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 http://www.ussc.org/pdf/USSCSignStandardsJune102015FINALEDITION.pdf
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:
- Determine speed of travel (MPH) in feet per second (FPS): (MPH x 1.47).
- Determine Viewer Reaction Time (VRT).
- Determine Viewer Reaction Distance (VRT x FPS).
- Determine Letter Height in inches by reference to the Legibility Index (LI): (VRD/LI).
- Determine Single Letter Area in square inches (square the letter height to obtain area occupied by single letter and its adjoining letterspace).
- Determine Single Letter Area in square feet: Single Letter Area in square inches/144.
- Determine Copy Area (Single Letter Area in square feet x total number of letters plus area of any symbols in square feet).
- Determine Negative Space Area at 60% of Copy Area (Copy Area x 1.5).
- Add Copy Area to Negative Space Area.
- Result is Area of Sign in square feet.