Hawaii has four Interstate Highways built to freeway standards. Since Alaska and Puerto Rico's Interstates are not built to freeway standards, Hawaii is the only place is the only place not in the Lower 48 to have freeway interstates.   

The four Interstates are numbered H-1, H-2, H-3, and H-201. H-1 is the longest and most important of the freeways, passing close to downtown. H-2 connects Honolulu and Wahiawa. H-3 connects Honolulu to the Marine Corps base on the North side of the island through a series or viaducts and tunnels. The project cost over $1 billion, and is criticized for its expensivesness, mostly due to the elevated roadway, which was built to avoid a burial ground. H-201 serves as a bypass of H-1 as the latter goes past the busy airport.

This video shows an aerial view of Interstate H-3. Note the elevated road deck and tunnels