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Determining Sign Size
Ed. Note: The following excerpt is an expanded version explaining to our readers how they determine perpendicular sign size, as detailed in Andrew Bertucci’s “Sign Legibility: Perpendicular Signs” that appears in the February 2007 issue of Sign Builder Illustrated. Andrew Bertucci is the executive director of the United States Sign Council.
The size of a sign is determined by the size and length of the message and the time required to read and understand it. It can be calculated once the numerical values of the five size determinants—Viewer Reaction Time, Viewer Reaction Distance, Letter Height, Copy Area, and Negative Space—have been established.
The step-by-step process to determine sign size, which is explained below, is useful not only as a calculation method, but also as a means of understanding the elements involved in the calculation (Figure 5).
Figure 5.
Area of Sign/Computation Process:
1. Determine speed of travel (MPH) in feet per second (FPS): (MPH x 1.47).
2. Determine Viewer Reaction Time (VRT).
3. Determine Viewer Reaction Distance (VRT x FPS).
4. Determine Letter Height in inches by reference to the Legibility Index (LI): (VRD/LI).
5. Determine Single Letter Area in square inches (square the letter height to obtain area occupied by single letter and its adjoining letterspace).
6. Determine Single Letter Area in square feet: Single Letter Area in square inches/144).
7. Determine Copy Area (Single Letter Area in square feet x total number of letters plus area of any symbols in square feet).
8. Determine Negative Space Area at 60 perecent of Sign Area (Copy Area x 1.5).
9. Add Copy Area to Negative Space Area.
10. Result is Area of Sign in square feet.
Area of Sign - Equation/Specific Usage
In addition to the computation method above, the USSC has developed an algebraic equation to determine the Area for signs containing letters only, which will provide the same result but will simplify the process (Figure 6).
Figure 6.
The equation allows for insertion of all of the size determinants, except for Negative Space, which is fixed at the standard 40/60 ratios. (Note: If numbers are rounded off in the computation process, a very slight difference in result may occur between the computation process and the equation).
Here’s how to work the equation:
1. Start with the first portion of the equation, which is three times the number of letters divided by 80. Three times 23 letters is 69; when divided by 80 the result is .8625. Keep this number ready for later use.
2. Compute the second part of the equation in brackets by multiplying VRT (Viewer Reaction Time), which is 10 by the MPH (miles per hour), which is 40. The multiplication product is 400.
3. Divide 400 by the LI (Legibility Index), which is 31, and the result is 12.90.
4. Square the 12.90 by multiplying it by itself (12.90 x 12.90) for a product of 166.
5. Finally, multiply the 166 by the .8625 obtained from the first part of the equation, and the resulting square footage is 143.
Area of Sign - Equation/Broad Usage
To allow for a broader scientific evaluation of sign size and satisfy the minimal legibility requirements across a full range of reaction times and speed zones, USSC has also developed a second more simplified equation (Figure 7).
Figure 7.
This formula fixes the average sign size determinants, leaving only Viewer Reaction Time (VRT) and the speed of travel (MPH) as the sole variables. It can be used effectively as a broad rule-of-thumb to ascertain the general size of signs necessary to adequately and safely convey roadside information to motorists traveling at a given rate of speed as well as to establish size parameters for signs across an entire community and/or road system.
Here’s how to work the equation, assuming VRT of 10 seconds and speed at 50 miles per hour:
1. Compute the values in the brackets by multiplying the VRT of 10 seconds by the MPH (miles per hour), which is 50. The multiplication product is 500.
2. Square the 500 by multiplying it by itself (500 x 500) for a product of 250,000.
3. Divide 250,000 by 800 for the resulting square footage of 312.
—Andrew Bertucci
Note: This story can be read in full in the February 2007 issue of Sign Builder Illustrated.
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