Saddle Road (state/county route 200) is the closest thing to a cross-island highway on the Big Island. It is a famously bad and dangerous road despised by most rental car companies, which generally prohibit tourists from taking their vehicles on the road, making this highway perhaps the only paved state highway in the United States that is open to motor vehicles but off-limits to most rental cars. The road was built as a gravel road during World War II to provide access to an Army training area in the Big Island's interior, and also as an inland evacuation route if Japanese forces attacked the Big Island. The road was paved in 1949. At least the western ten miles feels like it hasn't been maintained or improved much since. However, it is an interesting road for intrepid travelers, with its close-up views of the Mauna Kea and Mauna Loa volcanoes and the lava fields covering the "saddle" between the two. It also takes travelers to side roads to the observatory complex on the Mauna Kea summit, and a weather observatory high on the north slope of Mauna Loa (which also is one of the two trailheads for the arduous hike to the Mauna Loa summit).

The state is realigning and improving Saddle Road (and perhaps ultimately will extend it to join state route 19 on the Big Island's west coast), which should make the road less "exciting," and more rental-car friendly, without diminishing the experience of driving through the stark landscapes of the Big Island's interior. These plans picked up speed after 9/11, when the Defense Department offered to pick up the entire tab for the part of the project realigning the road away from the Army base, to improve base security and reduce conflicts between civilian and military traffic. Construction on that phase of the road improvements is underway. The first new segment (on a new alignment between mile 28 and mile 35) opened in late May 2007. The next phase of the project reconstructed the existing road between mile 19 and mile 28, and was completed in 2008. A later phase, between mile 35 and mile 42, reroute the rest of the highway passing through the Army base. Improvements between mile 6 and mile 19, and realignment of the highway west of mile 42, are in the works, and all the Saddle Road improvements could be completed by 2013. In addition, a recently-completed southern bypass of downtown Hilo makes it easier to get to and from Saddle Road's eastern end.