President Eisenhower authorized the construction of the Interstate Highway system in the US. I forget the exact stipulation, but I seem to remember that after every curve there was required to be a straightaway of at least 6000 feet in order to land an aircraft should the need arise. That wasn't to accommodate an engine failure of course. The Defense Department figured they'd need the roads in case of a conflict with Russia.
It's a myth. Most of the military planes of that era needed more than 6000 feet to operate.
One Mile in Five: Debunking the Myth - Vol. 63· No. 6 - Public Roads