Part 2: The Science of Outdoor Signs and Banners

How to Calculate the Best Letter and Graphic Sizing for the Street, Road, or Highway Where You’re Advertising Your Business with a Sign or Banner

Original Question: We are designing some banners for our business, and are discussing what colors to use on them. What input can you give us regarding contrast and overall banner color design.

Continuing Answer: As stated in part one of this series, I am borrowing heavily from the USSC publication “Sign Legibility Rules of Thumb” in order to show you, dear reader, that you just can’t slap up a cheap, poorly designed sign and expect the world to beat a path to your business’ door.

If this were the case, the big corporations would not have huge signs with huge letters beckoning you from three counties away. They do it for a reason, and this article will continue to elucidate to you why it is important to have your outdoor business signs and banners professionally designed by designers who actually understand the “rules” of color, distances, contrast, and size. pavement sign

The USSC has developed a “Legibility Index” (LI) which is a number value which represents the distance (in feet) that a letter can be read. The LI for a one inch capital san serif letter would be 30, meaning that you should be able, with normal vision, to see that letter at about 30 feet, or a ten inch letter at 300 feet. Surprisingly, all capital letters need about 15% more height than do upper and lower case lettering combinations.

The chart was developed by the USSC to help illuminated sign and graphic designers determine viewing distance with factors such as the type of illumination, letter style, background color, letter color, and size of letter.

The calculation for your sign’s viewing distance, using the chart above, combined with our previous VRD formula (Viewer Reaction Distance), to create the letter height formula by dividing the VRD by the LI to calculate the letter height needed for a sign.

Reviewing our terms so far –

VRT = Viewer Reaction Time

VRD = Viewer Reaction Distance

LI = Legibility Index

To illustrate this, let’s say your target driver needs to see your sign at 600 feet away, based on our previous calculation (VRD = MPH x VRT or 1.47), and the LI is 30, your letters should be 20 inches in height.

The calculation for this would look like this – VRD (600′) divided by the LI (30) equals 20 feet. In strictly numeric values, 600/30 = 20. In word form, VRD (in feet) / LI = Letter Height (in inches. Yeah, it’s a bit complicated, but it takes the guesswork out of your sign design.

And most of the time, by the way, the LI used is 30, which keeps it simple if slightly less accurate. In our design, we simply err size-wise about 20% larger than what the index states, so that if it says we need a 20″ letter, more often than not, we’ll prescribe a 24″ letter. With exterior signs or banners, bigger IS better (if allowed by your local sign codes).

So, to review everything in Parts 1 and 2 that I’ve outlined, we’ve learned that your prospective sign viewer is affected by –

1) Number of Traffic Lanes

2) Speed of Traffic

3) Viewer’s Reaction Time (based on the previous two factors)

4) Viewer’s Reaction Distance (also based on the first two factors)

5) Type of Letters Used on Your Sign

6) Colors Used on Your Sign Letters

7) Background Color of Sign

8) Type of Illumination Used – or not used

And you thought it was just a banner!?

There are more exact legibility calculations put out by the USSC – the “On-Premise Sign Standards” – which provides more precise formulas and guidelines for complex areas. However, for most exterior banners or signs, in our opinion, these guidelines suffice quite well.

Of course, we cannot leave the design simply at six words with letters that contain 24 inch letters. We need to design your sign so as to be maximally readable. That means that with your 24 inch letters, we also will need to calculate the “negative space” – or the area between and outside the letters – to properly calculate the overall size of the sign.