Stop sign has a unique shape for the reason. Humans would recognise it by shape only – and that's the reason for it being unique. Even if it is covered in snow,

Stop sign has a unique shape for the reason. Humans would recognise it by shape only – and that's the reason for it being unique. Even if it is covered in snow, you know that's it is the STOP sign. Similarly, the bottom-up triangle can only mean "yield".