A person on my Facebook has asserted that "We have interstates because Ike sold them as back up runways in case an enemy destroyed military bases."
I've heard that we designed the Interstate system to have enough straightaways that military aircraft could land on them, but I had always thought that that was mostly urban legend (emergency landings need a lot more than a straightaway, they need the right direction, enough clearance, etc.). His statement got me to thinking that I don't know the full story, so I thought I would ask. What was the intent behind the interstate system? Was it designed with military needs in mind? Was that the primary focus?
Serving as landing strips for aircraft is a myth, but there was a national defense component to the Interstate Highway System.
In 1919 Eisenhower participated in an Army Convoy across the US. https://www.eisenhower.archives.gov/research/online_documents/1919_convoy.html Ostensibly the purpose of the trip was to see how well military equipment could be transported cross-country by road. However, it showed how bad roads were in general compared to those in Europe. This trip left a major life-long impression on Ike.
During WWII, Eisenhower also saw first hand how quickly military forces could be transported over the German Autobahn.
While trying to build support in Congress for the Interstate Highway bill, the Defense Department came out in support of the plan and stated it was important for military mobilization. During testing of possible road surfaces and dimensions of curves, onramps, etc.; the Defense Department provided military equipment and they monitored and gave input for the Interstate standards.
So, while the Interstate Highways were primarily a civilian project for civilian purposes, national defense was a secondary purpose and was used to help get it passed in Congress.
You can read more about the history of it here: https://www.fhwa.dot.gov/publications/publicroads/96summer/p96su10.cfm