ADAIR, Iowa — The blue-and-white highway sign for the eastbound rest stop near here displays more than the standard icon of a person in a wheelchair, indicating facilities are accessible to people who can’t walk. The sign also shows a person standing behind a horizontal rectangle, preparing to perform a task. The second icon signals that this rest area along Interstate 80 in western Iowa has a bathroom equipped with a full-size changing table, making it an oasis for adults and older children who use diapers because of disabilities. “It’s a beacon of hope,” said Nancy Baker Curtis, whose 9-year…