Why does Hawaii have interstate highways?
Hawaii has three interstates - H1, H2, and H3 - which connect important military facilities on the island of Oahu.
The real name of the interstate highway system is "The Dwight D. Eisenhower National System of Interstate and Defense Highways", the Defense part of this name means they are intended primarily as Military Highways for the transportation of military equipment and troops in times of war and other emergency. Therefore such highways do not actually have to be "interstate" if they serve a valid "military" purpose. In fact many of the specifications for these highways derive directly from military equipment in the US inventory during the 1950s, when the system was in planning (e.g. the minimum clearance on overpasses comes from the height of the 280mm M65 Atomic Cannon the largest Army artillery piece ever made - in service from 1953 to 1963 it could fire a shell with a 15 kiloton yield about 20 miles).