Years ago I learned that our national highway system has built-in runways for emergency landing strips. Is this still true?
Not only is this not "still true," it was never true in the first place. This is a classic urban legend. Many tellings of the legend claim that one mile in ten has to be a straight shot, so that it can be used as a landing strip in times of domestic emergency.
The interstate highway system was derived as part of the Federal-Aid Highway Act of 1956. This was one of President Dwight Eisenhower's greatest achievements as president. While the Federal-Aid Highway Act deals in part with the layout of the Interstate highway system, there is no text in the bill that suggests that the highways could double as airplane runways, and there is no edict that one-mile strips need to be straight (for use by airplanes or for any other reason).
Sure, there are stretches of various highways that are straight, but that's because of the lay of the land and the logistics of traveling from point A to point B, not because they serve an alternativee purpose.
This myth might have originated because of World War II. In 1944 (before the Federal-Aid Highway Act), Congress considered using federal highway funds to build landing strips next to some highways. The idea was never to clutter the highways by allowing planes to land on them, but to build airstrips next to some major highways. (The highways themselves, naturally, would have been used to move troops and supplies to the landing strips.) However, the bill that contained this suggestion was quickly dropped, and it's never been proposed again.
If interstate highways were to be used as airplane runways, no doubt they would have been used as such on September 11, 2001. As it became clear that the U.S. was under attack, the government had an urgent need to get every airborne plane on the ground immediately. Yet there were still no planes landing on our highways.