ISO 3864-1:2011: Graphical symbols - Safety colours and safety signs - Part 1: Design principles for safety signs and safety markings.

Size-to-Distance Principles:
• Emergency signs: Width = 15 × viewing distance, minimum 100mm
• Warning signs: Width = 20 × viewing distance, minimum 150mm
• Mandatory/Prohibition: Width = 12 × viewing distance, minimum 100mm
• Text height = sign width ÷ 8

This international standard ensures visibility at intended distances through scientifically validated ratios for different sign categories.