In 1967, the population density was still mainly concentrated in Cook County... Much of DuPage, esp. western Du Page, like Naperville, Wheaton, and other towns were still considered "the sticks" back then, because large parts of DuPage were farmland. Same for Lake Co IL as well. Will Co. outside of Joliet was farm country as well. 44 years later, and DuPage has a large population as do all of the other counties around Chicago. I remember going to visit family friends who moved to Lisle in the mid 70's we used to drive I-5 (now I-88) out there, and all I remember seeing was farms, and open land. The area started to really grow in the late 70's through the 80's.
If an EF-4 touched down at say, I-55 and Route 53, and tracked NE even for 10 miles, the damage, and loss of life would be horrendous. If it tracked farther say 15-20 miles, it would be grinding through a very densely populated area not only full of people, but industries as well... I don't even want to think about it.