Symbol signs ­– signs without words – are often used to communicate safety or public information messages. The International Organization for Standardization (ISO) provides standards ISO 3864-1 and ISO 3864-3 giving design principles for both graphical symbols and completed signs in the safety arena. The present study shows that for difficult signage circumstances a two-part sign, showing the desired and undesired circumstances with a tick and a cross, can be much more effective than the normal single-part sign provided for by these ISO standards. The present study also suggests that ISO’s sign assessment methods could be efficiently combined and simplified.