How the United States Sign Council's Sign Legibility Index was Used to Secure a Variance

A sign company wanted to build a 90-square-foot, double-faced, internally illuminated sign for a shopping plaza near Harrisburg, Pennsylvania. The sign code only allowed a 25-square-foot sign. The sign company also wanted to build a 250-square-foot, 80-foot-tall freeway sign for a Holiday Inn Express. There, signs were limited to a 40-foot height and 80 square feet. So variances were needed.  

The sign-company owner was very familiar with the United States Sign Council's Sign Legibility Index (SLI), which calculates the distance in feet from which a 1-inch letter can be read. The distances are directly proportionate, so the legibility distance would double for a 2-inch letter, quadruple for a 4-inch letter, etc. Slight distinctions are made for certain fonts and background/foreground color combinations. The owner also used a standard reaction time of 10 seconds to read a sign.

For the plaza sign, the street has a 45-mph speed limit, which is also 66 feet per second. With the 10-second rule, this means a sign would need to be legible from 660 feet away at that speed. The SLI stated that 1-inch letters need to be legible from a distance of 26 feet. To be viewed from 660 feet, the letters would need to be 25 inches tall.

The owner created proposed signs that would satisfy the existing sign code, and ones that would adhere to the SLI suggestion. These were then superimposed onto photographs taken from a 660-foot distance. These presentations indicated that signs that adhered to the existing sign code would not be legible soon enough for motorists to react to them. In each case, the variance was granted.