of the housing. The number indicates the bulb's wattage: "5" means 50 watts, "15" means 150 watts, and so on. You can get a good feel for how much it costs to operate a streetlight by checking the comparison table in the Bulb Basics section.

The good news is that many municipalities and utility companies are now installing dark-sky-friendly streetlights that are fully shielded — all of their light is directed below horizontal. These are easy to spot because they have flat glass on their undersides (as shown at upper right). Because none of its light is wasted, a fully-shielded fixture can often deliver the same intensity of light (illuminance) on the ground using a lower-wattage bulb.

The bad news is that very little streetlighting is done with a logical lighting design in mind. You'll often see streetlights here and there along the roadway with no logical reason for them being placed where they are. Or you'll encounter streets and highways that are continuously lit, with evenly spaced poles stretching mile after mile. Transportation engineers have begun to question such wasteful practices. For example, the State of California, which maintains thousands of miles of freeway, rarely employs continuous lighting.

The worse news is that many cities and towns have purchased antique-style streetlights in an effort to dress up their downtown areas. Many of these "period" fixtures are attractive by day but horrible light polluters at night, as illustrated in the photos below.

Light-emitting diodes are a promising new technology that, in theory, can provide well-directed, fully shielded streetlighting with very low energy consumption. Currently the cost of LED streetlights is quite high, though some cities (such as Ann Arbor, Michigan) have begun testing them.