The woman suffered a fractured spinal column and sternum, her daughter-in-law told the Sun-Times.
Carolyn Janke was returning to Naperville with her husband from a second home they share in Arizona. He suffered a blow to the head and needed more than 70 stitches, the daughter-in-law said.
Police said they didn't know of any previous problems with the sign.
The owner of the McDonald's franchise issued this statement, saying he's genuinely concerned about the victims and is cooperating fully with authorities. The owner, Roy Talker, said it would be "inappropriate to speculate on what may have happened."