It is determined by the 25th amendment
To keep the price of us automobiles competitive with others
B. The Gadsden Purchase was the last major land acquisition prior to the Civil War. In 1853, the ten million dollar purchase made by U.S. ambassador James Gadsden completed the continental U.S. as we know it. Initially, the strip of land was viewed as necessary for the completion of a southern trancontinental railroad. Eventually, the Southern Pacific Railroad was constructed here.