R 408.13708 Lettering.

Rule 3708. (1) A letter shall be block style, upper case for the upper panel, and upper or upper and lower case for the lower panel. (2) Letter size shall be determined by the length of the message and maximum visibility and readability. See table 1 for a guide for minimum letter height at safe viewing distance. (3) Table 1 read as follows: TABLE 1 Minimum Viewing Height of Distance Letters (Feet) (Inches) 5...............................................200-250 4 1/2...........................................150-200 4...............................................120- 150 3 1/2...........................................105-120 3...............................................90-105 2 1/2...........................................75-90 2...............................................60-75 1 1/2...........................................40-60 1...............................................30-40 3/4.............................................20-30 1/2.............................................10-20 1/4.............................................Less than 10 Note 1: Crowding of letters and words reduces legibility. Note 2: A rule-of-thumb of 35-40 feet viewing distance per inch of letter height may be used for heights of letters greater than 5 inches.