Jun 28, 2019

If a sign stands by the highway but nobody can read it, does it say anything? That’s kind of like the old riddle about the tree falling in the forest, but with practical consequences. After all, what’s the point of a sign that few or none can read? If we put our minds to it, we could probably come up with some exceptions. Generally speaking, however, the entire purpose of signage is communication of one kind or another. That’s why experienced designers and installers of signage have to be experts in sign legibility.


Familiar with the eye chart in the doctor’s office? They ask patients to read a series of increasingly smaller letters. From a standard distance. The size of the smallest letters the patient can read is the measure of his or her vision. Maybe, then, that chart’s a good starting point for a discussion of signage legibility.

The standard (Snellen) eye chart’s 8th line is the one a patient reads to show 20/20 vision or better. The upper-case letters D E F P O T E C on that line are 8.73 mm in height. The chart is at a distance of 20 feet from the patient. This, then, is our marker. A person with normal vision can read letters 8.73mm/0.34 inches tall from a distance of 20 feet. But note! This is under ideal conditions. There’s no motion, the chart is illuminated, the letters are upper case block font, high contrast against chart background, etc. Nevertheless, it’s a useful benchmark.


Working from this standard, signage professionals have developed their own letter height charts. These guide sign design under real-world conditions. A highway sign, for example, is meant to be read by people traveling at high speeds, under variable conditions of illumination. In contrast, a sign in a hospital waiting room has a captive audience and constant lighting. It’s not nearly as simple as a non-professional might imagine. Everything matters. To pick a legibility factor at random, consider the orientation of the sign in relation to its intended audience. Will the audience be facing the sign directly, head on? Or will the sign be parallel to the reader’s direction of movement? Somewhere in between?

The United States Sign Council (USSC) has published the definitive reference book on the subject. The title acknowledges that signage is an art as well as a science. Sign Legibility Rules of Thumb isn’t by any means light summer reading. It’s worth a look, though, to anybody interested in gaining an appreciation of the knowledge and skill professional sign designers and installers bring to the table.

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