One of the great urban legends is that one mile in every five of U.S. interstate highways is completely straight. The myth goes that under the Eisenhower Interstate Highway System, these straight stretches of road would be used as improvised airstrips in times of crisis or times of war. This is a definite myth, but it has certainly inspired air forces in other nations to employ the concept.
Air bases are among the most vulnerable targets in war, and depending on how close you are to international rivals, the prospect of losing an air base to a military strike is a very real possibility. As such, an airstrip baked into the design of a highway is a must.
These were first built late in WWII by Nazi Germany. Following the war, they were also built in many places in Europe during the Cold War. These were built in East Germany and West Germany, Sweden, Switzerland, Poland and Czechoslovakia.
Other nations that use highway airstrips as part of their strategy include North Korea, Taiwan, Singapore and the former USSR. Here is a clip from the Polish film Na Niebie i Na Ziemii (In the Sky & on the Ground), where Polish Mig-21s use a highway landing strip for operations:
The key hallmarks of many runway strips are medians that are not grass, but cement, and minimal space between the two traffic directions. The space is typically split by jersey barriers (the Russians probably do not call them that), which can be quickly removed, creating one large landing zone out of the two traffic lanes.
Despite the fact that many stretches of highway in the United States look long enough to land a plane on (and sometimes a plane will land on one in an emergency), it is not part of the U.S. highway plan, but is certainly part of the strategy in other nations.