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Publication Number: FHWA-RD-03-082
Date: December 2003

Minimum Retroreflectivity Levels for Overhead Guide Signs and Street-Name Signs

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Figures

Figure 2. Graph. Weathering Degradation of Retroreflective Sheeting. This graph illustrates how weathered and unweathered retroreflectivities change with the observation angle of the ASTM Type 3 sheeting.  On the X-axis is the observation angle in degrees from 0.1 to 1, and on the Y-axis is the retroreflectivity of the sheeting in candela per lux per meters squared from 0 to 400. This graph shows a downward trend for both trend lines as the observation angle increases. However, the unweathered sheeting consistently has more retroreflectivity than the weathered sheeting. This is to be expected, as sheeting starts to lose its retroreflective properties as it is weathered.  There is also a consistent reduction in the retroreflectivities between the two lines as the observation angle increases, meaning that the effect of the observation angle on the retroreflectivity of the sheeting does not change between weathered and unweathered sheeting. Return to Figure

Figure 3. Chart. Overhead Sign Retroreflectivity Values.  This bar chart shows the average of the retroreflectivity readings taken on the white legend of the signs used in the study.  On the X-axis is the word displayed on the sign. These words, from left to right, are barley, bishop, dearly, eatery, flange, forget, nerves, nurses, ounces, plunge, season, senior, sensor, series, and shapes. They are written in capital letters. On the Y-axis is the average retroreflectivity for the legend in candela per lux per meters squared from 0 to 350.  The average retroreflectivity levels for the legends ranged anywhere from 230 candela per lux per meters squared for the word "bishop" to 290 candela per lux per meters squared for the words "nurses" and "ounces."  Return to Figure

Figure 4. Drawing. Layout of Overhead Sign Panel and Legend. This drawing shows how two words, "Flange" and "Felony" were spaced on the overhead sign for the study.  All the dimensions are given in millimeters and inches.  The words and letters were spaced according to the FHWA's standard highway alphabet.  The capital letters were 400 millimeters tall and were spaced 350 millimeters from the top and bottom of the sign.  The words were spaced 1,100 millimeters apart.  The sign measured 3.6 meters wide by 2.7 meters tall, and had a green background with white lettering. Return to Figure

Figure 5. Graphs.  Supplied Legend Luminance Graphs.  This figure contains six graphs, labeled A through F, which show the luminance values that were used to illuminate the signs during the testing.  Graphs A and B show the plots for the top word on an overhead sign for low and high beams, respectively.  Graphs C and D show the plots for the bottom word on an overhead sign for low and high beams, respectively.  Graphs E and F show the plots for a street-name sign for low and high beams, respectively.  Each graph has the same layout, with the X-axis displaying the dial position on the power supply from 1 to 16, and the Y-axis displaying the associated sign luminance value in candela per meters squared with different ranges. Graph A has a luminance value range of 0 to 3; graph B has a luminance value range of 0 to 25; graph C has a luminance value range of 0 to 4; graph D has a luminance value range of 0 to 50, graph E has a luminance value range of 0 to 7; and graph F has a luminance value range of 0 to 80. Graphs A through D have 3 plots; one each for 320, 480, and 640 feet, which are considered thresholds for certain sign legibility indices. Graphs E and F also have 3 plots; one each for 120, 180, and 240 feet.  In all the graphs, the trend lines increase as the dial position increases, because there is more light traveling to the sign, making the sign brighter. Return to Figure

Figure 6. Graph. Ford Taurus Headlamp Output. This graph shows the percent of light output from a Ford Taurus headlamp.  On the X-axis is the percent of light from the headlamps from 0 to 90 percent, and on the Y-axis are the BS2 counts used to maintain the percent of light output from 0 to 300.  The general trend of this graph is an increase in the BS2 counts as the percent of light increases.  Return to Figure

Figure 7. Picture. Control Box. This picture displays the box used to control the amount of illumination the headlamps produced.  The box contains a dial switch to change the headlamp output to predefined settings, a power switch for the laser, and a power switch for the box itself.  Return to Figure

Figure 8. Picture. Aiming Laser. This picture shows a closeup view of the aiming laser used in the study. The laser was mounted to the left of the hood latch in the car's grill area using a special bracket.  Return to Figure

Figure 9. Picture. Laser Location.  This figure shows the location of the laser in relation to the rest of the car.  This picture shows the front end of a white car. The car's grill has been removed because the laser has been mounted in the grill area. The laser is hard to distinguish in this photo because of the darkness in the grill area.  Return to Figure

Figure 10. Picture. Use of Laser for Aiming. This figure shows how the laser was used to position the car so that the car was centered on the sign as accurately as possible. This picture shows the overhead sign in the distance, in position along with the pavement markings that were used to align the car. A red laser dot on the pavement in front of the sign marks the actual position of the middle of the car.  Return to Figure

Figure 11. Picture. Luminance Readings. This figure demonstrates how the luminance readings were measured.  It shows a side view of a person sitting in the driver's seat of the test vehicle and looking into an LMT1900, which looks like a small video recorder. The device is on the person's lap, and he is taking luminance readings of the overhead sign, which is not shown in the picture.  Return to Figure

Figure 12. Graph. Chromaticity Color Shift (CIE, 1931). This graph shows the bounds of the chromaticity color shift, the FHWA limits for white, and the data points from the test vehicle.  On the X- and Y-axes are coordinates that represent the two-dimensional position in the color region enclosed by the curve, from 0 to 0.8 on the X-axis and from 0 to 0.9 on the Y-axis.  The FHWA limits on the color white begin around coordinates 0.35, 0.4 and end around coordinates 0.5, 0.4.  Inside the box defined by these coordinates are the illuminance readings taken from the headlamps.  This section of the graph is better displayed in the following figure.  Return to Figure

Figure 13. Graph. Closeup Chromaticity Color Shift (CIE, 1931). This graph shows a closeup view of the FHWA requirements for white headlamps and the data points taken from the headlamps of the test vehicle used in this study.  On the X- and Y-axes are coordinates that represent the two-dimensional position in the color region. The X-axis ranges from 0.35 to 0.55, and the Y-axis ranges from 0.3 to 0.5.  On the far left edge of FHWA's box, represented by a red dot, is the illuminance output when the dial box was set on 16. This output decreases from left to right.  Dial settings 1 and 2 are outside the FHWA limits, but the rest of the settings are within the FHWA requirements. This shows that varying the voltage to the headlamp is not affecting the color of the headlamps significantly, and that the procedure used in this study could be implemented.  Return to Figure

Figure 14. Graph. Color Temperature Shift.  This graph shows how the temperature of the chromaticity and color change with the dial position of the control box.  In this graph, the X-axis represents the dial position of the control box, from 1 to 16, and the Y-axis represents the chromaticity and color temperature in Kelvin, from 1,500 to 2,900.  The trend line on the graph shows a steady increase in the temperature as the dial position is increased, which means that, as the illuminance increases, the temperature also increases.  Return to Figure

Figure 15. Diagram. Test Course. This diagram shows the location of the overhead sign and street-name signs used in the study as well as the driving path used by the vehicle.  The diagram shows the street-name sign to the right of the driving path, and the overhead sign is directly in front of the driving path to simulate an actual overhead sign on a roadway.  Return to Figure

Figure 16. Picture. Overhead Sign. The is a picture of the overhead sign used for testing with the word "Shapes" in the top position and the word "Flange" in the bottom position. The sign has a green background with white lettering. Return to Figure

Figure 17. Picture. Street-Name Sign. This is a picture of a street-name sign used during the study with the word "Airplane" in white, capital letters on a green background.  Return to Figure

Figure 18. Graph. Overhead Sign Results.  This graph shows how the different size fonts used for overhead traffic signs affect the luminance required by a certain percentage of the population. 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 accommodated by the given log of the luminance from 0 to 100.  The graph shows that 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.  The graph represents a typical overhead guide sign.  Return to Figure

Figure 19. Graph. Street-Name Sign Results. This graph shows how the different size fonts used for street traffic signs affect the luminance required by a certain percentage of the population. 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 accommodated by the given log of the luminance from 0 to 100.  The graph shows that 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.  The graph represents a typical street-name sign. Return to Figure

Figure 20. Graph. Results for Overhead Signs. This graph shows the percentage of the driving population that is accommodated at certain luminance levels at a given legibility index.  On the X-axis is the legibility index from 20 to 40 feet per inch, and on the Y-axis is the log of the minimum luminance in candela per meters squared from 0.1 to 100.  Shown on this graph are seven lines that represent the expected percentage of drivers that would be accommodated by a given luminance and legibility index.  The percentages are 10, 25, 50, 75, 85, 95, and 98.  In all cases, the luminance values increase as the legibility index increases (from left to right), because a larger legibility index requires more luminance for the same percentage of the population to be able to read and interpret the sign.  Return to Figure

Figure 21. Graph. Results for Street-Name Signs. This graph shows the percentage of the driving population that is accommodated at certain luminance levels at a given legibility index.  On the X-axis is the legibility index from 20 to 40 feet per inch, and on the Y-axis is the log of the minimum luminance in candela per meters squared from 0.1 to 100.  Shown on this graph are seven lines that represent the expected percentage of drivers that would be accommodated by a given luminance and legibility index. The percentages are 10, 25, 50, 75, 85, 95, and 98. In all cases, the luminance values increase as the legibility index increases (from left to right), because a larger legibility index requires more luminance for the same percentage of the population to be able to read and interpret the sign.  Return to Figure

Figure 22. Graphs. Isocandela Plots of CARTS50 (Top) and UMTRI25PC Headlamp.  This figure shows two graphs that compare the headlamp output for the CARTS50 and UMTRI25 PC headlamps.  On the X-axis of both graphs is the horizontal distance away from the center of the headlamp in inches from 0 to 22,000, and on the Y-axis is the vertical distance away from the center of the headlamp in inches from 0 to 22,000.  The center of the headlamp was positioned at coordinates 0, 0.  These graphs demonstrate how headlamps have changed over the years and how their outputs above the X-axis have fallen because of the shift in the intensity between the top and bottom graphs.  This means that less light is making it to the overhead signs, and that as the vehicle fleet changes, the minimum retroreflectivity levels will have to be reevaluated.  Return to Figure

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 minimum 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).  Return to Equation

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. Return to Equation

Equation 3. Equation. Demand Retroreflectivity at the Standard Geometry.  This is the same equation as equation 2. 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.  Return to Equation

Equation 4. Equation. Minimum Retroreflectivity Level at the Standard Geometry.  This is the same equation as equation 1.  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 minimum 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). Return to Equation

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