I'm assuming the rumor was intending for war planes (bombers/fighters, etc) to be landed on interstates. So...if they were able to land on the highway, then what would be done with them? No room to turn around, or no where to park off to the side so others could be landed Yes, an emergency landing could be performed by jets, but it would be risky assuming the winds would allow it. No wires or overpasses obviously.

I remember when I-57 was being built in the mid-'60s thru our area of NE IL that it's true intention was the mass evacuation of Chicago in case of nuclear attack. Might have been relevant then, but now no  intruding country would bother wasting a nuke on Chicago. But, even if it were true, how many vehicles could you get out of town with a few minutes notification?