The National System of Defense and Interstate Highways was created in 1956 in concept as a system of superhighways that would connect all major metropolitan areas. Much of the system as originally conceived was completed, and much work today involves maintaining and upgrading the existing system. However, as population has increased, the need for additional high-capacity routes resulted in additions to the network. Many of these routes, such as the Interstate 69 extension to the Rio Grande Valley and the Interstates 73/74 corridor, are identified as high-priority corridors by Congress through the series of surface transportation acts passed during the 1990s and 2000s.