Designing signs for legibility: Viewing time

By Ken Millar

Posted on Monday, February 15th, 2021

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Outdoor advertising companies usually have a salesperson to sell potential clients on using billboards at existing locations for an advertising contract. Often the salesperson takes a beautiful layout to the client, who holds it at arm’s length for viewing.

The client seldom knows much about the issue of readability at a given distance. The salesperson may not be aware of the guidelines that will guarantee good, readable signage, either. The end result is often that the salesperson is happy to sell the location—but the client is not happy when he drives by the billboard location he has rented and cannot read the message.

I see this failure almost daily and wonder who is to blame. The same problem is apparent on fascia signs, vehicles, freestanding signs and most any other type of sign. The primary message of many signs simply can’t be read from the distance at which they can be seen.

Letter size is a big factor, as is color and contrast. [See Designing signs for legibility: Letter size.] The amount of time the viewer has to read the message is also critical. Finally, it is also important to be able to present the drawing to the customer effectively and explain your design in terms of readability.

By becoming familiar with the formulas for readability, the sign designer and the salesperson will both gain knowledge that has been proven over time to produce better signage. It is a crucial skill if you want to provide effective signs for your customers.

Viewing time is a critical factor

Viewers typically have three to five seconds to read a sign once they are close enough to see it adequately. This assumes the lettering is done in a very legible style and color combination. Often we must read signs from a moving vehicle—or the sign itself is on a moving vehicle. These factors affect the time we have to read the sign and introduces other distractions that can reduce this time.

Let’s look at a couple typical scenarios. First, assume you are driving a vehicle at 30 mph on a city street. You’re moving at 44 ft. per second.

Consider these three possible different locations of your vehicle relative to a sign that is on a building at the corner of an upcoming intersection:

At 220 ft. from the intersection, you have five seconds to view the sign

At 132 ft. from the intersection, you have three seconds to view the sign

When you reach the intersection, there are two possible events: You either go through the intersection on a green light with no viewing time at all or you are stopped by a red light and have a long viewing time.

A driver may not be aware of the sign at all during the travel space of 220 ft. He or she may be looking only at the road or may be looking in another direction—say looking for a street sign or a business address. To catch the eye, the message on the sign must be direct and to the point, with minimal copy.

What letter size will be necessary for the driver to read the sign—if he or she chooses to do so—from 220 feet? From 132 feet? Again, this is critical information if your customer expects the sign to be read.

Now consider you are driving on a highway at 60 mph. You’re moving at 88 ft. per second. This drawing indicates two potential locations for your vehicle when approaching the sign:

At Point A the viewer is 440 ft. from the sign with five seconds of reading time

At Point B the viewer is 264 ft. from the sign with three seconds of reading time

Now review the table below, which shows what size lettering you can read as you pass by a sign at various speeds. Remember that all readability charts assume a dark letter in a simple, legible style on a light background. Depending on letter style and color, you may have to increase the letter size to insure readability at these distances.

At a viewing distance of 440 ft., the smallest lettering of the sign’s primary message should be 16 in. or more in height, with strong contrast to the background color. If the viewer chooses to read the secondary copy, he will be closer in a few seconds and will be able to read the smaller copy. Again, the total message must be direct and to the point with minimal copy.

Of course, there may be more than one sign at this site, which means less reading time available for the viewer to read any one sign. Or the viewer may be concentrating on the traffic, passing or being passed, and thus have no reading time at all. Getting the attention of viewers is always a challenge.

Test the design from a distance that is to scale

When you create a design and present the drawing to the customer, the proper approach is to estimate the typical viewing distance at that scale and look at the drawing from this distance. If more sign designers started doing this, the effectiveness of signs in general would skyrocket. Most likely, the primary message on most signs would get larger.

In the early 1950s, the standard poster size for printed-paper outdoor advertising was approximately 12-ft.-high-by-24-ft. long—a ratio of 1 to 2. The sketches were typically done at a scale of 1⁄2 in. equals one ft.

The designers often used a simple formula for reviewing a sketch or drawing of an outdoor advertising sign before making it into a full-sized printed poster: Viewing the sketch from about 17 ft. away simulated seeing the sign at a typical viewing distance of about 400 ft. So looking at the drawing for three to five seconds gave them an idea of what the view could be expected to read.

Here is a set of guidelines based on that traditional formula that you can use to review your drawings at a viewing distance that relates to the scale of the drawing:

Here’s a sign we see every day

Consider a STOP sign on a 35 mph street: white lettering, white border, red background, done in all reflective material. The word STOP is the message. The message is enclosed in a panel.

You can read a 10-in. letter at 250 to 300 ft. Thanks to the designers who developed the STOP sign, you have seven to eight seconds to respond to the message and hit the brakes.

Good designers know this matters

Information about letter sizes required for readability, viewing time and how to look at a drawing from the proper distance is vital knowledge for the sign designer. The customer usually has never even thought of them. They look at the sign design on a drawing or computer screen from 20 inches away, study it for a minute or two, and decide that it is what they want. It’s likely that no viewer will ever look at the sign that way and for that long. We need to educate our customers.

The factors that go into a successful sign are these: Words, Panel, Colors, Reading Distance.

For fun, we can put this information to a formula:

That equation reads like this:

WORD(s) divided by PANEL(s) over COLOR(s) times DISTANCE equals SOLUTION

When designing signs, if the SOLUTION is not right—that is, the sign doesn’t work because it isn’t readable from the viewing DISTANCE—we must change the WORD/PANEL/COLOR portion.

That’s what makes us designers. We don’t just come up with a clever or creative layout—we consider its use so that the sign does its job for the client.

Ken Millar spent many years as a sign designer in the Chicagoland area and was also the instructor at the International Brotherhood of Painters and Allied Trades sign school in Chicago.

SignCraft, January/February 2011