Overview
When a driver is driving at 45 miles per hour, he or she travels 66 feet per second. That's because if it takes longer than three seconds to read your sign, that customer is long gone. Many businesses receive a mockup of the sign, display it on a laptop screen, and discover that the font size or the font is too small to be seen from the roadside. We discuss the 1 inch for every 10 feet rule below, as well as the impact of speed on the math and what we measure on a site before a structure is ever erected. Our custom outdoor business signs can establish a strong presence on a property, but each letter still needs to be sized so drivers can read it in time.
The Golden Rule of Sign Legibility: The 1-Inch-per-10-Feet Formula
The base (or starting point) of the sign industry is simple. A person with 20/20 vision can see about 10 feet away with one inch of letter size. A 10" letter can be read from about 100' away.
This figure decreases when the viewer is moving. Two working ranges are utilized by sign professionals for a driver to be able to watch traffic, brake lights, and lane markings. At approximately 30 to 40 feet per letter height, the "best legibility" is where the text is easily read and can be read at a rapid pace. Maximum usable is 50 or 60 feet per inch, where words are readable, but take effort to read. Produce the initial design first, and do not rely on the second range.
For planning purposes of moving traffic:
- 4-inch letters: approximately 100 feet
- 8-inch letters: approximately 200 feet
- 15-inch letters: approximately 400 feet
- 24-inch letters: 600 feet or more
All of these can change: font, color, lighting, approach angle; and therefore, a measured site visit has a better chance than any chart.
Speed-Adjusted Reaction Time: Matching Letter Height to Roadway Limits
Driver Reaction Window: Most drivers require 1.5- 2 seconds to see the sign, another 3- 5 seconds to read it and respond, such as signal, brake, or turn in. At the speed limit, that window translates into the distance a driver travels while seeing and reading the sign.
Low-Speed Urban & Pedestrian Zones (15–25 MPH)
In downtown blocks and walking plazas, signs are often viewed from roughly 30 to 100 feet away, with more time to take in the surroundings. Pedestrians notice things at eye level, and letters may be smaller here. Double-sided commercial blade signs perpendicular to the sidewalk provide instant recognition for shoppers in dense areas where foot traffic prevails.
Arterial Commercial Corridors (35–45 MPH)
The strip centers, the independent restaurant, and the auto service shop are sometimes 150 to 350 feet from the multi-lane road. If it is a main name, expect to use 12 to 18 inches of lettering. The most common sizing error we see is going smaller. To ensure customers don't overlook the turn, high-visibility commercial storefront signs are scaled to the actual setback on roads like this.
Highway & Interstate Corridors (55–70+ MPH)
At interstate speeds, signs may need to be readable from roughly 500 to more than 1,000 feet away. Along I-15, properties may require freestanding elevated structures with capital letters roughly 24 to 48 inches tall. Structure height matters too, because trees, overpasses, and terrain can block an otherwise large letter. Interstate properties require custom commercial pylon signs that are high above the surrounding tree lines to ensure high-speed motorists can read them in time.
Typography, Negative Space & Visual Contrast Factors
But a sign is not necessarily readable because of its size. If the 24" letter is not in the correct font, it can fail to perform as well as the 15" letter in the correct font.
Font choice and stroke width: Bold, clean sans-serif fonts like Helvetica, Montserrat, and Futura maintain their straight lines and wide spaces at a distance. The thinnest sections of script fonts, fancy lettering, and hairline weights are the first to become smudged.
Kerning and negative space: Letters at high speed merge together if they are packed too close together. Allow space for characters and margin around the text. Short beats crowded. A sign which attempts to convey five messages will say none.
Color Luminance Contrast (The 30% Rule): The difference in brightness is more important than the color. Separation is good in strong light when black on yellow, white on deep blue, or green on white. The aim is to achieve a 30 percent or greater luminance difference between letters and background. Custom front-lit channel letters with high contrast stay crisp in low lighting and in bright afternoon sun in Utah.
Day vs. Night Readability: The Role of Internal Illumination
A sign that works during daylight hours may not work at night. The common cause is typically "halation. If the light is too intense for the letter face, then the light will leak out of the face, and the characters will grow together. A well-designed illuminated sign features calibrated diffusers and a balanced LED output, which will ensure even light on the faces and not on the edges. The aim is clear text on a dark sky background so as not to dazzle an oncoming motorist. More light will not help, while even light is helpful.
Frequently Asked Questions About Sign Sizing & Legibility
How large should our storefront letters be if our building is set back 150 feet from the road?
Main brand lettering should be a minimum of 12 to 15 inches tall on a road with a 35-40 mph speed limit. This allows drivers a sufficient amount of time to see the sign and respond at ease.
Does sign color choice affect readable distance?
Yes, absolutely. A high-contrast combination, such as a black background and white illuminated letters, can increase the readable distance by up to 30 percent over a weak-contrast combination, like a red background and white illuminated letters.
Can we use decorative or script fonts for exterior business signs?
They can be utilized for a secondary tagline as well as interior branding. They're not recommended for your primary identity because fancy serifs and uneven strokes make them hard to read and become hard to see at driving speed.
How does Visibility Signs determine the exact size needed for our building?
Optical laser rangefinders, camera passes while you're driving by, and digital mockups testing your sightlines to the local speed limit and city zoning codes.
Optimize Your Signage for Maximum Visibility
Please do not estimate sign sizes. It's much more expensive to correct the "sizing" error after installation than it is to get it right the first time! It works as follows:
- Please provide your facility address and approximate setback from the main road.
- We simulate your sightline obstructions, travel speeds, and viewing angles.
- Scaled dimensional proofs are sent to you; signage is fabricated for reading.
Call us at (801) 212-9987 or use the Request a Free Sizing & Visibility Audit button to get started. We provide commercial corridors and industrial campuses throughout Salt Lake City, West Valley City, Sandy, Draper, Lehi, American Fork, Orem, and Provo.