Yeah, I'm really not sure about this - stories of the strange history of the Eisenhower freeway system are extremely common and there's just a surprising amount of myth around them, like the airplane landing thing that we've seen elsewhere in the thread. It's probably unnecessarily conspiratorial to think that the motivations were anything other than what was officially said (e.g. economic advantages), but I give a certain weight to evacuation as a motive because the vast majority of the efforts made towards survivability were remarkably secretive and it was common, at the time, for the government to use cover stories as justification for survivability efforts. Consider, for example, how FEMA used the FBI as a "front" for the crisis relocation program in order to deny their involvement. It's the kind of secret-keeping that seems really pointless and strange now (how much does it even matter to anyone if it's the FBI or FEMA inventorying relocation sites?), but is an artifact of the ultra-paranoid and minimally accountable atmosphere of the time. Most of these things remain pretty secretive today, not necessarily because there's some ongoing push to keep it a secret, but through the more boring mechanism of once secret, now obscure documents being hard to get declassified.

So I sort of suspect that evacuation was a bigger factor than the government admits, but that's purely speculative. Just like suburbanization there are definitely a lot of reasons the interstates happened.