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Publication Number: FHWA RD-03-081
Date: June 2003

508 text for Updated Minimum Retroreflectivity Levels

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Figures

Figure 1. Graph. Minimum Luminance Data for Warning Signs. This graph shows an example of the required luminance of a typical traffic warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required in candela per meters squared from negative 1.5 to 3.0. Subject data, CARTS predicted, and luminance at recommended minimum are charted. Luminance at recommended minimum is represented as a straight, horizontal dotted line across the graph at approximately 0.9 log luminance. The subject data and CARTS predicted both show a general upward trend in luminance as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figures 2A and B. Graphs. Minimum Luminance Requirements. These graphs show how different size fonts used for traffic signs affect the luminance required by a percentage of the population. Graph A represents a typical overhead guide sign, and graph B represents a typical street-name sign. On the X-axis of both graphs is the log of the luminance in candela per meters squared from 0.1 to 100, and on the Y-axis of both graphs is the cumulative percentage of drivers accommodated by the given log of the luminance from 0 to 100. In graph A, a log luminance value of 1 will accommodate 85 percent of drivers for signs with a legibility index of 20 feet per inch, 60 percent of drivers for signs with a legibility index of 30 feet per inch, and only 15 percent of drivers for signs with a legibility index of 40 feet per inch. In graph B, a log luminance of 1 will accommodate 89 percent of drivers with a legibility index of 20 feet per inch, 60 percent of drivers with a legibility index of 30 feet per inch, and 15 percent of drivers with a legibility index of 40 feet per inch.

Figure 3. Graph. Scatterplot of Data from Mercier et al. and TTI. This graph compares the luminance requirement for white and green signs for people at different ages. On the X-axis is the age of the driver in years from 20 to 80, and on the Y-axis is log of the luminance in candela per meters squared required for the driver to see and read the sign from 0.1 to 100. Three sets of data are being compared: CARTS predicted, Mercer et al., and TTI. The CARTS data shows required luminance levels from 1.5 to 8 as the driver ages, the Mercer et al. data shows required luminance levels from 0.2 to 1.5, and the TTI data shows required luminance levels from 0.3 to 1.8, which are similar to the Mercer et al. data. Comparing these data sets on the graph shows that the CARTS predicted luminance levels are far above what drivers actually need.

Figure 4. Graph. Comparison of Data for Older Drivers Only. This graph compares different critical detail times for Mercer et al. and TTI data based on the percentage of drivers accommodated by the different critical detail times given the specified luminance level. On the X-axis is the log of the luminance in candela per meters squared from 0.1 to 100, and on the Y-axis is the cumulative percentage of drivers that are accommodated by the given log of the luminance from 0 to 100. The data represented are from Mercier et al., critical detail time (CD) equals 1.32 minutes; Mercier et al., CD equals 1.56 minutes; TTI, CD equals 2.0 minutes; TTI, CD equals 1.32 minutes; and TTI, CD equals 1.0 minute. In this graph, the TTI data is similar to the Mercer et al. data for all critical detail times; four of the five require no more than 10 candelas per meters squared to accommodate 100 percent of the drivers. The exception is the TTI with a CD of 1.0 minute, which requires more luminance to accommodate the same percentage of drivers than the other plots-approximately 80 candelas per meters squared were required to accommodate 100 percent of the drivers.

Figure 5. Graph. Effect of Contrast Ratio on Legibility. This graph compares the contrast ratio of signs based on the legibility index of the letters used for the sign. On the X-axis is the log of the contrast ratio of the legend to the background from 1 to 10,000, and on the Y-axis is the legibility index of the sign in both feet per inch from 20 to 80, and meters per centimeter from 2.4 to 9.6. Four authors' data are represented on this graph: SOP-10; OB-10; MGH-8E lowercase M; and MGH-12E lowercase M. Except for some of the SOP-10 data, all of the data in this graph have contrast ratios greater than 3 to 1 for a legibility index of 40 feet per inch. This graph supports a ratio of at least 3 to 1, if not greater.

Figure 6. Graph. Cumulative Percentage of Driver Population as a Function of Driver Age for Trips at Different Times of Day (Source: National Personal Transportation Survey, 1995). This graph shows the percentage of drivers based on the age of the drivers that travel during different times of the day. Three sets of data are charted: number of licensed drivers, N equals 241,675,000; number of trips from 6 AM to 7 PM, N equals 192,523,609,978; and number of trips from 7 PM to 6 AM, N equals 37,190,250,230. The age of the driver in 5-year increments from 15 to 85 and older is on the X-axis, and the cumulative percentage of the driver population from 0 to 100 is on the Y-axis. This graph shows that more than 89 percent of the drivers are under 55, and 96 percent are under 65 years old. This graph also shows that there appears to be no difference between the number of drivers who drive during the day or night.

Figure 7. Graph. Rotational Sensitivity of Four Types of Retroreflective Sheeting Materials. This graph compares the measured retroreflectivity to the rotational sensitivity of five different types of signs-626 control, 630 control, 651 control, 657 control, and 101 control-and how they compare with aged signs of the same type. On the X-axis is the rotational angle from 0 to 360 degrees, and on the Y-axis is the measured retroreflectivity in candela per lux per meters squared from 0 to 1,200. The aged signs closely track their control counterparts for types 626, 630, and 101. The measured retroreflectivity for the 101 control and aged signs remains constant at 300 and 250 candelas per lux per meters squared, respectively. The measured retroreflectivity for the 630 control and aged signs fluctuates between a low of 500 and a high of 1,050 candelas per lux per meters squared, across different rotation angles. The measured retroreflectivity for the 626 control and aged signs fluctuates between a low of 200 and a high of 500 candelas per lux per meters squared, across different rotation angles.

Figures 8A and B. Graphs. Observation Angle Profiles as a Function of Weathering. This figure includes two graphs. Graph A shows the data from a test panel in Louisiana with a beta, or entrance, angle of 4 degrees, and graph B shows data from the same test panel with a beta, or entrance, angle of 30 degrees. In both graphs, the X-axis is defined as the observation angle in degrees from 0 to 1, and the Y-axis is the deviation from uniform deterioration in percentages from 50 to negative 50. These graphs plot four sheeting types (types 3, 7, 8, and 9) and show how the prismatic sheetings are very sensitive to the observation angle when compared to a glass beaded sheeting (type 3). Types 7 and 8 vary from 20 to almost 40 percent, depending on the observation and entrance angles, but the type 9 and type 3 sheetings do not vary as much (only 5 percent in either direction).

Figure 9. Graph. Right Curve (E1). This graph shows an example of the required luminance of a typical right curve warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2.5. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 10. Graph. Right Curve (A1). This graph shows an example of the required luminance of a typical right curve warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 11. Graph. Right Intersection (E2). This graph shows an example of the required luminance of a typical right intersection warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2.5. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 12. Graph. Right Intersection (F3). This graph shows an example of the required luminance of a typical right intersection warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 13. Graph. Right Intersection (A2). This graph shows an example of the required luminance of a typical right intersection warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 14. Graph. Right Intersection (B3). This graph shows an example of the required luminance of a typical right intersection warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2.5. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 15. Graph. Narrow Bridge (E3). This graph shows an example of the required luminance of a typical narrow bridge warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 16. Graph. Narrow Bridge (A3). This graph shows an example of the required luminance of a typical narrow bridge warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 17. Graph. Right Lane Ends (E4). This graph shows an example of the required luminance of a typical right lane ends warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 18. Graph. Right Lane Ends (F4). This graph shows an example of the required luminance of a typical right lane ends warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 19. Graph. Right Lane Ends (A4). This graph shows an example of the required luminance of a typical right lane ends warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1 to 2. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 20. Graph. Right Lane Ends (B4). This graph shows an example of the required luminance of a typical right lane ends warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 21. Graph. Bicycle (E5). This graph shows an example of the required luminance of a typical bicycle warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 22. Graph. Bicycle (F5). This graph shows an example of the required luminance of a typical bicycle warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 1.5. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 23. Graph. Bicycle (A5). This graph shows an example of the required luminance of a typical bicycle warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 24. Graph. Bicycle (B5). This graph shows an example of the required luminance of a typical bicycle warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 25. Graph. Pedestrian (F1). This graph shows an example of the required luminance of a typical pedestrian warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 26. Graph. Pedestrian (B1). This graph shows an example of the required luminance of a typical pedestrian warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 1.5. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 27. Graph. Exit 25 MPH (F2). This graph shows an example of the required luminance of a typical exit 25 MPH warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2.5. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 28. Graph. Exit 25 MPH (B2). This graph shows an example of the required luminance of a typical exit 25 MPH warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2.5. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 29. Graph. Do Not Pass (G2). This graph shows an example of the required luminance of a typical do not pass warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 30. Graph. Do Not Pass (C2). This graph shows an example of the required luminance of a typical do not pass warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 31. Graph. Keep Right (G3). This graph shows an example of the required luminance of a typical keep right warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 32. Graph. Keep Right (C3). This graph shows an example of the required luminance of a typical keep right warning sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 33. Graph. No Right Turn (G4). This graph shows an example of the required luminance of a typical no right turn sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 34. Graph. No Right Turn (C4). This graph shows an example of the required luminance of a typical no right turn sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 35. Graph. One Way (G5). This graph shows an example of the required luminance of a typical one way sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 1.5. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 36. Graph. One Way (C5). This graph shows an example of the required luminance of a typical one way sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 37. Graph. Stop (H1). This graph shows an example of the required luminance of a typical stop sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 2 to 1.5. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 38. Graph. Stop (D1). This graph shows an example of the required luminance of a typical stop sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 1.5. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 39. Graph. Do Not Enter (H3). This graph shows an example of the required luminance of a typical do not enter sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 2 to 1.5. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 40. Graph. Do Not Enter (D3). This graph shows an example of the required luminance of a typical do not enter sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 1.5. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 41. Graph. Corning 12 (H2). This graph shows an example of the required luminance of a typical Corning 12 sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 1.5. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 42. Graph. Corning 12 (D2). This graph shows an example of the required luminance of a typical Corning 12 sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 2 to 1.5. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 43. Graph. Gravity (H4). This graph shows an example of the required luminance of a typical gravity sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 1.5. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 44. Graph. Gravity (D4). This graph shows an example of the required luminance of a typical gravity sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 1.5. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 45. Graph. Speed Limit 50 (G1). This graph shows an example of the required luminance of a typical speed limit sign. On the X-axis is the age of the subject from 0 to 90, and on the Y-axis is the log of the luminance required from negative 1.5 to 2. The graph has a general upward trend as age increases, meaning that as drivers age, they require more luminance to perceive and read the sign. The main point of this graph is that it shows that the minimum luminance levels from the CARTS program are higher than the luminance levels required for most drivers based on experimentation with subjects.

Figure 46. Graph. Results for the Curve Sign (I.E., Bold Warning Sign). This graph compares the recommended retroreflectivity level for a typical curve sign with human subjects. On the X-axis is the percent of the subjects, from 0 to 100, that could successfully read and interpret the sign at the given log of the retroreflectivity level on the Y-axis, from 1 to 1,000 candela per lux per meters squared. The recommended minimum retroreflectivity level for this type of sign is 50. At this level, 42 percent of the subjects could successfully read the sign, using the best-fit line for the data. There is a text box next to the graph that reads, "Recommended Level (R lowercase A equals 50)" and "Best-Fit Line R squared equals 0.82."

Figure 47. Graph. Results for the Divided Highway Ends Sign (I.E., Fine Warning Sign). This graph compares the recommended retroreflectivity level for a typical divided highway ends sign with human subjects. On the X-axis is the percent of the subjects, from 0 to 100, that could successfully read and interpret the sign at the given log of the retroreflectivity level on the Y-axis, from 1 to 1,000 candela per lux per meters squared. The recommended minimum retroreflectivity level for this type of sign is 70. At this level, 52 percent of the subjects could successfully read the sign, using the best-fit line for the data. There is a text box next to the graph that reads, "Recommended Level (R lowercase A equals 75)" and "Best-Fit Line R squared equals 0.9387."

Figure 48. Graph. Results for Stop Sign (I.E., White-on-Red Iconic Sign). This graph compares the recommended retroreflectivity level for a typical stop sign with human subjects. On the X-axis is the percent of the subjects, from 0 to 100, that could successfully read and interpret the sign at the given log of the retroreflectivity level on the Y-axis, from 1 to 1,000 candela per lux per meters squared. The recommended minimum retroreflectivity level for this type of sign is 7. At this level, 61 percent of the subjects could successfully read the sign, using the best-fit line for the data. There is a text box next to the graph that reads, "Recommended Level (R lowercase A equals 7)" and "Best-Fit Line R squared equals 0.9486."

Equations

Equation 1. Equation. Minimum Retroreflectivity Level at the Standard Geometry. This equation gives the minimum required retroreflectivity level at an observation angle of 0.2 degrees and an entrance angle of negative 4 degrees, or R subscript A. To calculate the minimum R subscript A, the average retroreflectivity of new sheeting at the standard geometries (New R subscript A, SG) is multiplied by the quotient of the retroreflectivity required to produce the demand luminance (Demand R subscript A, NSG) divided by the retroreflectivity of the new sheeting at a nonstandard geometry (Supply R subscript A, NSG).

Equation 2. Equation. Demand Retroreflectivity at the Standard Geometry. This equation gives the demand retroreflectivity at the standard geometry. The demand retroreflectivity at the standard geometry (R subscript A, NSG) is calculated by multiplying the minimum luminance required by the driver by the cosine of the viewing angle (Nu) then diving the total by the illuminance.

Equation 3. Equation. Supply Luminance. This equation gives the supply luminance, L, meaning the amount of light that makes it to the sign face. The supply luminance is found by adding the product of the illuminance from the left headlamp (R subscript A, left) and the coefficient of retroreflection for the left headlamp (E subscript left) to the product of the illuminance from the right headlamp (R subscript A, right) and the coefficient of retroreflection for the right headlamp (E subscript right) then dividing the total by the cosine of the viewing angle (Nu).

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