After world war ii, many people in the united states moved to the suburbs. which form of transportation contributed most to the growth of the suburbs

Automobile transportation on roads and highways.

There was a rise of automobile ownership after World War II.  In 1940, sixty percent of Americans owned cars.  By 1960, eighty percent owned cars.   There also was a huge push toward new highway construction pursued after World War II by the Eisenhower administration.  By 1956, the federal government was paying up to 90% of the cost to construct new highways in states across the country.

An article in The Atlantic (March 18, 2016) by Alana Semuels details how this expansion of road construction not only helped in the solidifying of suburbs, but also contributed to the poverty of African Americans in the cities.  The suburban sprawl wasn't an entirely positive development.