This argument is tough to swallow. The real push to build interstate highways was in early 1950s and began in earnest in 1956. National defense was only one piece of the justification for the project.
The origins of the interstate highway system in defense is well-documented in the historical record, and the title of the bill authorizing it was the "National Interstate and Defense Highways Act":
There were of course other civilian concerns as well, but Eisenhower's arguments at the time of the plan primarily cited the importance of Germany's autobahn system during WW2 as a justification.
