Guide signs are an important part of navigating roads safely and efficiently. The colors and blues used on guide signs are carefully chosen to optimize visibility and communicate important information to drivers. In the United States, standards for guide sign colors are set by the Manual on Uniform Traffic Control Devices (MUTCD) published by the Federal Highway Administration.
Blue Backgrounds
The most common background color for guide signs on highways and major roads in the US is blue. The specific blue that is used is officially called “Retroreflective Sheeting Blue” in the MUTCD standards. This refers to the retroreflective properties that make the signs shine brightly under vehicle headlights at night. The blue also provides good contrast with the white and black text used on guide signs.
The exact specifications for the blue color are:
- Chromaticity coordinates:
- x: 0.097
- y: 0.210
- Luminance factor (Y%): 10 min.
These coordinates define a specific shade of blue on the CIE 1931 color space that has been chosen for optimal visibility. By standardizing the specific blue across all guide signs, drivers become familiar with this color and learn to quickly recognize the important navigational information signs provide.
Green Backgrounds
While blue is the most common, green backgrounds are also used on guide signs for certain applications. The green color provides similarly good contrast and visibility as the blue. Green guide signs are often used on low-speed roads, parkways, in recreational areas, and for general information or cultural interest signs.
The specific “Retroreflective Sheeting Green” used on signs has the following Chromaticity coordinates:
- x: 0.030
- y: 0.398
The luminance factor (Y%) is also above 10 minimum just as with the blue. The green shade provides visibility and communicates a different navigational context to drivers than the more common blue guide signs.
White Lettering
The letters, numbers, arrows, and symbols on guide signs are specified to be white. White provides the maximum contrast against the dark blue or green background colors. This ensures that the informational content of the signs can be read from as far away as possible.
The specific white used is officially called “Retroreflective Sheeting White” with the following chromaticity coordinates:
- x: 0.303
- y: 0.287
Black lettering is occasionally used for low-speed guide signs where reduced visibility distances are acceptable. But for all major guide signs, high-contrast white letters are required.
Borders
Guide signs typically have a white retroreflective border around the outer edge. This border frames the sign and separates it visually from the background landscape. The white provides good contrast against both the blue and green sign backgrounds.
For rectangular signs, the border width must be at least 0.5 inches on all sides. The corners are also required to be rounded with radii between 1 inch and 1.5 inches.
Retroreflectivity
Retroreflectivity is an important functional property of guide sign colors. This refers to the ability to reflect light from a source like vehicle headlights back towards the origin. Retroreflectivity makes signs appear bright at night when illuminated.
All the colors used on guide signs, including the blue, green, white, and black are required to meet minimum retroreflectivity standards. This ensures visibility of the signs 24 hours a day.
There are three grades of retroreflective sheeting defined by the MUTCD:
| Grade | Minimum Coefficient of Retroreflection (cd/lx/m^2) |
|---|---|
| Engineering | 250 |
| High Intensity | 380 |
| Highest Intensity | 575 |
Guide signs typically use High Intensity or Highest Intensity grade sheeting to ensure good retroreflectivity and visibility at night. The specific grade needed depends on the functional classification of the roadway and speed limits.
Contrast Ratios
In addition to meeting retroreflectivity standards, guide sign colors also must meet minimum contrast ratio requirements. This specifies the difference in luminance between the sign’s legend (letters, numbers, symbols) and its background.
The minimum contrast ratio for guide signs is 3:1. So the luminance of the white legend must be at least 3 times greater than the luminance of the blue, green, or black background. Contrast ratios of 5:1 or more are recommended for improved visibility.
Luminance
The luminance of a sign’s retroreflective colors also must meet standards. Luminance is a measure of how bright the sign appears when illuminated at night. Minimum luminance values ensure signs are visible and legible in nighttime driving conditions.
The white legend elements on guide signs must have a luminance factor of at least .8 when illuminated by standard headlights. Blue, green, and black sign backgrounds must have a luminance factor of at least .2.
Daytime Requirements
In addition to nighttime retroreflectivity and luminance, guide sign colors also have to meet standards for daytime visibility. During daylight, a sign’s appearance is determined more by its pigment colors rather than retroreflectivity.
The daytime specifications require that the pigment colors fall within the following chromaticity boundaries:
| Color | Chromaticity Coordinates (Corner Points) |
|---|---|
| White |
(x: 0.305, y: 0.295) (x: 0.355, y: 0.355) (x: 0.285, y: 0.325) (x: 0.335, y: 0.375) |
| Blue |
(x: 0.078, y: 0.171) (x: 0.150, y: 0.220) (x: 0.210, y: 0.160) (x: 0.137, y: 0.038) |
| Green |
(x: 0.030, y: 0.398) (x: 0.166, y: 0.364) (x: 0.286, y: 0.446) (x: 0.207, y: 0.771) |
| Black |
(x: 0.385, y: 0.355) (x: 0.300, y: 0.270) (x: 0.260, y: 0.310) (x: 0.345, y: 0.395) |
This ensures the colors are within the optimal ranges for visibility and legibility under varying daytime lighting conditions.
Conclusion
Guide signs in the United States follow very specific standards for colors defined in the Manual on Uniform Traffic Control Devices. The blue, green, white, and black used have properties optimized for visibility, legibility, and retroreflectivity. Adhering to the MUTCD specifications for hue, luminance, contrast ratios, and retroreflectivity helps guide signs effectively communicate the information necessary to safely navigate roads in all conditions.
