Stop signs are the only octagonal (eight-sided) road signs. They
are like that because the government wants them to be easily
distinguished from other road signs, even when viewed from the
back.
The first stop signs appeared in 1915. At the time, they were
square with a white background. “STOP” was written in a black
font. The sign worked until more cars began to crowd the roads
during the 1920s.
The American Association of State Highway Officials came up with
a new eight-sided 'STOP' sign that drivers could easily
recognize, even if they did not understand or could not see what
was written on it.
The old eight-sided stop signs had yellow backgrounds with black
lettering and outlines. The background became red with white
lettering during 1954. They changed to a red background, because
traffic lights were using red to mean 'STOP'.