When was the First Stop Sign Installed?
The first stop sign was installed in 1915, in Detroit, though some accounts, including the clipping we have and the University of Michigan's Project on the Automobile in American Life and Society place its first installation in 1914. There are multiple competing claims to who actually invented the stop sign, but its invention is usually attributed to William Eno, a New York businessman also credited with the invention of the one-way street, the traffic circle, and the taxi stand. Eno's version looked nothing like the modern octagon. Instead, it was a 2' x 2' square, with a white background and black letters.
Signage at the dawn of the automobile era was in its infancy, as were traffic laws. There were no speed limits, no lane markings, no directional signage, and often no street name signage. Eno's actual implementation would not stay in place, but the concept would. (Even the concept of a painted line in the middle of the road was revolutionary at the time!)
The forerunner of the modern stop sign was designed by the Mississippi Valley Association of State Highway Departments in 1923. Their stop sign had a yellow octagon with black lettering. The general theory of the Mississippi Valley planners was that the more corners a particular sign had, the more hazard a driver would recognize. Thus, the most dangerous hazard of the 1920s, railroad crossings, received a circular sign, since circles have an infinite number of corners, and the second-most-dangerous hazard, a mandatory stop at an intersection, received an octagonal sign. Ordinary hazard signs were diamond-shaped, and are generally recognizable as the progenitor of modern hazard signs.
The Mississippi Valley stop sign was incorporated into the first federal Manual on Uniform Traffic Control Devices in 1935, and black-on-yellow stop signs remained standard for the next 19 years. In 1954, the Mississippi Valley stop sign was revised to its modern white-on-red form to match the color used in stoplights. The modern stop sign was then adopted internationally as the international standard in 1968 by the Vienna Convention on Road Signs and Signals.