The frequently repeated "fact" that United States highways were designed to work as emergency runways is little more than an urban legend, despite its prevalence (a lot of people seem to know about it) and its longevity (no one seems to know when it actually originated, but it can be traced to legislation that dates back to the 1940s). At first glance, the idea seems like both common sense and a total head-scratcher. Of course a military plane should be able to land on a nice, wide road in an emergency! But then what about all the roads that are too curvy or too hilly or have an otherwise unsuitable landing surface? And what about the cars and trucks that are probably already on the highway with no way of being warned of this emergency?
The people who originally thought up this urban legend accounted for some of those questions. For example, the whole road isn't supposed to be suitable for emergency runway use; just one mile out of every five miles. This ratio is supposedly enough to account for turns, elevation changes and densely populated areas. And like all good urban legends, this one does have a historical basis. The one-in-five-mile rule actually goes back to misinterpretations or misquotations of the Defense Highway Act of 1941, the Federal-Aid Highway Act of 1944 and the Federal-Aid Highway Act of 1956, depending on the source.
Though all three of these laws are real, none actually contain such language. The Defense Highway Act provided flight strips, mostly for military use, that were near highways. The first Federal-Aid Highway Act came close to including another flight strip program but ultimately did not, and it wasn't part of the second Federal-Aid Highway Act, either [source: Weingroff].
So no such law has ever been passed, and furthermore, such a strategy wouldn't be practical. Since using a highway as a runway would only happen in the kind of unprecedented theoretical emergency in which planes wouldn't even have time to divert to the closest commercial airport, there's absolutely no way local law enforcement could close and clear the highways quickly enough to provide a safe emergency landing.
What's the plan, then, if there's ever an emergency that requires airborne planes that are unable to reach their destinations to land immediately? Small municipal and private airports are the most obvious solution. Military bases are another option. There are actually little-used and little-known (that is, not for public commercial travel) runways all over the place that are a logical location for an emergency plane landing in a war or terrorist situation.