I've heard that Eisenhower had a requirement for the highway system to have stretches long enough that a commercial airplane could land on it in an emergency. Is this true, if so, was there a specific event that inspired this requirement, or did he just think it was a good idea? Also, has this requirement been maintained over the years as new highways have been built? It seems with overpasses, signs, light/telephone poles, concrete medians, etc, that it would be extremely difficult to it safely, even with no emergency. Is this a true story?
Not true. Somewhat tackled before here: https://www.reddit.com/r/AskHistorians/comments/5mq2tw/whats_the_real_story_behind_the_interstate/
There is a grain of reality behind the story, though. Dispersed basing is a real thing in Europe, with occasional stretches of Autobahn which can be converted to airstrip use. It's a skill which the US Air Force hasn't been practicing in years, though. https://theaviationist.com/2016/06/20/four-a-10-tankbusters-have-landed-on-a-highway-in-estonia-its-the-first-time-since-1984/
However, such strips are in use (even on narrow non-freeway roads) particularly in Scandanavia, with the aircraft in service being selected for their short-field capability: Swedish aircraft are designed from the outset for dispersed basing, and Finland bought Hornets, which have reinforced landing gear compared to land-based aircraft, to deal with the hard landings one usually ends up with from a short strip with no room for a flare.