According to an article in this week’s New York Times, the first center line came in 1911 in Michigan. The first electric traffic signal was in 1915 in Cleveland. The first proper stop sign , also 1915, appeared in Detroit. It wasn’t the 8-sided red sign we’re all used to; it was square with black letters on a white background.
In 1923 the Mississippi Valley Association of State Highway Departments thought up some recommendations about street signs, their colors, their shapes. They recommended a Stop sign with 8 sides, but their color of choice was yellow.
The red sign didn’t come along until 1954. Red had been the preferred color much earlier (after all, stop lights are red), but a good, reflective red paint did not exist until the early Fifties. A non-reflective yellow, on the other hand, could be seen better at night.
So I’ve had to be careful in my novel, set in 1925, not to mention red stop signs. In fact, considering how slowly ideas spread back then, I’ve concluded that there were few stop signs at all in the mid-Twenties, expect perhaps in the larger cities.