The US Department of Transportation reported to the Department of Defense when they were building the interstate highway system to make many sections of it capable of converted to air bases in short order (at least one location every 300 miles).
You happen to have a good source for that? Because I've seen the "one airstrip every X km" story being called an urban legend several times, but I've heard the same about the manholes for demolition charges on German bridges (which definitely DO exist), so I'm curious what the true story is.