Legibility IndexLI

The Legibility Index (LI) expresses over what distance a sign is legible per unit of character height, in feet per inch (ft/in). LI = 30 means that 1 inch of character height is legible up to 30 ft (≈ 3.6 m per cm). The old ‘gold standard’ LI = 50 assumed 20/20 vision; because of an ageing population, LI = 30 is now recommended (20/40 vision).