If a sign is to be effective, then it must be legible at enough of a distance to help a driver respond. Generally, letters with a taller and wider “stroke” can be read from further away. These are the recommended letter heights to enable legibility at various speeds of highway traffic:

Speed of Traffic	Recommended Letter Height (inches)
55 mph (88 kph)	16.5
50 mph (90 kph)	15
45 mph (72 kph)	13.5
40 mph (64 kph)	12
35 mph (56 kph)	10.5
30 mph (48 kph)	9
25 mph (40 kph)	7
Signs for Success Seminar Handbook, Nevada SBDC, 2001.

Letters of this height at these speeds should be legible to a driver for six seconds. However, if your business appeals mostly to tourists, they might need more reaction time because of unfamiliarity with your area. The longer the reaction time required, the greater the recommended letter height.