thumb|290px|Proposed 2009 alignment
Route 200, known locally as Saddle Road, traverses the width of the Island of Hawaii, from downtown Hilo to its junction with Hawaii Route 190 near Waimea. The road was once considered one of the most dangerous paved roads in the state, with many one-lane bridges and areas of marginally maintained pavement. Most of the road has now been repaved, and major parts have new re-alignments to modern standards. The highway is mostly one-lane in each direction, but there are two lanes on the uphill portions. The highway reaches a maximum elevation of and is subject to fog and low visibility. Many rental car companies used to prohibit use of their cars on Saddle Road, but now allow use of the road. The highway experiences heavy use as it provides the shortest driving route from Hilo to Kailua-Kona and access to the slopes of Mauna Loa and the Mauna Kea Observatories.
Route description
Waiānuenue Avenue
The mile marker 0 is posted in Hilo on the traffic signal at the intersection of Waiānuenue Avenue, Kamehameha Avenue and Bayfront Highway at coordinates . The route continues inland along Waiānuenue Avenue to a little over a half-mile past the mile 1 where it veers left onto Kaūmana Drive near Gilbert Carvalho Park. Further along, Waiānuenue Avenue is Rainbow Falls Park (Wai means "water" (fresh) in the Hawaiian language; ānuenue means "rainbow". Thus "Rainbow Falls" is rendered as Waiānuenue).
- Total miles = 1.7 (2.7 km)
Kaūmana Drive
Starting at the “Y” junction adjacent to Gilbert Carvalho Park, Highway 200 continues mauka (uphill) on Kaūmana Drive and provides access to neighborhoods overlooking Hilo. The road is quite narrow and windy with many blind corners, hidden driveways and open drainage ditches. Near mile 4 it passes Kaumana Cave, a lava tube. Just past mile 6 (coordinates ) is the junction with Pūainakō Street Extension, (Hawaii Route 2000), completed in September 2004 as a bypass of the above-mentioned windy sections. The intersection with Ua Nahele Street at mile 8 marks the mauka terminus of Kaūmana Drive.
- Total miles =
Saddle Road
The official start of Saddle Road is at the “T” intersection of Ua Nahele Street at mile 8. This is the last neighborhood through which the route will pass. As it has from its beginning in Hilo, Route 200 continues to climb towards the Humuula Saddle between Mauna Kea and Mauna Loa. The rainforest of the Hilo Forest Reserve and Upper Waiākea Forest Reserve surround the roadway and begin to thin as the elevation increases. Quality of the asphalt surface is quite good on this side of the crest but there are many curves and rises with limited visual distances. There are no tourist services or other infrastructure on the Saddle Road.
A new section starting near mile marker 42 that bypasses Waikii and connects to Māmalahoa Highway near mile marker 14, opened September 7, 2013. At this time, the highway was officially renamed the Daniel K. Inouye Highway in honor of the late senator from Hawaii.
This new section avoids the old route, where from milepost 44, near Kilohana, to the Māmalahoa Highway the road retained its original character, a narrow ribbon of poorly maintained pavement with crumbling edges. On the old section, there are several one-lane bridges, blind curves and hills. It is common for drivers to negotiate the center of the road to avoid the rough shoulders, moving back into the lane only when necessary to pass traffic proceeding in the opposite direction. The route is quite scenic with views of the coastline, the Hualālai and Kohala volcanoes, winding its way across Parker Ranch and through the development of Waikii.
The original western terminus of Route 200 comes at its junction with Māmalahoa Highway (state route 190) toward Kona of Waimea (coordinates ).
- Total miles =
thumb|right|Route 200 westbound near its closest approach to Mauna Kea, at over above sea level.
History
In May 1849, Minister of Finance Gerrit P. Judd proposed building a road directly between the two population centers of the Island of Hawaii. Using prison labor, it started near Holualoa Bay at and proceeded in a straight line up to the plateau south of Hualālai. After ten years only about were completed, when work was abandoned at when the 1859 eruption of Mauna Loa blocked its path.
Although destroyed at lower elevations due to residential development, it can still be seen on maps as the "Judd Trail".
While planning for the defense of the Hawaiian Islands in the wake of the attack on Pearl Harbor, the U. S. Army hastily built an access road in 1943 across the Humuula plateau of Parker Ranch at . Since it was not intended as a civilian road, the simple gravel path was built by the Civilian Conservation Corps and the US Army Corps of Engineers in case of an invasion. Military vehicles of all types and treads traversed the Island for the next three years.
Following the end of World War II in 1945, the Army turned over jurisdiction of the road to the Territory of Hawaii and it was designated "State Route 20". However, the territorial government had few funds to maintain the road, let alone upgrade it to civilian standards. Much of the paving dates from 1949.
About the same time, Tom Vance, who had earlier supervised building a highway up Mauna Loa named for Governor Ingram Stainback, secretly used his prison laborers to start a more direct Hilo-Kona road. He started at a camp (still called "Vance" on USGS maps) which was exactly midway between Hilo and Kealakekua. The road extended in a straight line, heading for the pass between Hualālai and Mauna Loa.
In 1950, the camp caught fire after construction reached . The public refused to allocate more funding when they discovered about US$1 million had already been spent, so the project was also abandoned.
After the islands became the State of Hawaii in 1959, Saddle Road was handed to the County of Hawaii and for many years only minimal maintenance was performed, leading to generally poor conditions and the source of the road's notorious reputation.
Since 1992, there has been increased attention on the road, with efforts to rebuild and renovate the highway into a practical cross-island route. This resulted in repaving some sections and complete rebuilding of others. The entire road is now well paved, and in the uphill sections there is a separate lane for trucks and slower cars.
Future
There are plans to complete a section past Māmalahoa Highway which will intersect the Queen Kaahumanu Highway (state route 19) to support cross-island commuting by tourists and resort employees. The route was changed after the 2006 explain of military exercise areas. This represents a realignment of the island’s traffic patterns and converts it into a modern state highway.