The growth of suburbs in the united states following world war ii caused

The growth of suburbs in the United States following World War II caused  massive governmental expenditures on roads programs.

Population growth after World War II was a cause of expansion of cities into suburbs.  The prices of homes in suburbs were more affordable to middle class families, due to lower land prices and new building practices like tract housing.  (The song, "Little Boxes," sang about those kinds of homes, row after row of the same sorts of construction.)

With the growth of the suburbs, improvement of roadways became a priority.  Highway improvement was also a priority of President Eisenhower for the sake of national security.   The Federal-Aid Highway Act passed in 1956 allocated $26 billion (in 1956 dollars!) to a monumental road-building effort that created the interstate highway system.  

The growth of the suburbs can be viewed as a good thing or a bad thing. It was good in that it was part of a dynamic picture of economic growth and prosperity in America.  But suburban culture had the tendency to segregate white Americans in the suburbs from blacks in the cities' inner core neighborhoods, leading to racial segregation and inner city poverty issues that we're still dealing with today.

4.a depression is the answer