Guidelines for Nighttime Visibility of Overhead Signs (2016)

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62 Guidelines for Nighttime Overhead Sign Visibility A P P E N D I X D This appendix contains recommendations for revised material specifically for the AASHTO Roadway Lighting Design Guide. The latest edition (2005) includes a chapter focused on roadway sign lighting (Chapter 10). For ease of implementation, the key research findings have been integrated into the chapter from the AASHTO Roadway Lighting Design Guide. The revised chapter is shown below. Chapter 10 Overhead Sign Lighting 10.1 OVERVIEW Introduction Traffic signs are placed along the roadway in strategic locations and are used to convey specific, consistent messages to motorists. The standards used in the design of traffic signs are described in the Manual on Uniform Traffic Control Devices (MUTCD). The intent of these standards is to ensure that all traffic signs are designed and maintained to provide information that can quickly and accurately convey the necessary information and to provide national sign design consistency. The MUTCD states “signs shall be retroreflective or illuminated to show the same shape and similar color by both day and night” (1). Nighttime sign legibility can be achieved in one of two ways: Using retroreflective sheeting materials for the legend and background. Using either internal or external sign illumination. Almost all signs are made with retroreflective sheeting materials. Only some signs are illuminated, and generally those are overhead guide signs and overhead street name signs. The added sign illumination helps compensate when the vehicle headlamps and retroreflective properties of the sign sheeting materials are inadequate by themselves. A sign designed to be legible under daylight conditions can be illuminated to fulfill its basic purpose at night. A properly designed sign lighting system can aid motorists with the rapid and accurate recognition of the sign’s shape, color, and message. This serves to improve safety

63 Signs in areas where atmospheric conditions create condensation or frost on the sign face and reduce the effectiveness of the retroreflective sheeting. Key Visibility Considerations for Overhead Sign Lighting The following considerations should be addressed to assess nighttime sign visibility. Visual Complexity—Traffic signs are designed to be easy to read, but they also have to be conspicuous so that they are quickly recognized. The nighttime conspicuity of signs depends on the surrounding visual environment, which includes all other competing visual stimuli such as roadway lighting, vehicle lighting, other signs, and especially the roadside development and associated lighting. In this document, the term visual complexity is used to describe the surrounding visual environment. Visual complexity in the scene surrounding the sign impacts the sign luminance needed for nighttime motorists. If the visual complexity is high, then retroreflective material alone may not provide enough luminance, and therefore sign lighting may be needed. Sign Luminance—Sign luminance is a measure of the brightness of a sign. Sign luminance can be defined as either the legend luminance or the background luminance. The legend luminance is generally used as a key performance metric of guide sign visibility. Retroreflectivity of the Sign Legend and Background—The retroreflective material used for the legend as well as the background should be carefully considered. In many conditions, the proper selection of retroreflective sheeting materials can provide adequate visibility for nighttime motorists. The MUTCD contains minimum maintenance levels for sign retroreflectivity. Sign Contrast—The contrast between the sign legend and the sign background is an important factor to maintain adequate visibility. Generally speaking, adequate contrast is provided if the signs are fabricated using the color combinations as specified in the MUTCD and with materials meeting the color specifications established in 23 CFR Part 655, Traffic Control Devices on Federal-Aid and Other Streets and Highways; Color Specification for Retroreflective Sign and Pavement Marking Materials (1). 10.2 GUIDELINES FOR OVERHEAD SIGN VISIBILITY The guidelines for overhead sign visibility are designed to accommodate the needs of nighttime motorists by establishing a threshold level of legend luminance based on specific levels of visual complexity (defined later). The contrast of the sign legend and sign background is accounted for with the matching of retroreflective legend and background materials. The by reducing the possibility that motorists will significantly reduce their speed at locations where signs may be otherwise difficult to read. Overhead sign lighting is generally considered under the following locations: Signs in areas having a high level of visual complexity. Signs beyond sag vertical curves and outside the influence area of vehicle headlamps. Signs in horizontal curves that are outside the influence area of vehicle headlamps.

64 Table D-1. Luminance Levels for Overhead Signs. Visual Complexity Level Minimum Legend Luminance Candelas per Square Meter Candelas per Square Foot 1 2.5 0.23 2 8 0.75 3 14 1.31 4 20 1.87 5 25 2.34 The minimum luminance levels in Table D-1 are meant to be applied at a distance of 40 ft per inch of letter height. Thus, if the uppercase letters on a guide sign are 16 in., then these recommendations would apply at a distance of 640 ft (16 x 40 = 640 ft). For the highest visual complexity rating, Level 5, analyses of overhead sign legend luminance based exclusively on retroreflective sheeting materials available in 2015 indicate that additional luminance is needed beyond what can typically be provided by headlamps alone. In other words, for signs in areas with a visual complexity level of 5, sign lighting should be considered. For signs in areas with a visual complexity rating of 4 and below, the use of ASTM D4956-13 Type XI retroreflective sign sheeting materials can provide sufficient legend luminance in nearly all conditions. The exceptions are along horizontal curves in rural areas with radii of 880 ft or less and horizontal curves in urban areas with radii of 2,500 ft or less. In these conditions, sign lighting should be considered. For visual complexity ratings of 3 and below, the use of ASTM D4956-13 Types VIII and IX retroreflective sign sheeting materials can accommodate nearly all conditions with the same geometry restrictions as noted for visual complexity ratings of 4 and below. For visual complexity ratings of 1 and 2, the use of retroreflective sheeting ASTM D4956-13 Types IV, VIII, IX, and XI will provide adequate legend luminance. The analyses supporting this paragraph are based on an assumed 20 percent loss of retroreflective performance due to the impacts of sign weathering. Visual Complexity Levels for Overhead Guide Signs The level of visual complexity approaching a sign has a large impact on the visibility of the sign. Areas with high visual complexity dictate higher sign luminance levels in order to maintain adequate nighttime visibility. Although visual complexity can be computed from calibrated digital images of the nighttime scene, doing so requires a high level of expertise and expensive equipment. Therefore, the images in Figures D-1 and D-2 were developed and tested to show five levels of visual complexity. It is intended that these images be used to determine the visual complexity that best represents a jurisdiction or specific section of highway. guidelines incorporate five unique levels based on the latest research, which links sign legend luminance needs to visual complexity (2). For each level of visual complexity, there is an associated minimum legend luminance. The minimum level of luminance for the lowest visual complexity level (i.e., Level 1) has been appropriately matched with the Federal Highway Administration’s research that was used in the MUTCD to establish minimum retroreflectivity levels (since that work was completed in a dark, rural environment with low visual complexity). The recommended minimum maintained luminance levels for overhead signs are provided in Table D-1.

65 Level 1 Visual Complexity: Minimal objects and light sources. Low traffic. Level 2 Visual Complexity: Low commercial activity, some nearby light sources and signs. Low traffic. Level 3 Visual Complexity: Illuminated commercial signs, moderate number of other signs and light sources. Low to moderate traffic. Level 4 Visual Complexity: Moderate commercial activity with illuminated signs and businesses. Moderate to heavy traffic. Level 5 Visual Complexity: Heavy commercial activity with illuminated signs and businesses. Heavy traffic and glare from vehicle lights. Figure D-1. Five levels of visual complexity for Overhead Guide Signs (Set 1).

66 Level 1 Visual Complexity: Minimal objects and light sources. Low traffic. Level 2 Visual Complexity: Low commercial activity, some overhead lighting. Low to moderate traffic. Level 3 Visual Complexity: Moderate commercial activity with illuminated signs. Some other signs and light sources. Low to moderate traffic. Level 4 Visual Complexity: Moderate commercial activity with illuminated signs and businesses. Moderate to heavy traffic. Level 5 Visual Complexity: Heavy commercial activity, advertising signs and businesses. Heavy traffic and glare from vehicle lights. Figure D-2. Five levels of visual complexity for Overhead Guide Signs (Set 2).

67 Visual Complexity Levels for Overhead Street Name Signs The images in Figures D-3 and D-4 were developed and tested to show five levels of visual complexity for overhead street name signs (at signalized intersections). It is intended that these images be used to determine the visual complexity that best represents a jurisdiction or specific intersection.

68 Level 1 Visual Complexity: Minimal light sources, signs, and objects. Low traffic. Level 2 Visual Complexity: Some lighting, signs, or other objects. Low traffic. Level 3 Visual Complexity: Minor commercial and roadway lighting. Some objects in view. Low to moderate traffic. Level 4 Visual Complexity: Moderate commercial and roadway lighting. Several illuminated objects in view. Moderate traffic. Level 5 Visual Complexity: Several light sources from commercial activity and roadway lighting. Several illuminated objects in view. Heavy traffic. Figure D-3. Five levels of visual complexity for Overhead Street Name Signs (Set 1).

69 Level 1 Visual Complexity: Minimal light sources, signs, and objects. Low traffic. Level 2 Visual Complexity: Some lighting, signs, or other objects. Low traffic. Level 3 Visual Complexity: Minor commercial and roadway lighting. Some objects in view. Low to moderate traffic. Level 4 Visual Complexity: Moderate commercial and roadway lighting. Several illuminated objects in view. Moderate traffic. Level 5 Visual Complexity: Several light sources and heavy commercial activity. Several illuminated objects in view. Heavy traffic. Figure D-4. Five levels of visual complexity for Overhead Street Name Signs (Set 2).

70 10.3 ILLUMINATED SIGN TYPES Signs can be illuminated in a variety of different ways in order to make the sign message visible and legible to the passing motorist during the hours of darkness. The two main ways of providing illumination to a static sign are as follows: Externally Illuminated—Externally illuminated signs are static traffic signs that are uniformly illuminated by a source of light that is mounted external to the sign. This technique is generally used for overhead guide signs. Internally Illuminated—Internally illuminated signs are static traffic signs that are illuminated by a source of light that is enclosed within the sign and the sign message becomes visible when illuminated from within because of the difference of color and transparent nature of the material that makes up the sign face. This technique is generally used for overhead street name signs. 10.4 SIGN LIGHTING RECOMMENDATIONS Once it has been determined that sign lighting is warranted, the lighting engineer should select a light source that will light the sign so that it exhibits similar color rendering properties during the hours of darkness as it did under daylight conditions. The amount of light that is required to adequately light the sign during the hours of darkness is defined in Table D-1. Several different types of light sources that can be used to light roadway signs are available. Each light source has its own set of unique characteristics that may make it more desirable than others for a given sign installation. Energy consumption is a major factor in choosing a light source and should be considered. However, there are other factors such as color rendering, operating temperature, efficiency, and maintenance ease that are equally important and should also be evaluated. The light source that is selected should be able to adequately light the face of the sign without interfering with the contrast between the letters that make up the legend and the background of the sign that they are installed on. The contrast between the letters and the background will determine how quickly and accurately a passing motorist can recognize the shape and color of the sign as well as the interpretation of the message that is being displayed. Lighting Uniformity Uniformity of lighting is an indication of the quality of illumination and can be defined as either the average-to-minimum, maximum-to-minimum, or maximum-to-average ratios of light levels that are present on the face of the sign. In performing sign-lighting calculations, the maximum-to-minimum ratio has been established as the standard means of determining the uniformity of light levels that appear on the face of a sign. The uniformity of the light levels that appear on the face of the sign should be controlled if the sign is to be effective in conveying the sign message to motorists at night. Suitable uniformity over the entire face of the sign will provide consistent and proportional contrast that is similar to daytime conditions. Maximum and minimum points that are spaced too close

71 together will provide poor contrast between the letters that make up the legend and the background of the sign, making it more difficult to read. A maximum-to-minimum uniformity ratio of 6 to 1 is recommended as an acceptable ratio of lighting levels on the face of the sign. Since lower ratios will produce a more pleasing appearance and a more legible sign, lower maximum-to-minimum uniformity ratios are preferred. Light Source Selection There are several options for the light source selected to light a sign. Each of the sources has individual characteristics that can be desirous for the sign lighting application. The two primary considerations for lighting are energy consumption and the color characteristics. Other characteristics such as temperature of operation and ease of maintenance are secondary but should also be considered. The standards that are used as a basis for sign colors are coded in the MUTCD. The colors have been standardized nationwide so it is essential that the face of the sign be properly illuminated in order to retain the colors for identification purposes. The lighting can impact these color appearances as shown below: High-Pressure Sodium Lighting LED Lighting Sign faces should be lighted to maintain these color appearances through the selection of a light source that has a high enough color rendering index to maintain the color index (recommended CRI > 70). Of special note is the advent of solid state lighting. This newer technology allows for a light source that provides both higher energy efficiency and good color rendering. This source is an attractive choice for a light source on a sign. However, LED luminaires typically emit less heat and as such, melting of snow or frost may be different with LED and may present a maintenance issue. Placement of Lighting Units The location of the lighting units impacts the distribution of light on the sign, affecting the amount of illuminance on different areas and the resultant uniformity across the sign face. The lighting units that illuminate the face of a sign may be located on either the top of the sign,

72 the bottom of the sign, or an adjacent support. The lighting engineer should evaluate the following considerations before selecting the mounting arrangement that is to be utilized. The luminaire housing should not obstruct the view of the sign message. The reflected light should not reduce the visual performance of the sign message. Contribution to sky-glow should be limited as much as is practicable. The spill light should not be directed into the eyes of motorists. The luminaire mounting arrangement should not create maintenance problems. Locating the lighting units on the bottom of the sign, if practical, is generally the preferred alternative for the following reasons. The reflected light is less likely to reduce the visual performance of the sign message or produce reflected glare into the eyes of motorists. The lighting units do not produce daytime shadows and reflections from the sun on the face of the sign. The lighting units are easier to access for maintenance. The lighting units may collect snow and dirt but may also be cleaned by rain. The face of the sign may only partially shield the light that spills onto traffic approaching from the rear of the sign. However, a separate shielding mechanism that will minimize this effect can be provided on the lighting units. Excess sky-glow or light pollution may be inherent. However, a separate shielding mechanism can be provided on the lighting units or optical control equipment can be utilized in order to minimize these effects. The lighting units may obstruct the view of the sign message at some viewing angles. However, proper placement and installation of the lighting units can minimize this problem. In addition to the above considerations, the lighting engineer should also verify that the adjacent roadway lighting system, if present, does not adversely impact the lighting levels on the face of the sign or physically block the face of the sign. The adjacent roadway lighting system is not intended to perform the lighting of the adjacent overhead retroreflective signs. 10.5 REFERENCES 1. Manual on Uniform Traffic Control Devices (MUTCD). Federal Highway Administration, U.S. Department of Transportation, Washington, DC, 2009. 2. Carlson, P. J., B. K. Brimley, J. Miles, S. Chrysler, R. B. Gibbons, and T. Terry. NCHRP Report 828: Guidelines for Nighttime Visibility of Overhead Signs. Transportation Research Board, Washington, DC, 2016.

Abbreviations and acronyms used without definitions in TRB publications: A4A Airlines for America AAAE American Association of Airport Executives AASHO American Association of State Highway Officials AASHTO American Association of State Highway and Transportation Officials ACI–NA Airports Council International–North America ACRP Airport Cooperative Research Program ADA Americans with Disabilities Act APTA American Public Transportation Association ASCE American Society of Civil Engineers ASME American Society of Mechanical Engineers ASTM American Society for Testing and Materials ATA American Trucking Associations CTAA Community Transportation Association of America CTBSSP Commercial Truck and Bus Safety Synthesis Program DHS Department of Homeland Security DOE Department of Energy EPA Environmental Protection Agency FAA Federal Aviation Administration FAST Fixing America’s Surface Transportation Act (2015) FHWA Federal Highway Administration FMCSA Federal Motor Carrier Safety Administration FRA Federal Railroad Administration FTA Federal Transit Administration HMCRP Hazardous Materials Cooperative Research Program IEEE Institute of Electrical and Electronics Engineers ISTEA Intermodal Surface Transportation Efficiency Act of 1991 ITE Institute of Transportation Engineers MAP-21 Moving Ahead for Progress in the 21st Century Act (2012) NASA National Aeronautics and Space Administration NASAO National Association of State Aviation Officials NCFRP National Cooperative Freight Research Program NCHRP National Cooperative Highway Research Program NHTSA National Highway Traffic Safety Administration NTSB National Transportation Safety Board PHMSA Pipeline and Hazardous Materials Safety Administration RITA Research and Innovative Technology Administration SAE Society of Automotive Engineers SAFETEA-LU Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (2005) TCRP Transit Cooperative Research Program TDC Transit Development Corporation TEA-21 Transportation Equity Act for the 21st Century (1998) TRB Transportation Research Board TSA Transportation Security Administration U.S.DOT United States Department of Transportation

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