Guidelines for Nighttime Visibility of Overhead Signs (2016)

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16 Closed-Course Study As discussed in Chapter 2, recent trends indicate a move- ment away from lighting overhead guide signs toward relying on retro reflective sheeting and vehicle headlamps assumed to provide nighttime drivers with sufficient sign luminance for visibility. There are various guidelines sug- gesting how much illuminance or luminance should be provided, and the MUTCD provides minimum mainte- nance standards for retroreflectivity when sign lighting is not used. The literature summarized in Chapter 2 identified the fol- lowing controllable elements that influence the nighttime visibility of a sign: the use and type of sign lighting, the type of sign sheeting material, the vehicle’s headlamps, the back- ground complexity, and the use of street lighting. Most of these factors have been evaluated in separate contexts but not in a full-factorial study identifying the specific influence of single elements. This chapter describes the design and results of a closed-course study that was performed to evalu- ate how combinations of various lighting sources and types of sheeting contribute to the nighttime visibility of overhead guide signs. Experimental Design The purpose of this study was to evaluate the nighttime visibility of guide signs constructed of different materials and illuminated under various lighting conditions. The lighting conditions included the use of different types of sign lighting and intensities, as well as the use of overhead street light- ing. The study involved having drivers on a closed course read the legend of an overhead guide sign while approaching the sign. Each participant made multiple runs. A different combination of lighting was used with each run. The legend was also changed each time to ensure the study involved legibility rather than recognition. Facilities and Equipment Closed Course The research was conducted on the Virginia Smart Road at the Virginia Tech Transportation Institute. It was completed in 2013. The Smart Road is a 2.2-mi closed-access test track that simulates a typical stretch of highway with pavement markings and guardrails and includes a frame for mounting a guide sign. Figure 3 is an illustration of the Virginia Smart Road and iden- tifies where the street lighting was located with respect to the overhead guide sign, which was viewed from only one direc- tion. There were more street lights behind the sign than in front of the sign. The luminaires in front of the sign increased the illuminance on the sign (and resulting luminance viewed by the driver), while the luminaires behind the sign acted as glare sources and increased the visual complexity of the scene. Vehicles Two Ford Explorers from 1999 and 2000 were the test vehi- cles. The vehicles had the same body style and internal layout. Each vehicle was similarly instrumented for data collection with digital audio and video recorders, global positioning system (GPS) receivers, and buttons for experimenters to identify critical points in the data stream. The two vehicles’ headlamps were identical, and their aim was calibrated prior to each session. The study protocol involved two participants driving at the same time in short succession. The rear view and side mirrors were covered to prevent headlamp glare from the other test vehicle. Sign Lighting Systems Two separate lighting systems were used to illuminate the guide signs and compare driver responses with the type of C H A P T E R 3

17 lighting. The two systems consisted of HPS and LED lights. The HPS lighting was a GE Versaflood II luminaire with a correlated color temperature (CCT) of 2,100 K. The LED lighting was two Cree OL Series Flood luminaires with a CCT of 5,700 K. The two lighting systems were closely matched for illuminance, but there were differences in the resulting lumi- nance. Researchers mounted both systems on the sign’s frame to eliminate the need to physically change the lighting system during the study. A single HPS luminaire was mounted in the center of the lighting fixture, and two LED luminaires were placed on each side of the HPS luminaire. The lighting fix- ture was mounted 75 in. from the guide sign. Figure 4 shows one of the study signs illuminated by the two different types of sign lighting and a third condition without sign lighting (headlamps only). The cooler temperature of the HPS light- ing compared with the LED lighting can be seen in Figures 4a (HPS) and 4b (LED). The research team adjusted the orientation, aim, and spac- ing of the HPS and LED luminaires 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. The intensity of each light was adjustable, which added another factor to the study. Three intensity levels (25, 50, and 100 percent) were selected. There was also a fourth intensity level (off). To select a particular intensity for an experimental run, the researchers designed a method to control the inten- sity of the sign lighting based on the system’s power output. It was observed that different levels of power were needed to produce a comparable illuminance using the two lighting sys- tems. Figure 5 shows how illuminance (measured at the sign face) of the two systems changes with a defined power level and how much luminance (measured at 300 ft from the sign) is provided for a specific amount of illuminance. The lumi- nance (which is affected by the type of retroreflective sheet- ing) reported in Figure 5 was measured from Type XI green background sheeting with no illumination other than from the specified sign lighting. Figure 5 shows that the light produced by the LED system is more efficiently reflected as luminance than the light from the HPS system. The amount of illuminance provided by the two lighting systems for a given power level is also incon- sistent. Each lighting system was adjusted to a unique power level to achieve a similar illuminance when set at one of the three intensities for the study (25, 50, and 100 percent). With maximum power producing about 600 lx for both systems, the 50 and 25 percent levels produced approximately 300 and 150 lx, respectively. Table 6 indicates the specific values of illuminance for the selected intensity levels. These values closely match the AASHTO and IESNA recommended illu- minance levels for low, medium, and high ambient lighting as described in Chapter 2 (Tables 2 and 3). Retroreflective Sheeting The sign backgrounds measured 8 ft × 12 ft and were constructed of one of the three sheeting materials: ASTM Type III beaded, Type IV prismatic, or Type XI prismatic. The sign legends were constructed of either ASTM Type IV or Type XI sheeting. The Type IV legend was placed only on the Type III background. The Type XI legend was applied only to the Type IV and Type XI backgrounds. Close-up photos of the sheeting materials are shown in Figures 6–8. Figure 3. Illustration of the closed-course test road (Virginia Smart Road). Bottom Turnaround Top Turnaround Lighting Figure 4. Test sign illuminated under different lighting conditions: (a) HPS, (b) LED, and (c) headlamps only. (a) (b) (c)

18 Figure 5. Measures of illuminance and luminance (at 300 ft with Type XI green sheeting) by type of lighting. 0 2 4 6 8 10 12 14 0 100 200 300 400 500 600 700 10 9 8 7 6 5 4 3 2 1 0 Lu m in an ce (c d/m 2 ) Ill um in an ce (lx ) Power Level LED Illuminance HPS Illuminance LED Luminance HPS Luminance Intensity Level Illuminance (lx) HPS LED 100% 622 590 50% 333 300 25% 145 144 Table 6. Measured illuminance for each intensity level and lighting system. Figure 6. Type IV prismatic legend on Type III beaded background. Figure 7. Type XI prismatic legend on Type IV prismatic background. Figure 8. Type XI prismatic legend on Type XI prismatic background.

19 The legends have mixed-case letters with a size correspond- ing to an uppercase letter height of 16 in. The typeface was Clearview 5WR font. Clearview 5WR (“R” for reduced) is a narrower version of the Clearview font and has a footprint similar to Series E (Modified) (82). Overhead Street Lighting The street lighting system consisted of 12 LED luminaires spaced 250 ft (80 m) apart along the road. The three luminaires located in front of the test guide sign were placed starting approximately 650 ft (200 m) before the sign. The remaining luminaires extended 2,500 ft (760 m) beyond the guide sign. The color temperature of the overhead LED lights was 6,000 K. These roadway lights were used for half of the trial runs. Participants Twenty-four participants were recruited, with an even split of gender and age (six older males, six younger males, six older females, and six younger females). The data collected for two of the 24 participants were discarded during post-processing when the researchers discovered the vehicle headlamps had not been properly configured before the trials. Each partici- pant passed vision tests that included measurements of acu- ity, color vision, and contrast sensitivity. No participant was colorblind or had acuity worse than 20/40. Contrast sensi- tivity was above 25 percent using a Snellen eye chart with an illuminator. Experimental Procedure Each participant adjusted basic settings upon sitting in the driver’s seat. A researcher sat in the vehicle with the partici- pant to give instructions. The participant drove at a constant speed of 35 mph and with the headlamps on, reading the guide sign’s legend aloud as soon as it was legible. The GPS receiver continuously recorded the location of the vehicle. When the participant correctly read a sign legend, the researcher in the vehicle pressed a button to record in the data stream the moment when the sign was legible. After passing the sign, the participant turned around to repeat the test under different lighting conditions and with a different legend. The legend was selected from words divided into two differ- ent groups, as shown in Table 7. One word from each group was placed on the sign for each lap. The sheeting material was constant throughout a single participant’s tests, though the legend changed with each approach to the sign. Two participants drove at one time, and the timing was arranged so the vehicles never crossed paths. In addition to changing the legend with each lap, the on-site researchers also adjusted the lighting configuration. The two types of sign light- ing systems were set to three different intensity levels (100, 50, and 25 percent). These six possible sign lighting settings were matched with whether or not overhead street lighting was used, producing 12 possible combinations when sign light- ing was on. Two additional options that involved no sign lighting (and street lighting was either on or off) resulted in 14 total lighting configurations. Each participant in the study completed up to 14 laps. An example protocol for a single participant is shown in Table 8. Table 7. Guide sign legend word groups. Group 1 Group 2 Lake Camp Long Port Gray Cape Bear Road Oven Park East Bend Table 8. Example experiment protocol. Lap Sign Sheeting Sign Lights Sign Lighting Intensity Overhead Street Lighting Legend 1 XI on XI LED 100% OFF Camp Lake 2 XI on XI LED 50% OFF Port Long 3 XI on XI LED 25% OFF Cape Gray 4 XI on XI LED 25% ON Road Bear 5 XI on XI LED 50% ON Park Oven 6 XI on XI LED 100% ON Bend East 7 XI on XI HPS 100% ON Long Road 8 XI on XI HPS 50% ON Gray Park 9 XI on XI HPS 25% ON Bear Bend 10 XI on XI HPS 25% OFF Oven Camp 11 XI on XI HPS 50% OFF East Port 12 XI on XI HPS 100% OFF Lake Cape 13 XI on XI None 0% OFF Bear Port 14 XI on XI None 0% ON Park Lake

20 Participants were assigned a secondary task in which they were asked to read aloud the speeds posted on a speed limit sign. The speeds shown were either 35 or 55 mph. This increased the complexity of the driving task to better simulate a realistic driving scenario and helped the participants maintain their focus on road signs when the only other task would be to read the guide sign. Data for the speed limit sign legibility distance are not reported here. With legibility distance as the dependent variable, the inde- pendent variables were sorted in a 4×3×2×2×2 mixed facto- rial design from four combinations of sign lighting levels (off, 25 percent, 50 percent, and 100 percent intensities), three combinations of retroreflective sign sheeting for the legend and background, two types of sign lighting (HPS and LED), two conditions of street lighting (on and off), and two age groups of study participants. Table 9 identifies the different possible settings. Each driver viewed the signs under condi- tions of the four lighting levels, two types of sign lighting, and two overhead street lighting settings. Each participant, however, only viewed one combination of legend and back- ground sheeting material because the background was too cumbersome to change during the experiment. Legend and background material was a between-subjects variable. Data Legibility distance was calculated as the distance between the location of the vehicle at the moment when the researcher pressed the button in the GPS data stream and the station- ary location of the guide sign. Audio and video recordings of the participants were used to increase the accuracy of the vehicle’s identified location since there was an inherent delay from when the participant read the sign legend and the researcher pressed the button. In addition to considering the individual factors listed in Table 9, and separate from the data collection with partici- pants in the vehicle, researchers measured the luminance of the target sign at 100-ft intervals from the sign using a Radi- ant Imaging ProMetric photometer. A 300-mm lens was used to achieve a detailed image from all measurement distances. Each pixel in the image has a corresponding luminance value. The device was mounted inside the vehicle where the driver sits and aimed in the direction where a driver typically looks. At each 100-ft interval, the photometer captured images for each of the three sign background and legend combinations with each possible scenario of sign lighting type (HPS or LED), intensity (25, 50, or 100 percent), use of street lighting (on and off), and even use of headlamps (on and off, though headlamps were always on in the legibility tests). These combi- nations resulted in 24 images for each sign sheeting combina- tion when sign lighting was used. There were three additional images captured when sign lighting was turned off: one for when street lighting and headlamps were both on, one for when street lighting was off but headlamps were on, and one for when street lighting was on but headlamps were off. An additional (fourth) image of no sign lighting, no street light- ing, and no headlamps was not sensible since there would be no illuminance from any light source. Twenty-seven images for each of the three signs at nine locations resulted in 729 total images. With a luminance value associated with each pixel in each photometric image, average luminance values for the sign’s legend and background were derived from rectangles cover- ing multiple areas of the image, as shown in Figure 9. The Variable Number of Levels Values Sign Lighting Level 4 100 percent, 50 percent, 25 percent, off Sign Lighting Type 2 HPS, LED Legend/Background Sheeting Combination 3 Type IV legend on Type III background, Type XI legend on Type IV background, Type XI legend on Type XI background Overhead Street Lighting 2 On, off Age 2 Younger (25–35) and Older (65+) Table 9. Independent variables and values. Figure 9. Rectangles selected for calculating sign luminance and Weber contrast.

21 rectangles covered four regions on the legend and four on the background. A Weber contrast value for each image was calculated from the average luminance of the legend and background, according to Equation 1: = − (Eq.1)C L L L L B B where C is the Weber contrast ratio, LL is the luminance of the sign legend, and LB is the luminance of the sign background. Figure 10 illustrates how legend luminance changes with distance from the sign using different types of sign lighting (HPS and LED) and illumination levels (50 and 100 percent). Type XI sheeting is used for those comparisons. Headlamps were always on, and there was no overhead street lighting. A condition with no sign lighting (headlamps only) is also shown using Types IV and XI sheeting. There are minor dif- ferences between the luminance measures on the Types IV and XI legends. In all instances, luminance increases as the vehicle approaches the sign, though there are fluctuations along the approach and a notable decrease starting at about 300 or 400 ft from the sign. This unevenness is due to the complex interaction of the retroreflective sheeting and light at the angles formed between the light sources, the sign, and the observer (photometer). It appears that the LED lighting at a 50 percent illuminance level produces nearly the same amount of luminance as the HPS lighting at 100 percent. The contribution of individual light sources on the luminance provided to a driver is described in Appendix A. Since luminance and, subsequently, Weber contrast are dependent on the distance from the sign, the analyses that consider the effects of luminance and contrast on legibility distance should use luminance measured from a consis- tent location from one trial to another. Legend luminance and contrast 640 ft from the sign were used in the analyses because 640 ft represents a legibility index of 40 ft/in. for the 16-in. uppercase letter in the legend. The values were inter- polated from measurements at 600 and 700 ft. Figure 11 is a histogram of legend luminance at 640 ft using the 42 factorial combinations of sign sheeting, sign lighting type, sign light- ing intensity, and overhead street lighting use. The minimum is 8 cd/m2, the median is 21.6 cd/m2, and the maximum is 46.7 cd/m2. The minimum luminance of 8 cd/m2 is about 3.5 times brighter than the minimum luminance level that was used to derive the FHWA minimum retroreflectivity levels for overhead guide signs (2.3 cd/m2). Figure 12 shows the distribution of Weber contrast at 640 ft with the same combinations of lighting, intensity, and sheeting. The mini- mum is 5.2, the median is 9.0, and the maximum is 20.1. There are also 42 observations. The study obtained 261 total observations of legibility dis- tance from the 22 participants with usable data. Figure 13 illus- trates the distribution of the data. The minimum distance was 197 ft, the maximum was 1,252 ft, and the median and mean were 705 ft and 718 ft, respectively. The median and mean leg- ibility distances were slightly greater than the 640-ft distance representative of a 40-ft/in. legibility index. Most of the obser- vations therefore have a legibility index greater than 40 ft/in. The data are right skewed, an expected feature since the leg- ibility distance cannot be less than 0. The dependent variable in the analyses described in the next section was the legibility distance from the sign when the drivers correctly read the legend. Variation in the legibility Figure 10. Luminance measurements of sign legend (Type XI) by distance from sign (no street lighting; headlamps on). 0 10 20 30 40 50 60 2003004005006007008009001,000 Lu m in an ce (c d/m 2 ) Distance from Sign (ft) LED 100% (Type XI) LED 50% (Type XI) HPS 100% (Type XI) HPS 50% (Type XI) No Sign Lighting (Type XI) No Sign Lighting (Type IV)

22 Figure 11. Distribution of legend luminance measured at 640 ft under the closed-course factorial conditions. 10% 5% 33% 17% 12% 7% 5% 10% 2% 0% 5% 10% 15% 20% 25% 30% 35% 5 10 15 20 25 30 35 40 45 50 Fr eq ue nc y Legend Luminance (cd/m2) at 640 ft Figure 12. Distribution of Weber contrast values measured at 640 ft under the closed-course factorial conditions. 45% 17% 17% 19% 2% 0% 10% 20% 30% 40% 50% 4 8 12 16 20 24 Fr eq ue nc y Weber Contrast at 640 ft Figure 13. Distribution of legibility distances from the closed-course study. 0.4% 1.1% 1.5% 6.1% 21.5% 17.6%18.4% 12.6% 11.5% 6.9% 1.9% 0.4% 0% 5% 10% 15% 20% 25% Fr eq ue nc y Legibility Distance (ft) 200100 300 400 500 600 700 800 900 1,0 00 1,1 00 1,2 00 1,3 00

23 distance from one observation to another was analyzed with respect to the categorical variables from the factorial experi- ment (sign lighting type, lighting intensity level, use of over- head street lighting, and sign sheeting material) and the values of luminance and contrast at 640 ft that resulted from the experiment design. The presence of categorical and contin- uous independent variables meant than an analysis of vari- ance (ANOVA) and an analysis of covariance (ANCOVA) could be performed to identify the factors that influence legibility distance. If lighting or sheeting configurations or luminance or contrast values were found to influence leg- ibility, such findings would support guidelines for signs to be constructed with a certain type of sheeting, illuminated by a specific type of lighting, or provide a defined level of luminance or contrast. Analyses The analyses described in this section focus on the fac- tors that affect legibility distance. One of the hypotheses was that legibility distance is dependent on the luminance of the sign legend and/or the Weber contrast of the sign. The first analysis investigated these relationships. Figures 14 and 15 are scatterplots showing the legibility distance versus legend luminance and Weber contrast, respectively. Based on visual inspection of the plots, legibility distance was not dependent Figure 14. Scatterplot of observations of legibility distance and values of legend luminance measured at a distance of 640 ft. There is no relationship between legibility distance and luminance from these data collected on the closed course. 0 200 400 600 800 1,000 1,200 1,400 0 10 20 30 40 50 Le gi bi lity D ist an ce (ft ) Legend Luminance (cd/m2) Figure 15. Scatterplot of observations of legibility distance and values of Weber contrast measured at a distance of 640 ft. There is no relationship between legibility distance and contrast from the closed-course data. 0 200 400 600 800 1,000 1,200 1,400 0 5 10 15 20 Le gi bi lity D ist an ce (ft ) Weber Contrast

24 on either of the two measurements. Additionally, statistical analysis indicated that there was no relationship, even when considering a potential interaction. Since there was no relationship between legibility distance and either luminance of the legend or Weber contrast of the sign, an ANOVA test was appropriate for identifying whether or not the categorical factors significantly affect legibility. Legibility Distance versus Study Factors An ANOVA test was performed to determine if changes in legibility distance could be attributed to independent variables of age, retroreflective sheeting, sign lighting type, sign lighting intensity, and overhead street lighting use. Each study factor and potential interactions were considered. The study participants were incorporated into the error term to account for the variability between participants and focus on the within-subjects effects. Table 10 contains the results of the test. The ANOVA test identified only two factors (which are interactions) that significantly affect the legibility distance: Sign Lighting Type × Sign Lighting Intensity and Sign Light- ing Type × Roadway Lighting × Age. The legibility distances for the observations under those conditions are shown below. Sign Lighting Type × Sign Lighting Intensity It was shown in Figure 5 that each type of lighting and intensity level results in a unique luminance. Based on that information, it may not be surprising that the interaction of sign lighting type and intensity is a significant factor in the legibility distance. However, the lack of a statistical relation- ship between luminance and legibility distance (Figure 14) suggests that the effect is not directly related to the addi- tional luminance from the sign lighting. Figure 16 shows mean legibility distances with standard deviations for each sign lighting type and intensity. The difference between the mean legibility distances for LED lighting (which range from 725 to 734 ft) and the mean for no lighting is about 50 ft. The mean legibility distance for no lighting is nearly equal to the mean for HPS lighting at 25 percent intensity. The mean legibility distances for HPS lighting at intensities of 100 and 50 percent differ by about 50 ft. Sign Lighting Type × Roadway Lighting × Age The second significant factor in the ANOVA test was the interaction of sign lighting type, roadway lighting, and age. There were 12 possible combinations characterizing the inter- action of these three main effects. The mean legibility distance Source F Ratio Pr > F Age 2.12 0.164 Sign Sheeting 0.02 0.982 Sign Sheeting × Age 2.87 0.086 Sign Lighting Type 0.13 0.724 Sign Lighting Type × Age 3.97 0.065 Sign Sheeting × Sign Lighting Type 3.52 0.056 Sign Sheeting × Sign Lighting Type × Age 2.85 0.090 Sign Lighting Intensity 1.26 0.298 Sign Lighting Intensity × Age 0.32 0.725 Sign Sheeting × Sign Lighting Intensity 1.01 0.416 Sign Sheeting × Sign Lighting Intensity × Age 0.33 0.858 Sign Lighting Type × Sign Lighting Intensity 3.77 0.038a Sign Lighting Type × Sign Lighting Intensity × Age 1.07 0.359 Sign Sheeting × Sign Lighting Type × Sign Lighting Intensity 0.95 0.452 Sign Sheeting × Sign Lighting Type × Intensity × Age 0.28 0.890 Roadway Lighting 4.43 0.053 Roadway Lighting × Age 0.04 0.850 Sign Sheeting × Roadway Lighting 0.37 0.695 Sign Sheeting × Roadway Lighting × Age 0.29 0.754 Sign Lighting Type × Roadway Lighting 0.2 0.667 Sign Lighting Type × Roadway Lighting × Age 6.3 0.027a Sign Sheeting × Sign Lighting Type × Roadway Lighting 0.84 0.456 Sign Sheeting × Sign Lighting Type × Roadway Lighting × Age 2.89 0.095 Sign Lighting Intensity × Roadway Lighting 0.08 0.924 Sign Lighting Intensity × Roadway Lighting × Age 0.93 0.407 Sign Sheeting × Sign Lighting Intensity × Roadway Lighting 1.21 0.329 Sign Sheeting × Sign Lighting Intensity × Roadway Lighting × Age 0.24 0.911 Sign Lighting Type × Sign Lighting Intensity × Roadway Lighting 0.89 0.442 Sign Lighting Type × Sign Lighting Intensity × Roadway Lighting × Age 0.36 0.709 Sign Sheeting × Sign Lighting Type × Sign Lighting Intensity × Roadway Lighting 0.06 0.942 Sign Sheeting × Sign Lighting Type × Sign Lighting Intensity × Roadway Lighting × Age – – Note: A dash (–) means that the effect could not be determined. a Variable significant at the 95 percent confidence level. Table 10. ANOVA results for legibility distance.

25 and standard deviations are shown in Figure 17. Based on visual inspection of each condition group in Figure 17, legibil- ity distances were consistently longer for younger drivers and when roadway lighting was on. The average legibility distance of younger drivers for all conditions was 787 ft; the average for older drivers was 667 ft. The average legibility distance of all drivers without roadway lighting was 694 ft; the average when roadway lighting was on was 743 ft. There appears to be no consistent difference in legibility distance between lighting types when split across the other interacting factors shown in Figure 17. The mean legibility distance without any sign lighting was 681 ft. For the HPS and LED lighting systems, the mean legibility distances were 715 ft and 729 ft, respectively. Summary of Findings Statistical analyses could not identify a relationship between legibility distance and values of either sign luminance or Weber contrast. Another iteration of the legibility distances for each Figure 16. Mean legibility distances with standard deviations for the combination of sign lighting type and intensity. 400 500 600 700 800 900 1,000 100% 50% 25% 100% 50% 25% 0% HPS LED NONE Le gi bi lity D ist an ce (ft ) type and intensity of sign lighting (previously shown in Fig- ure 16) is provided in Figure 18 with average values of lumi- nance for the given lighting conditions. It is clear that the differences in legibility distance from one lighting condition to another are not only quite small, they have no noticeable relation to the amount of luminance provided by the lighting. The luminance consistently decreases as the intensity of the lighting decreases from 100 to 0 percent, but there is no con- sistent change in legibility distance. The luminance values were measured from 640 ft and with the headlamps on to represent the amount of light provided to drivers at a 40-ft/in. index. The younger study participants had a mean legibility dis- tance 120 ft longer than the older study participants. This is not a surprising finding, as the measured visual acuities of the younger participants were generally better than those of the older participants. Legibility distance also improved by approximately 50 ft when overhead street lighting was on. The material construction of the study sign (whether Type XI legend on Type XI background, Type XI legend on Type IV Figure 17. Mean legibility distances with standard deviations for the combination of sign lighting type, roadway lighting, and age. 400 600 800 1,000 1,200 H PS LE D N O NE HP S LE D N O NE HP S LE D N O NE HP S LE D N on e Roadway Lighting Off Roadway Lighting On Roadway Lighting Off Roadway Lighting On Older Younger Le gi bi lity D ist an ce (ft )

26 0 10 20 30 40 50 500 600 700 800 900 1,000 100% 50% 25% 100% 50% 25% 0% HPS LED NONE Lu m in an ce (c d/m 2 ) Le gi bi lity D ist an ce (ft ) Figure 18. Mean legibility distance for each lighting condition with luminance values measured from 640 ft. Each lighting condition impacts the measured luminance, but there is no consistent relationship with legibility distance. background, or Type IV legend on Type III background) did not significantly impact legibility distance, whether evaluated alone as a main effect or in an interaction. Conclusions While the findings from the closed-course study may appear to contradict the prevailing assumption that sign legibility is dependent on factors such as sign luminance, contrast, and sign sheeting materials, the important element to keep in mind is that the visual complexity of study location can be described as low because the testing facility track generally resembles a stretch of rural highway (with roadway lighting). In addi- tion, the lowest luminance level observed by a driver was 8 cd/m2, which is about 3.5 times more than the luminance level used by FHWA to derive minimum maintenance lev- els for retro reflectivity of overhead guide signs. Additionally, the legends contained only two 4-letter words and the par- ticipants drove at a constant speed of 35 mph, there was no inter ference involving maneuvers of other vehicles, and the drivers were required to read only one guide sign (in addi- tion to two speed limit signs). In short, the workload of the drivers on the closed course was much lower than what is typically experienced when reading overhead guide signs, allowing the drivers to focus more effort than normal on reading the target sign. This is not atypical of closed-course study designs. With the context described above, it may be said that the legibility distance of a reasonably bright sign will not increase with sign lighting or a particular type of sign sheeting in an environment where drivers experience low workload and low visual complexity. Although the relationships between legibil- ity and the factors evaluated in the closed-course study did not necessarily result in significant new discoveries, the lack of significance is still useful information, indicating there is no benefit to high levels of sign brightness in areas with low visual complexity. Information in Appendix A provides context on the quantitative effects of the multiple light sources evaluated in the factorial study. Figure 5 shows measurements of the guide sign illumi- nance and luminance. As expected, and verified with these measurements, the illuminance reaching a sign is not a good indicator of luminance. This is a particularly useful verifica- tion of results that can be used to revise sign lighting guide- lines (i.e., removing the illuminance levels). Illuminance level design would be justified if sign sheeting materials were dif- fuse reflective, which they were a long time ago. However, since essentially all overhead guide signs are made with retro- reflective sheeting materials, luminance is a much more appropriate performance metric for establishing guidelines. The open-road study described next in Chapter 4 was designed to expose drivers to the variety of conditions encoun- tered when they typically read signs, potentially addressing some of the limitations of the closed-course study. Neighbor- ing businesses, other vehicles (including oncoming vehicles), and complex geometric features are just some of the elements drivers experience on the open road that were not possible in the closed-course study.