Late 1950s under president Eisenhower. The full name of the system is "The National Defense and Interstate Highway System".
The last stretch of the original Federal Highway Act (FAHA) of 1956, interstate 105 in Los Angeles, was finished in 1993.
They made it easier for people to live in the suburbs.
Eisenhower
President Dwight D. Eisenhower encouraged the building of this system.