![Crossing sign](https://cdn.statically.io/img/static.wikia.nocookie.net/memoryalpha/images/7/73/Crossing_sign.jpg/revision/latest/scale-to-width-down/180?cb=20160315164600&path-prefix=en)
Two "do not enter" signs above Kirk's head
A "do not enter" sign advised individuals not to enter a specific area.
In 1986, the city of San Francisco used these signs. James T. Kirk and Spock stood near a pair while time traveling back to that year. (Star Trek IV: The Voyage Home)