Was this maybe what Conelrad was for? (Remember the 640 and 1240 triangles on AM radios?) How else is everyone expected to get the word that they’re to pull off the highway? What’s the point?
What indeed? As a defensive move, pulling off the highway might be of some small use in a conventional war, but perfectly pointless in a nuclear attack. I don’t remember any exercises like that where I lived in 1957 (Oregon) but I was only 8 so I might not have been aware.
Conelrad was to facilitate radio communication from the government to the people in the event of nuclear war, and I guess other possible major catastrophes (invasion from space, giant meteor wiping out the west coast, that sort of thing).
Not just that, but also to make it difficult for Soviet bombers to use radio stations as navigational beacons, supposing that was ever even a thing they’d try.
In an emergency, all stations would go off the air. A select station would then broadcast emergency info for just a couple minutes at a time before going quiet so the next station in the rotation could takeover. In theory, a regular stream of emergency info would be delivered from locations that bounced around.
It basically didn’t work.