During the Jim Crow area, many towns and cities put up signs that read

  N*****, Don’t Let the Sun Set On You in $CITY_NAME

African-American laborers could come into town for work during the day, but any who found themselves there after dark were liable to be lynched or otherwise hassled out of town. While African-Americans were the most common target, Asian-Americans[0] and Jews were sometimes the subject of these warnings/threats. The cities that put up these signs and highway billboards became known as Sunset Towns (although in the last 2 decades the term "Sundown town"[1] became more popular[2]).

The Negro Motorist Green Book was a travel guide that was written to help black people plan roadtrips. It acknowledged that "the White traveler has had no difficulty in getting accommodations, but with the Negro it has been different". The Green Book warned about Sunset towns and had pointers for finding friendly lodgings. Sunset towns weren't limited to the Deep South: New England and California also took part in the practice, and Route 66 was notoriously difficult for black travelers.

Afroman immortalized a Californian sundown town and its racial housing covenant in his song "Colt 45"[3]:

Well, it was just sundown in a small white town

They call it East Side Palmdale

When the Afroman walked through the white land

Houses went up for sale

Well, I was standing on the corner sellin' rap CDs

When I met a little girl named Jan, I let her ride in my Caddy

'Cause I didn't know her daddy was the leader of the Ku Klux Klan

[0]: http://1.bp.blogspot.com/_BLlJwLdIRpk/RZ1Bfy6G8CI/AAAAAAAAAB...

[1]: https://en.wikipedia.org/wiki/Sundown_town

[2]: https://books.google.com/ngrams/graph?content=sunset+town%2C...

[3]: https://www.youtube.com/watch?v=SH2ukQdF-0U&t=1m21s