James Loewen, author of “Sundown Towns: A Hidden Dimension of American Racism,” said he has been astounded by his research on the prevalence of sundown towns, all-white communities where unofficial rules forbade black Americans after dark. In some cases, signs posted at the cities’ entrances warned black out-of-towners, “N—–, Don’t Let The Sun Go Down On You.”

“I don’t think this is a case of black paranoia for a minute,” he said.

Loewen estimates that the nation had no fewer than 10,000 locales with these rules.

In particular, black drivers in the North had to be on high alert. Sundown towns were a Northern phenomenon, said Loewen, who continues to locate municipalities with such histories.

“In Illinois, I’m up to 507. In Mississippi, I’m at three,” he said.