Historic Streetscape Lighting: Integration of Aesthetic Concerns with Modern Technology

Street lighting can contribute significantly to the visual environment of a historic downtown. Lighting technologies used in modern street lighting systems are very different from those used in street lighting a century ago. The present paper compares the lamps, luminaires and design approaches used for street lighting when it was first introduced to those of the present day. Also provided are some preliminary considerations for the selection of lighting equipment to optimize the benefits of historic streetscape lighting in terms of safety, context sensitivity and historic compatibility, against the costs of street lighting in terms of equipment, energy, maintenance and light pollution concerns. Carefully designed street lighting can be an important component of maintaining historic character as well as contributing to safety in historic downtowns. TRB 2013 Annual Meeting Paper revised from original submittal. Bullough and Bullough 2 INTRODUCTION Street lighting can contribute significantly to a context-sensitive and appropriate visual environment of a historic downtown containing shopping, dining and entertainment (Leslie and Rodgers, 1996). At the same time, technologies used in present and future lighting systems differ substantially from those used near the introduction of electric illumination systems for public lighting. Understanding the technical and visual issues related to historic street lighting can help design professionals and transportation engineers ensure that lighting enhances the rich history many downtowns have while at the same time providing optimal visibility, safety, and energy efficiency. BENEFITS AND COSTS OF STREET LIGHTING It is important to understand why streets are illuminated in the first place. Vehicle and pedestrian safety and personal security as well as that of property are among the issues likely to be most important to government entities. Many governments include lighting as part of their strategy to bring people into downtowns for shopping, dining and entertainment. And because many downtowns have some historic character, lighting is often deployed as a way to reinforce that character during the daytime as well as the nighttime. These are the supposed benefits of street lighting, but they have to be balanced by the costs, including the cost of luminaries and poles, lamps, energy costs, lamp replacement costs, and even environmental costs such as light trespass that may disturb neighbors, glare, and light pollution – the loss of the dark nighttime sky. Several investigators have studied whether, and how much, street lighting actually achieves the benefits that it is intended to achieve. It seems only logical that light helps vehicle drivers see at night, and that this should improve safety. The evidence suggests that this is indeed the case. A review of studies of traffic safety conducted by the Commission Internationale de l’Eclairage (CIE, 1992) found that the presence of roadway lighting was often associated with statistically significant reductions in nighttime crashes. Bullough and Rea (2011) compared visibility improvements from roadway intersection lighting (Rea et al., 2010) and nighttime crash reductions (Donnell et al., 2009, 2010) in the state of Minnesota. Figure 1 indicates, for a few different types of intersections, that when visibility is improved by installing street lighting, the night-to-day crash rate was reduced approximately proportionally to the visibility improvement. FIGURE 1 Relationship between visibility improvements from street lighting (∆RVP) and reductions in the nightto-day crash ratio (∆N/D) for several intersection types (Bullough and Rea, 2011). For other issues, like crime prevention (Tien et al., 1979) and economic development, the evidence is not as strong. Some well-lighted locations have very high crime rates, and lighting alone will not automatically generate foot traffic and associated increases in business. Many other factors are critical, of which lighting is only one, and probably not the most important. Without frequent police patrols, lighting may only help the burglar or vandal. And people need more reason than attractive luminaires to visit a downtown. Still, lighting can be a part of an intelligent security and economic development strategy, and reinforcing historic character is certainly a worthwhile objective. LIGHTING TECHNOLOGIES Luminaire Types Lantern, acorn and teardrop fixtures are common in historic downtowns, but usually these are not themselves historic fixtures. Rather, they are modern ones with a historic-looking appearance to match ones like the gas street TRB 2013 Annual Meeting Paper revised from original submittal. Bullough and Bullough 3 lights in Figure 2. Street lighting technologies have evolved substantially since it first emerged more than 100 years ago. Light levels were much lower 100 years ago than recommended at present (IES, 2000), being provided by less efficient gas or incandescent lamps, rather than the high pressure sodium (HPS) lamps common today (NYSDOT, 1995) and the light emitting diodes (LEDs) of tomorrow, and fixtures were typically spaced less than 100 ft apart. Most functional street lights today have the well known “cobrahead” shape shown in Figure 3, mounted 25 to 30 ft above the road and often spaced 150 ft or more apart, whereas older street lights were only 10-15 ft above the ground. In addition, as part of efforts to help reduce light pollution, cobrahead luminaires now use flat glass lenses instead of dropped lenses so that they emit all their light downward instead of both up and down. Almost all new cobrahead luminaires used to light highways will have flat lenses like the one shown in Figure 3. FIGURE 2 Appearance of historic gas street light luminaires (from American City magazine, 1920). FIGURE 3 Appearance of a modern cobrahead luminaire. Timing Control Another difference between street lighting of a century ago and that of the present has to do with timing. Most street lights burn from dusk to dawn (Bullough, 2010). A century ago, when street lights could barely compete with the full moon in terms of brightness, many cities and towns used them on a “moonlight” schedule in order not to waste gas or electricity, which was not the commodity often taken for granted in the present day. It was often considered not only redundant but even wasteful to illuminate streets when the moon was full. A scanning tour of roadway lighting practices in Europe (Wilken et al., 2001) found that dynamic roadway lighting, using lower light levels during periods of reduce traffic, was increasing in use. Interestingly, in these days of fiscal challenges, many local governments are looking to reduce costs, and street lighting is often one of the top municipal expenses along with water and wastewater treatment. Present recommendations for light levels along roadways are well above those provided by moonlight, but the idea of adaptive lighting makes some sense when considering that there are usually fewer pedestrians and cars on the road at 4 a.m. than at 10 p.m. (Ivan et al., 2002), and perhaps less need for higher light levels for safety throughout the entire night. It has been estimated by the National Lighting Product Information Program (NLPIP) that adaptive or dynamic street lighting might save 30% to 50% of the energy costs compared to conventional dusk-to-dawn lighting (Bullough, 2010). TRB 2013 Annual Meeting Paper revised from original submittal. Bullough and Bullough 4 Light Sources As mentioned previously, first gas and then incandescent lamps were the main light sources available for street lighting when it was first used. Relatively speaking, these sources are much less efficient and produce substantially less light than the primary street lighting lamps of today (Rea, 2000), and mantles and incandescent bulbs have much shorter lives, as well. Both gas light and incandescent illumination is often a “warm” yellowish-white color. The most common lamps used for street lighting today are HPS lamps (NYSDOT, 1995). These sodium vapor lamps are even yellower in color appearance than gas and incandescent. HPS lamps are also very efficient, and have long rated lives, a feature that makes them popular among highway engineers. The efficiency and high light output of lamps such as HPS means that street lights can be located hundreds of feet apart, and their long life means the lamps can last 6 to 9 years between replacements, instead of 6 to 9 months for gas and incandescent sources. However, the yellow color of HPS illumination is usually perceived as less bright and judged as less safe than “white” illumination from metal halide (MH) lamps (Rea et al., 2009), which have begun to be used more frequently for outdoor lighting. It is also easier to see different colors under the whiter light (Rea et al., 2009). However, highway engineers do not prefer MH lamps because they are less efficient and have shorter lives than HPS lamps. LEDs are already starting to be used in some street lighting installations (Radetsky, 2010, 2011). These newer sources have promise for very long lives, up to 20 years, and also can produce a range of white illumination colors from warm to cool white. Table 1 lists the properties of several light sources used for street lighting in the past, present and future. TABLE 1 Properties of Light Sources Used for Street Lighting Integration of Lamps and Luminaires Using a MH or HPS lamp inside an acornor lantern-style luminaire mounted on a pole that is 10 or 15 ft high is a common way to provide a semblance of historically-accurate appearance during the daytime, but it is also important to consider that at night, such street lights can produce 20 or more times as much light than the gas street light of a century ago (Figure 4). The combination of higher light output (in lumens, lm) and relatively low (compared to typical overhead street lights) mounting heights can result in substantial discomfort glare. In addition, many historic-appearing acorn and other post-top luminaires that enclose the lamp with a decoratively shaped glass or acrylic refractor lens, might not only contribute to glare, but these luminaires can also emit substantial amounts of light upward, where it usually does not serve any purpose except to produce unwanted light pollution (Brons et al., 2008). However, not all luminaires with historic appearance are the same (Leslie and Rodgers, 1996). Some luminaires, known as decorative cutoff types, are similar in their optical design to cobrahead systems where the lamp is mounted below a metal reflector, which spreads the light mainly downward through a clear glass lens, reducing light pollution. TRB 2013 Annual Meeting Paper revised from original submittal. Bullough and Bullough 5 a. b. FIGURE 4 a. Gas luminaire producing 630 lm, b. Acorn luminaire with a MH lamp producing 14,000 lm. Sometimes there is a glass or acrylic chimney inside the lower part of the housing where a mantle might have been located in a gas street light of the early 20th century, to better simulate the historic daytime appearance of gas lighting. These types of luminaires may be better at reducing glare as well as light pollution because more of the light is directed downward. However, when viewed from a fairly close distance of approximately 20 to 30 ft, it can sometimes be possible to have a direct view of the lamp, which could create substantial discomfort glare, especially since the luminaire height is lower than a conventional cobrahead street light. In general, for both post-top and decorative cutoff luminaire types mounted at 10-15 ft, meeting the light levels usually recommended for roadways (IES, 2000) will require shorter luminaire spacing than needed with cobrahead luminaires mounted 25 to 30 ft high. OPERATION OF STREET LIGHTING For many municipalities, street lighting systems are leased from the local electrical utility, which installs, maintains and energizes the lights for a flat, monthly fee per street light. This allows the costs to be spread over a long time, which is attractive because a municipality often does not have to access large amounts of funding to install street lighting systems. However, in order to keep costs low, utilities generally have a relatively limited assortment of available luminaires. Mainly, streets are lighted with cobrahead luminaires but there is usually a small number of period luminaires that may evoke a generically old fashioned appearance. Among the types of period-style luminaires that may be available from an electric utility are several of the following types: