A stop sign was a traffic sign which instructed drivers to stop their vehicles briefly.
Stop signs were often located at intersections, so as to avoid collisions with other vehicles.
The cover image of the January 1953 issue of Incredible Tales of Scientific Wonder had a stop sign in it. (DS9: "Far Beyond the Stars")
There were stop signs in the city of Toronto in Ontario, Canada. (SNW: "Tomorrow and Tomorrow and Tomorrow")
The Human colony on Parnassus Beta, built to resemble a town in the old Midwestern United States, included stop signs. (SNW: "Hegemony")