How Big Do the Letters on Signs, Parallel to the Road, Need to Be?

As noted elsewhere on this website, “visual acuity” and “conspicuity” and “cone of vision” are very important for signs, because motorists must be able to detect signs, read them and then react to them in a few seconds. So how much does the visibility change when a sign directly faces the driver (perpendicular to the road) versus one that may be flat against a building fascia (parallel to the road)?

The Larson Transportation Institute at Penn State University studied this phenomenon in 2006 through a grant from the United States Sign Council. Much of the work involved a literature review of prior, related research.

Several factors impact the necessary minimum size for letters, so that they can be detected and read quickly enough for the driver to react:

  • Number of lanes of traffic
  • Distance from the road to the sign
  • The motorist’s speed
  • The angle at which the sign can be read

USSC has published a chart that ranges from a sign that’s 10 feet from the road when the motorist is in the curb lane, versus a five-lane road in which the sign is 400 feet from the road. In the first instance, the sign’s letters need to be at least 4 inches tall. In the latter instance, the letters need to be a minimum of 90 feet tall.

This study, entitled “On-Premise Signs: Parallel Sign Legibility and Letter Heights, may be accessed at

What Does “Conspicuity” Mean for Signage?

Conspicuity for signage is determined by the contrast between the sign and its background. A sign must be conspicuous first, because, without it, the sign’s legibility and readability are moot points. While the appropriate size for signs is addressed on this website under the heading “How big should a sign’s letters be?”, conspicuity includes factors that only indirectly relate to size.

Anything that impedes the ability to detect a sign impacts its conspicuity: Trees, telephone poles, parked trucks and other signs are physical objects, and other factors include lack of illumination at night, driving west at sunset or east at sunup.

The two types of conspicuity are search and attention. Search conspicuity occurs when the motorist is actively looking for a gas station, food, lodging, etc. Attention conspicuity involves unexpectedly important signage, such as construction of “lanes closed ahead” signs. Generally, signs are three times more likely to be seen in search mode than attention mode.

“Cone of vision” is another major factor for conspicuity. The Southern California College of Optometry determined, for a motorist, their peripheral vision while still looking straight ahead while driving, is a maximum of 10 degrees left and right. Consequently, a sign’s distance from the road, called the “setback,” is absolutely critical in terms of conspicuity.

For example, at the same 10-degree viewing angle, a sign that’s 42 ft. from the road “disappears” from view when the driver is closer than 240 ft. If the sign is only 5 ft. from the road, at the same viewing angle, it remains in the driver’s sight until within 30 ft.

The Larson Institute of Penn State University, from 10 studies it has conducted between 1996 and 2010, has calculated a formula for determining when signs come into view in relation to setback:  L = D x 0.176, where L equals 10 degrees of “lateral offset” and D is the distance in feet of the sign at initial detection. Thus, if initial detection distance from the sign is 300 feet, 10 degrees of lateral offset would be 52 ft. Note that this offset is from the driver’s eye position, and not from some variable point. This is included in a 2015 publication called the United States Sign Council Best Practice Standards for On-Premise Signs. Go to

Similarly, Penn State has calculated how tall a sign should be, based on the distance from which it would be viewed. Using five degrees of vertical elevation, plus 3.5 ft. representing elevation of the average driver’s eye position above the road, a calculation of vertical sign-height limits, capable of providing comfortable detection over both long and short ranges, can be derived from the following equation:

H = D x .088 + 3.5, where H equals the sign-height minimum and D equals the distance in feet from the sign at initial detection. This is also included in the 2015 USSC publication.

Thus, if initial detection distance from the sign is 400 ft., the sign height must be at least 38.5 ft.

Signs that are perpendicular (facing) to the motorist, such as freestanding and projecting signs, are significantly more conspicuous than signs that are parallel to the road, such as fascia signs or letters attached to a building.

Penn State studied this phenomenon and found that 50-60% of the parallel signs weren’t even seen by motorists, even if they were three times bigger than corresponding perpendicular signs. Overall, perpendicular signs are generally four times more conspicuous than parallel signs.

Penn State has also calculated to what extent signs are blocked from view at specific distances, based on both the number of lanes of traffic, the amount of traffic and the sign’s setback. Numerous tables are included for the variables in the 2015 publication: which of the four lanes the motorist is in, the speed of travel, etc.

In a worst-case scenario, when a driver is in the curb lane of a four-lane highway, and the traffic flow is 1200 cars per hour, and the sign is 10 ft. off the road, a sign might be blocked from 77% of the drivers.

How Big Should a Sign’s Letters Be?

Signs need to be legible and readable, for both pedestrians and motorists. But the safety consideration becomes paramount for the latter. Consequently, the Federal Highway Administration (FHWA) sets minimum standards for the letters that appear on the interstate signs that say “Cincinnati” and “Second St.” and “Next Exit.” These standards are outlined in the FHWA-produced Manual of Uniform Traffic Control Devices (MUTCD). The exhaustive manual specifies the minimum size for every type of interstate/freeway sign. For example, a three-digit exit sign with a single-letter suffix, such as Exit 105A (designated as E1-5P 2E.31 in the MUTCD), must be a minimum of 156 x 30 in.! (13 x 2.5 ft.)

The formulae vary, but they’re based on four primary factors:

  • Distance to the viewer
  • The motorist’s speed
  • The angle from which the sign would be viewed
  • The time necessary to detect and read the sign

So if you know that the care is traveling at 70 MPH, and you know a motorist needs 2 seconds to read the sign (but 5.5 seconds to react/maneuver) , and you know to what degree the driver must turn his/her head to detect/read the sign, you can calculate how far the car will travel while the driver reads the sign, how far away the driver will be when he/she first needs to detect the sign, and then you can calculate how large the letters must be. (Font , upper/lower case,  day/night and the contrast between the letters and the sign’s background are also factors.)

Richard Schwab, who served as chairman of the Transportation Research Board, calculated Minimum Required Legibility Distances (MRLD). For example if a car is traveling at 55 mph, a sign must be legible from a distance of 440 ft. in order to be detectable and readable. At a traveling speed of 25 mph, the MRLD drops to 200 ft.

For highway signs, the standard is that each letter in a sign must be at least 1 in. tall for every 40-50 ft. of viewing distance. Thus, in our 55-mph example above, a sign that’s 440 ft. away would need to have individual letters at least 11 in. tall.

And, then, the overall size of the sign would need to consider the number of letters, and the appropriate percentage of background (or “negative space”), which is generally considered to be 60% of the overall size. Also, the further away the driver is from the sign, the higher the sign needs to be to be legible.

Again, per Schwab, signs on an urban freeway need to be 75 ft. tall. On roads with a 55-mph traveling speed, signs must be 50 ft. tall. But if the traffic speed is 25 mph, signs only need to be 12 ft. tall.

The Pennsylvania Transportation Institute of the Pennsylvania State University conducted 10 sign-related studies between 1996 and 2010. In terms of viewing distance and visual acuity, it has calculated Viewer Reaction Time average in simple environments for pre-sign maneuver is 8 seconds; and for post-sign maneuvers, 4 seconds. In complex or multi lane-environments, the pre-sign maneuver average advances to 10 or 11 seconds, respectively, and the post-sign maneuver average advances to 5 or 6 seconds.

This is included in a 2015 publication called the United States Sign Council Best Practice Standards for On-Premise Signs. Go to

In this same publication, Penn State has also calculated Viewer Reaction Distance. The distance between the viewer and the sign at the point of initial detection determines the letter height necessary for the viewer to acquire and understand the message. By converting Viewer Reaction Time to Viewer Reaction Distance, a relatively precise calculation of initial detection distance can be established.

Viewer Reaction Distance, expressed in feet, can be calculated by first converting travel speed in miles per hour (MPH) to feet per second (FPS) by using the multiplier 1.47.

FPS = (MPH) 1.47

Viewer Reaction Distance (VRD) is then calculated by multiplying feet per second by the Viewer Reaction Time (VRT).

The following is the resultant equation:

VRD = MPH x VRT x 1.47

The contrast (distinguishing between the sign’s copy and the background) is also impacted by the color combination. The optimum combination is black letters on a yellow background. The standard for federal highway signs, white copy on a green background, ranks at #8.

Legibility is also better for words in a combination of upper- and lower-case, rather than all capital letters, so this affects visual acuity as well.

Penn State has similarly calculated a Legibility Index that accounts for variations in visibility based on the combination of background and foreground colors. This is also included in the 2015 publication.

Overall, on average, Penn State has calculated that the ratio of necessary sign height to viewing distance is 1 in. for every 30 ft. Thus, if a sign must be readable from 300 ft., its letter must be at least 10 in. tall.

(Similarly, Dawn Jourdan, the associate professor and director of regional and city planning at the University of Oklahoma, in her 2014 evidence-based sign code, also uses an average formula of 1 in. in letter height for every 30-ft. distance from which the sign would be read.)

Also, Penn State has created a 10-step succession of calculations to determine appropriate sign size:

  1. Determine speed of travel (MPH) in feet per second (FPS): (MPH x 1.47).
  2. Determine Viewer Reaction Time (VRT).
  3. Determine Viewer Reaction Distance (VRT x FPS).
  4. Determine Letter Height in inches by reference to the Legibility Index (LI):  (VRD/LI).
  5. Determine Single Letter Area in square inches (square the letter height to obtain area occupied by single letter and its adjoining letterspace).
  6. Determine Single Letter Area in square feet: Single Letter Area in square inches/144.
  7. Determine Copy Area (Single Letter Area in square feet x total number of letters plus area of any symbols in square feet).
  8. Determine Negative Space Area at 60% of Copy Area (Copy Area x 1.5).
  9. Add Copy Area to Negative Space Area.
  10. Result is Area of Sign in square feet.

Are On-premise Signs Important to Shoppers?

Better Homes & Gardens magazine conducts annual surveys with its subscribers as part of The American Grocery Shopper Study™. Over a three-year period (2011-2013), questions were added about the importance of on-premise signage. Here are the three-year summaries (presented chronologically) of “yes” responses to specific statements:

“I have driven by and failed to find a business because the signage was too small or unclear.” 49.7%, 60.8%, 64.0%

“I have been drawn into unfamiliar stores based on the quality of their signs.” 29.0%, 35.8%, 34.8%

“I have made quality assumptions based on a store having clear and attractive signage.” 34.5%, 41.5%, 42.2%

For another set of statements, respondents could answer “agree,” “neutral” or “disagree” (presented chronologically).


“One of the first things I notice about a new or unfamiliar business is the signage outside its building.”

2013 76.0%, 19.1%, 4.9%

“In addition to identifying a business, signs can convey the personality or character of the business.”

2012: 85.7%, 11.9%, 2.4%

2013: 83.9%, 14.5%, 1.6%

“The letters on signs should be large enough for passing motorists to read at a glance.”

2012: 90.9%, 8.4%, 0.7%

2013: 91.4%, 7.8%, 0.8%

“I get frustrated and annoyed when signs are too small to read.”

2012: 81.5%, 13.7%, 4.8%

2013: 83.0%, 13.9%, 3.1%

“Smaller signs are generally more attractive than larger signs.”

2012: 13.5%, 52.1%, 34.3%

2013: 14.1%, 51.3%, 34.7%

“Uniformity of signage within a business district looks attractive, but makes businesses harder to identify at a glance.”

2012: 62.7%, 30.15, 7.2%

2013: 69.5%, 26.9%, 3.6%

The survey also asked respondents: “What make signs difficult to read?” In order of importance, their answers were:

The letters are too small (83.3%)

The placement of the sign makes it hard to see (71.4%)

The sign is not sufficiently lit at night (63.6%)

The color of the letters does not stand out from the background (60.3%)

Digital signs change the message too fast (52.6%)