ADEQUATE street lighting is a basic requirement for safety and comfort in residential and commercial areas. As a crime deterrent, it provides security for residents and pedestrians. At highway interchanges, good levels of illumination will reduce nighttime vehicular accidents by giving the motorist visibility outside the path of his headlights.
With all these incontestable benefits, one might ask why our streets and highways are not more brightly lighted than they are. The answer, obviously, is economic, not necessarily due to the cost of the initial installation, but rather the ongoing cost of electric power. To offset continually increasing power costs, more and more communities are using cost-effective lamps such as high or low pressure sodium, which produce the same levels of illumination as alternative sources, but with greatly reduced power usage.
A good residential lighting system must provide adequate illumination on the roadway proper for vehicular traffic, on the sidewalks for pedestrians, and over the walkways and lawns for the local residents' safety, security, and comfort all the way to the front door. Because homeowners are sensitive to unwanted light and glare, the distribution must provide a precise control of the light with good uniformity and no objectionable glare. Also, lighting should be "topshielded" to reduce the loss of light and to help reduce the effects of "horizon glare" that is so prevalent around the country.
Since the American Standard Recommendation prescribes illumination in terms of footcandles on the horizontal plane and emphasizes reduced glare, there has been a tendency to use luminaires with sharp cutoffs of the main beam, which directs the light predominantly down onto the pavement surface. However, in so doing we have tended to overlook the importance of high angle emission, which reveals important vertical surfaces. There is a growing number of individuals who feel that photometric data should also be presented in terms of light intensity on vertical planes. This can very readily be done. Perhaps the most important thing is that we must recognize the fact that, especially on residential streets, it is desirable to have the light flux emitted in all directions, but it must be properly proportioned and precisely distributed to reveal all objects within the entire street scene without objectionable glare. This, then, is the ultimate challenge to the lighting engineer.
B-6.1 LIGHTING DESIGN
Candlepower is the luminous intensity of lamp or luminaire expressed in candles--a standard candle being the unit of luminous intensity. At a distance of one ft, a standard candle produces one footcandle upon a surface normal to the beam. The term candlepower is most commonly applied to the light intensity of a luminaire at given angles to a line normal to the road.
Lumen is a unit of luminous flux equal to the flux emitted through a unit solid angle (one steradian) from a uniform point source of one candle. The total output of lamps is usually expressed in lumens.
Footcandle is a unit of illumination defined as the illumination on a surface one square foot in area on which is uniformly distributed a flux of one lumen. A footcandle, therefore, is equal to a luminous flux of one lumen per square foot. The illumination of roadway surfaces is expressed in terms of average footcandles, a term that takes into account the variation of illumination of the roadway at all points.
There are two qualifications of average footcandles worthy of definition: a) Average initial footcandle level of illumination on a roadway describes the illumination when lamps are new and luminaires are clean; and b) average maintained footcandle level takes into account the depreciation of light output of the lamp throughout its normal life, dirt accumulation inside and outside of the luminaire, and other aging factors such as "frosting" or "sandblasting" of refractors and loss of reflector reflectivity. Since this term is more representative of actual conditions, the industry normally describes lighting recommendations only in terms of average maintained footcandles.
The first thing to determine is the number of horizontal footcandles (lumens per square foot) required on the pavement for various conditions. Both pedestrian and vehicular traffic counts are taken. Pedestrian traffic may be divided into heavy, such as on main business streets; medium, as on secondary business streets; and light, as on streets in an average residential district. Vehicle traffic units per hour for the maximum night traffic hour will be used. The amount of light on roadways is dependent on the type of roadway, the traffic volume, and the roadway surface classification. The recommended average maintained illumination levels range from 0.3 footcandles to 1.7 footcandles relative to the roadway classification. A measure of lighting quality is the uniformity of the lighting levels. An average to minimum ratio of 3:1 to 4:1 is recommended. Maintained values are determined by applying multiplying factors that model the effects of lamp lumen depreciation and dirt depreciation to the initial values.
Having obtained the number of footcandles, the next step is to determine the spacing of the lamps and their height. As a preliminary we need now to consider the various standard types of luminaires that are available. The vertical cone of maximum candle power is at an angle of about 75 degrees to the vertical.
Type I is a two-way lateral distribution having a preferred lateral width of 15 degrees in the cone of maximum candlepower. The two principal light concentrations are in opposite directions along a roadway. This type is generally applicable to a luminaire location near the center of a roadway where the mounting height is approximately equal to the roadway width.
Four-way Type I is a distribution having four principal concentrations at lateral angles of approximately 90 degrees to each other. This distribution is generally applicable to luminaires located over or near the center of 90 |degrees~ intersections.
Type II light distributions have a preferred lateral width of 25 degrees. They are generally applicable to luminaires located at or near the side of relatively narrow roadways, where the width of the roadway does not exceed 1.75 times the designed mounting height.
Four-way Type II light distributions, each with a width of 25 degrees. This distribution is generally applicable to luminaires located near one corner of right angle intersection.
Type III light distributions have a preferred lateral width of 40 degrees. This distribution is intended for luminaires mounted at or near the side of medium width roadways, where the width of the roadway does not exceed 2.75 times the mounting height.
Type IV light distributions have a preferred lateral width of 60 degrees. This distribution is intended for side-of-road mounting and is generally used on wide roadways where the roadway width does not exceed 3.7 times the mounting height.
The Type V light distributions have a circular symmetry of candlepower that is essentially the same at all lateral angles. They are intended for luminaire mounting at or near center of roadways, center islands of parkways, and intersections.
The visibility problem that occurs when a tunnel is entered during daylight hours is created by the rate of adaptation of the human eye adjusting from daytime intensities as high as 10,000 footcandles to the relatively low intensity in the tunnel. Even after the eye has adjusted to tunnel conditions, vehicles often appear only as silhouettes against the daylighted portal ahead. This silhouette effect is so great, that it is hard to judge the relative speeds of other cars in the tunnel.
B-6.11 High-Pressure Sodium (HPS)
Nurtured by concerns for fuel costs and energy conservation, the HPS luminaire is popular with lighting engineers. In some cases, HPS luminaires produce more illumination per dollar than low pressure sodium (LPS), which is more efficient in terms of lumens per watt.
In addition to energy economy, the compact arc tube of the HPS lamp allows controlled use of light output when used with efficient reflectors, refractors, and lenses. Life span of HPS lamps has increased steadily and is comparable with mercury vapor.
B-6.12 Low Pressure Sodium (LPS)
The LPS lamp, which produces more lumens per watt than any other roadway light source, provides light that is monochromatic yellow. It has a very large arc tube, which makes it difficult to efficiently control the light output of the lamp. Many LPS installations have been made on streets and highways, and LPS is particularly well suited for large parking areas.
B-6.13 Mercury Vapor/Fluorescent/Incandescent
Mercury vapor, the most common lighting element on our highways, produces a light that is bluish white.
Fluorescent units emit soft, diffused light at low brightness, which practically eliminates glare. Even when mounting heights are limited, such as on bridge superstructures or in tunnels and underpasses, the fluorescent source provides "comfortable seeing."
Incandescent luminaires have the advantage of using lamps of simple, dependable construction. The small size and weight of the units are advantageous where long bracket arms are required. Servicing and maintenance are simple.
B-6.14 Metal Halide
The value of metal halide luminaires has been their advantage in light output over some comparable units. This advantage has been offset in part by the shorter life expectancy of low-wattage metal halide lamps.
Manufacturers of street and area lighting luminaires include AmeriLite, A Div. of D&P American Products; Crouse-Hinds Lighting; GE Lighting Systems; Herwig Lighting; Holophane Co., Inc.; Kenall; OMJC Signal, Inc.; Osram Sylvania Inc.; Sentry Electric Corp.; Sternberg Vintage Lighting; Sun Valley Lighting, Inc.; Thomas Industries; WideLite |R~, A Genlyte.
Ornamental luminaires available with mercury, high-pressure sodium, or metal halide lamps for street, park, and urban restoration are supplied by AmeriLite, A Div. of D&P American Products; Classic Lamp Posts; Hanover Lantern, A Div. of Hoffman Products, Inc.; Herwig Lighting; Holophane Co., Inc.; Sentry Electric Corp.; Sternberg Vintage Lighting; Sun Valley Lighting, Inc.; Thomas Industries. Ornamental street light globes made of durable, vandal-resistant plastics are furnished by Classic Lamp Posts; Rotocast Plastic Products, Inc.; Sternberg Vintage Lighting. Vandal-proof lighting fixtures for all major lighting systems are manufactured by Kenall; Urban Systems "Streetscape" Inc.
Luminaires for use in hostile environments such as wastewater treatment plants, are furnished by GE Lighting Systems; Holophane Co., Inc.
Solar-powered, stand-alone lights for lighting remote sites such as parking areas, campgrounds, bridges, and hiking trails are available from Solar Electric Systems of KC, Inc.; Southwest PV Systems Inc.
Lighting for tunnels, overpasses, and subway systems is supplied by Thomas/Schreder Lighting.
High-pressure sodium lamps are supplied by C.E.W. Lighting, Inc.; Osram Sylvania Inc. Metal halide lamps are furnished by C.E.W. Lighting, Inc. Mercury vapor lamps are offered by C.E.W. Lighting, Inc. Retrofit lamps and replacement ballasts are supplied by C.E.W. Lighting, Inc. Replacement ballasts are also offered by OMJC Signal, Inc.
Lighting controls are available from Automatic Switch Co.; Precision Multiple Controls. Inc.; TORK.
Portable testing equipment for testing lumalox, high-pressure sodium, and mercury vapor lighting systems is supplied by Sunshine Instruments.
B-6.2 POLES AND HARDWARE
Luminaires are usually mounted on brackets supported by a wood, metal, or concrete pole. Many types and styles are available. Current emphasis is on taller poles, longer brackets on mast arms, and breakaway capabilities. From a safety standpoint, aluminum and stainless steel on breakaway hardware are the materials of choice.
Steel's popularity stems from reasonable prices, tradition, and color options. They do require a good grade of base paint and periodic repainting. Steel poles are available in many designs for all types of applications.
Stainless steel poles (with nickel content) are available from several sources. They are attractive, strong, and relatively maintenance-free.
The tapered aluminum pole is extensively used. Light weight makes installation easy and economical. Its light weight and no maintenance have made aluminum a favorite on bridge and overpass applications. Tests and experience have proved that aluminum poles resist corrosion even under industrial and salt spray conditions.
Centrifugally cast, steel-reinforced concrete poles have been improved by prestressing, which reduces the thickness of the steel rods necessary and increases the density of the concrete. They can have smooth or textured surfaces.
Concrete poles are practically maintenance free, remaining permanently unaffected by exposure to salt conditions and industrial atmospheres. They are attractive in appearance, fulfilling one requisite of the ornamental standard in that they blend well.
Fiberglass poles are available in lengths up to 45 ft, and are lightweight, requiring only a small crew for installation. They are also corrosion and stain resistant and come in a variety of colors.
Light poles and associated hardware are manufactured by Crouse-Hinds Lighting; GE Lighting Systems; Hanover Lantern, A Div. of Hoffman Products, Inc.; Herwig Lighting; Holophane Co., Inc.; Millerbernd Mfg. Co.; Shakespeare Co., Electronics and Fiberglass Div.; Spring City Electrical Mfg. Co.; Traffic Engineers Supply Corp.; Valmont Industries.
Fiberglass light poles are furnished by Shakespeare Co., Electronics and Fiberglass Div.; Sherman Utility Structures, Inc.
Breakaway highway lighting posts are available from Crouse-Hinds Lighting; Transpo Industries, Inc. Fiberglass breakaway poles are supplied by Sherman Utility Structures, Inc.
Street light foundations that require no excavation or concrete are supplied by A.B. Chance Co.
A spring-loaded banner frame (for displaying banners at holiday time or other special occasions) from Traffic and Parking Control Co. Inc. attaches to light poles.
Landscape architects are finding many inventive and appealing ways to impart a feeling of traditional comfort to modern structures by using antique reproduction lighting posts and bollards. Authenticity is the primary factor to achieve just the right blend of new and old that these lighting posts provide. After consulting with city or town planners as to what kind of "feel" the project needs, the architects look first to existing installation for inspiration, then to manufacturers' catalogs. The interchangeability of luminaires and various available post heights for antique reproduction lighting posts allow architects to tailor the amount of light required to meet any civic or pedestrian need. The result is an esthetically pleasing light source that conveys the flavor of the desired time period and locale.
Antique cast iron lamp posts are made by American Site Furniture, Bench Div.; Classic Lamp Posts; Sentry Electric Corp.; Spring City Electrical Mfg. Co.; Sun Valley Lighting, Inc. Spring City Electrical Mfg. Co. also furnishes cast iron bollards for use with matching posts. Antique cast aluminum lamp posts are supplied by Sternberg Vintage Lighting. Antique cast bronze and cast aluminum lamp posts are furnished by PeCo Inc.
Coordinated high-mast systems consisting of luminaire, lowering device, and pole are offered by Holophane Co., Inc.
Luminaires for use in high mast highway situations are available from Crouse-Hinds Lighting; Holophane Co., Inc.
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|Title Annotation:||1994 Public Works Manual|
|Date:||Apr 15, 1994|
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