We're discussing the decision-making of people in the US in the 1950s, correct? If communists take control of a dense urban area and throw up barricades to block the more narrow streets, there is military usefulness in having a freeway passing through that urban area.
It is also theoretically useful in case of a railway strike.
This was the 1950s. If it was even slightly conceivable that Communists would take a city somehow, after an amphibious landing across a distance the world had never seen, and the city had not already been reduced to rubble by incoming ICBMs or strategic bombing, it would have been destroyed by a stay-behind nuclear mine.
The Communists were big on revolution. As in, from the inside, done by the workers of the target country themselves, usually with a lot of foreign help but no full-blown foreign invasion. It may seem implausible now, but many countries had already fallen to revolutions, and it was not unreasonable at the time to think it might happen in the U.S.
It is also theoretically useful in case of a railway strike.