Migrants who left the great plains behind during the 1930s

Migrants who left the Great Plains behind during the 1930s did so mostly due to the Great Depression, which led crop prices to decline rapidly--forcing many farmers in the Great Plains out of work. 

You might think it too high to say 90% ... but it really was 90% of funding for the Interstate Highway system that was provided by Congressional funding.  This was a huge program signed into law during the presidency of Dwight Eisenhower.  It was considered essential to the national interest.  The Federal-Aid Highway Act passed in 1956 allocated $26 billion (in 1956 dollars!) to this monumental road-building effort.