<Li> September 12, 1991: I - 90 becomes the final coast - to - coast Interstate Highway (Seattle, Washington to Boston, Massachusetts) to be completed with the dedication of an elevated viaduct bypassing Wallace, Idaho . This section was delayed after residents forced the cancellation of the originally planned at - grade alignment that would have demolished much of downtown Wallace . The residents accomplished this feat by arranging for most of the downtown area to be declared a historic district and listed on the National Register of Historic Places; this succeeded in blocking the path of the original alignment . After the dedication residents held a mock funeral celebrating the removal of the last stoplight on a transcontinental Interstate Highway . </Li> <Li> October 14, 1992: The original Interstate Highway System is proclaimed to be complete with the opening of I - 70 through Glenwood Canyon in Colorado . This section is considered an engineering marvel with a 12 - mile (19 km) span featuring 40 bridges and numerous tunnels and is one of the most expensive rural highways per mile built in the United States . Although this was claimed the final section of Interstate Highway to open, at the time this section was dedicated there were still missing interchanges elsewhere in the system, making some Interstate Highways not contiguous . </Li> <P> The initial cost estimate for the system was $25 billion over 12 years; it ended up costing $114 billion (adjusted for inflation, $425 billion in 2006 dollars) and took 35 years . </P> <P> The system was proclaimed complete in 1992, but two of the original interstates--I - 95 and I - 70--are not continuous: both of these discontinuities are due to local opposition, which has blocked efforts to build the necessary connections to fully complete the system . </P>

What is the purpose of the interstate highway system