Not all road signs have lettering on them, which is true in the case of Tetris-shaped road signs. They’re almost always yellow diamonds, which means they’re a warning sign.

Road signs are even designed to overcome barriers, ... type of road, roadway features, sight distances ... Traffic warning notices like the Cross Traffic Does Not Stop sign are used to prevent ...

Road sign warning to drivers. Source: Instarmac, with permission. Used in several EU countries, this sign tells road users that they currently have the right of way.