When did American suburbs increase dramatically, at a rate of 40 times that of cities? (Between the years 1950s, 60s, 70s, or 80s)

That is because the US economy was the best in the world. They were manufacturing on an unprecedented level so they needed a lot of factories and a lot of workers. The suburbs started developing because the workforce that was larger than ever needed somewhere to live, so people started moving to the suburbs and they were built rapidly.