A critical aspect of any sign is viewing distance. The appropriate amount of detail varies greatly, depending on the distance from which the sign will be viewed. In digital printing, this "resolution" is determined by "dots per inch," or DPI. The more closely an image will be viewed, the higher its resolution needs to be, which means the dots produced by the inkjet printer would need to be closer together, and there would be more of them within a defined space.
The same concept applies to electronic message centers. The individual LEDs function the same as the inkjet dots. The more detail you want, the more LEDs you would need with a defined space, and the decision would be based on the anticipated viewing distance. An electronic billboard 600 feet from the highway is different than an electronic message center built into the cabinet of a freestanding pole sign next to a two-lane road.
For electronic signs, this resolution is called "pixel pitch," and it means the distance between the centers of individual LEDs, which are known as pixels. The distance also varies if the individual pixel is color (comprising different-color LEDs) or monochrome (one color). Here are some general guidelines for pixel pitch and viewing distance.
Pitch Range Viewing Distance
3-6mm up to 50 feet
6-12mm 50-100 feet
12-15mm 100-200 feet
15-20mm 200-400 feet
20-30mm 400-800 feet
30-40mm 800-1500 feet
More than 40mm More than 1500 feet
As for the size of letters and viewing distance, the standards for non-electric signs apply similarly -- approximately 1 inch of letter height for every 50 feet of distance from which it would be viewed. This should be coupled with the speed of traffic. Allowing a viewing time of 20 seconds is ideal. Thus, if a car is traveling at 60 mph, the sign should be legible from a distance of 1734 feet. Generally, electric highway signs should be a minimum of 10 x 30 feet.
An article on this topic appeared in the May 2004 issue of Signs of the Times magazine.