Why are stop signs octagons instead of round or square?

Stop signs are (with very few exceptions) very distinct -- bright red face, large white letters... and they're always the ...
Reader's Digest

Here’s What It Means If You See a Blue Stop Sign

Color us surprised: Blue stop signs really exist! Here's where you'll find them, what they mean and what to do if you see one on the road. In a world of confusing road signs, there’s one that’s never ...