The Interstate Highway Act of 1957 was the largest public works investment in US history, and basically built our entire national highway system. While it was done for military reasons, to move troops and supplies quickly from east to west and north to south, it also created hundreds of thousands of jobs, made transportation more efficient and gave rise to suburbs and cities. It greatly expanded our economy.
From a purely selfish point of view - securing our empire as we did from 1950 - 1990 also secured us the most consistent and reliable access to oil of any nation on Earth. 25% of all oil pumped out of the ground each day comes to the US, and we still pay only $3 a gallon for gas, as compared to $7 - $8 in most European countries. This has allowed us to live cheaply, to own lots of cars, to vacation and to expand industry.