Guidelines for Nighttime Visibility of Overhead Signs (2016)

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41 Incremental Effects of Light Sources and Sign Sheeting on Legend Luminance for Overhead Guide Signs This appendix describes an analysis of the photometric data collected during the closed-course study described in Chapter 3. The purpose of the analysis is to identify the effects on legend luminance as a result of the type of sheeting, use of sign lighting, and presence of roadway lighting. The infor- mation can help agencies understand how a defined level of luminance can be achieved using available sheeting products and types of lighting. Findings from the closed-course study (Chapter 3) indi- cate that increases in sign luminance when a sign is already reasonably bright and in a rural, dark area have no effect on legibility. However, in the open-road study (Chapter 4), where drivers were placed in situations with greater complex- ity, visibility from a recognition task improved with increased luminance. The analysis presented in this appendix investi- gated the contributions of light sources and sign sheeting on luminance provided to drivers and provides useful infor- mation to those responsible for ensuring signs are visible in urban and suburban environments. When it is unreasonable to take photometric measurements in the field, practitioners can use this information to make better-informed decisions by knowing how much luminance is added from a change in sign sheeting or lighting. Data The data used in the analyses were collected on a closed course where the lighting was restricted to the vehicle’s head- lamps, the roadway lighting (when used), and the sign light- ing (when used). Photometric measurements were taken at 100-ft intervals from the guide sign, as described in Chap- ter 3. The analysis presented in this appendix was concerned with the luminance provided at a single location 640 ft from the sign, where drivers were expected to be able to read a sign with 16-in. letters. The raw luminance data for the 600-ft and 700-ft intervals were interpolated to produce luminance val- ues for 640 ft. The sign lighting systems used were high-pressure sodium (HPS) and light-emitting diode (LED) lights. The HPS lighting was a GE Versaflood II luminaire with a correlated color tem- perature (CCT) of 2,100 K. The LED lighting was two Cree OL Series Flood luminaires with a CCT of 5,700 K. The orientation, aim, and spacing of the HPS and LED luminaires were adjusted to make the illumination from the light sources as uniform as possible based on AGI32 light modeling software. Illuminance was measured on the sign surface using a 12-point orthogonal grid. Three intensity levels for sign lighting were used, with values of illuminance identified in Table A-1. The sign legends were constructed of either American Society for Testing and Materials (ASTM) Type IV or Type XI sheeting. The roadway lighting consisted of LED luminaires spaced 250 ft (80 m) apart along the road. Three luminaires were located in front of the test guide sign starting approxi- mately 650 ft (200 m) before the sign. The color temperature of the LED roadway lights was 6,000 K. Legend luminance data were collected from combinations of the following factors: • Sign sheeting: – Type IV and Type XI. • Sign lighting: – Two types of sign lighting (HPS and LED) set to provide similar amounts of illumination. • Sign lighting level: – Four possible levels: 100 percent (600 lx), 50 percent (300 lx), 25 percent (150 lx), and off. • Roadway lighting: – Either on or off. Images captured by a Radiant Imaging ProMetric pho- tometer mounted inside a 2000 Ford Explorer from a driver’s view were used to determine luminance values. Measured luminance from the four areas of each photometric image, as shown in Figure A-1, were used to determine an average value A P P E N D I X A

42 of legend luminance. An average was used because, at a typi- cal mounting height for overhead signs, luminance from the top row of the legend is generally less than from the bottom row since the headlamp illumination decreases with greater height from the road. Average luminance values for 28 combinations of sheeting type, sign lighting, lighting level, and roadway lighting were used to evaluate the effects of lighting and sheeting. Table A-2 provides the luminance measurements for the combinations of the study factors at a distance of 640 ft from the sign. Analysis The luminance supplied to a driver is a result of the illu- minance provided by the various light sources, the efficiency of the reflective material, and the position of the vehicle. The luminance can be defined as a function of these influencing factors: =       , , , ,Luminance f sheeting headlamps vehicle location sign lighting type sign lighting intensity roadway lighting Luminance can be increased by using a more efficient type of sheeting, adding sign lighting, or changing the intensity or type of sign lighting. Headlamps were used in the analysis described below, and the vehicle location was kept constant at 640 ft from the guide sign. Therefore, the only influencing factors were the sheeting, type of sign lighting, intensity of sign lighting, and use of roadway lighting. An analysis of variance (ANOVA) was performed on the 28 data points from Table A-2 to model the effects of sign sheeting, type and intensity of sign lighting, and road- way lighting on legend luminance. Interactions between all the variables were tested. The interaction of sign sheeting and sign lighting was significant. The effects of the model are identified in Table A-3. A regression model produced from the effects is shown in Equation A-1, with some model esti- mates provided in Table A-4. The predicted luminance values closely matched the actual values, as illustrated by the plot in Figure A-2. The root mean square error of this model was 1.0 cd/m2, and the adjusted R2 value was 0.99. Intensity Level Illuminance (lx) HPS LED 100% 622 590 50% 333 300 25% 145 144 Table A-1. Measured illuminance for each intensity level and lighting system. Figure A-1. Sample areas for calculating averaging luminance. Sheeting Type Sign Lighting Lighting Level Roadway Lighting Luminance (cd/m2) IV HPS 100% Off 25.0 IV HPS 100% On 29.6 IV HPS 50% Off 17.0 IV HPS 50% On 19.2 IV HPS 25% Off 15.3 IV HPS 25% On 17.0 IV LED 100% Off 37.7 IV LED 100% On 40.7 IV LED 50% Off 21.3 IV LED 50% On 24.4 IV LED 25% Off 15.4 IV LED 25% On 15.5 IV None 0% Off 8.4 IV None 0% On 8.8 XI HPS 100% Off 31.6 XI HPS 100% On 31.8 XI HPS 50% Off 22.3 XI HPS 50% On 23.9 XI HPS 25% Off 17.1 XI HPS 25% On 18.0 XI LED 100% Off 42.9 XI LED 100% On 44.6 XI LED 50% Off 30.3 XI LED 50% On 30.3 XI LED 25% Off 18.2 XI LED 25% On 18.5 XI None 0% Off 9.5 XI None 0% On 9.6 Table A-2. Legend luminance values at 640 ft from sign. Parameter F Ratio Pr > F Legend Sheeting Type 0.864 0.370 Sign Lighting 198.8 <0.001 Roadway Lighting 14.21 0.002 Legend Sheeting × Sign Lighting 5.063 0.007 Table A-3. Parameter effect tests.

43 ( )= + × + + ×7.9 0.9 1.4 (Eq. A-1) 640L I f Sign Lighting ITypeXI Road Lighting Where L640 = legend luminance (cd/m2) measured at 640 ft; ITypeXI = 1 for Type XI sheeting, 0 for Type IV; f(Sign Lighting) = function of the main effect of Sign Light- ing and its interaction with Legend Sheet- ing Type, shown in Table A-4; and IRoad Lighting = 1 for using roadway lighting, 0 for no roadway lighting. Discussion From the variables in Equation A-1, the effect of roadway lighting is relatively small (increasing luminance by 1.4 cd/m2). Nearly 8 cd/m2 are provided when using Type IV sheeting, and luminance increases by nearly 1 cd/m2 with Type XI sheeting. The values for the use of sign lighting with the two types of sheet- ing in Table A-4 show that luminance can typically increase by 8 to 34 cd/m2, depending on the type and intensity of the lighting used. The different values in the table indicate that the two types of sign lighting and their three intensity levels uniquely interact with the two types of sheeting, which should be expected. Even though the amount of illuminance is nearly identical, the result- ing luminance is different, possibly a result of the color of the lighting and the position of the luminaires. The model estimating luminance of overhead guide sign legends is useful for agencies that need to ensure drivers are provided adequate sign luminance. At 640 ft, representing a 40 ft/in. index when reading a 16-in. legend, the combination of headlamps, the brightest sheeting, and the highest inten- sity of sign lighting produce over 40 cd/m2 of luminance. The different effects in the model show that agencies have options to provide luminance that is adequate for a given situation. One of the limitations of this analysis is that the only light- ing sources were the single vehicle in front of the sign, the sign lighting (when used), and the roadway lighting (when used). In suburban and urban areas, light from additional sources may provide more illumination than what is accounted for here. The most additional illuminance will be from other vehicles on the road. The lighting conditions at specific sites, including light from multiple vehicles, should be considered when evalu- ating the performance of a sign and determining the potential benefits of using sign lighting or a specific type of sheeting. Sign Lighting Type IV Type XI None 0 0 HPS, 150 lx 7.6 8.0 HPS, 300 lx 9.5 13.6 HPS, 600 lx 18.7 22.2 LED, 150 lx 6.8 8.8 LED, 300 lx 14.3 20.7 LED, 600 lx 30.6 34.2 Table A-4. Values for f(Sign Lighting) in Equation A-1. 0 10 20 30 40 50 0 10 20 30 40 50 Ac tu al L eg en d Lu m in an ce (c d/m 2 ) Predicted Legend Luminance (cd/m2) y=x R2adj = 0.99 Figure A-2. Actual-by-predicted plot for model of luminance at 640 ft.