Waipio Valley Road is a short steep road in the Big Island of Hawai, restricted to 4x4. It is the steepest road of its length in the United States. The road is steep enough to destroy brakes on the way down, with some 45% grade sections.
The road is difficult and it’s a nightmare in the wet or dark (or both). It links Waipio Overlook at the western end of Honokaa-Waipio Road (state route 240), down into the Waipio Valley, in the Hamakua District. While the road is now paved and only about ¾ mile long, the 25% average grade (said to be up to 45% at some points), taking the road about 900 feet down to the valley floor, is steep enough to destroy brakes on the way down, and stall engines on the way up. The road is therefore restricted to 4x4s (which you'll need anyway to navigate the unpaved roads on the valley floor), and hikers with strong legs. This road is not for the faint at heart. It is a forty-five degree angle all the way down and the only way to make it is with 4WD in low gear. It is a single lane road and folks going down have to yield to folks coming up.
This road has innumerable twists and turns. A paved, one-lane road does provide 4-wheel drive access, only. It is most likely one of the steepest roads in the world with a 25% grade for the entirety of its 900-foot descent, in less than one-mile. Do not even think about taking a regular 2-wheel drive car, as you will burn up your brakes on the way down. As a courtesy, at the very few wider locations in this extremely narrow road, downhill traffic yields to uphill traffic. Once on the valley floor, there are no paved roads. Non-4x4 vehicles are prohibited on this road, even though it is paved. Vehicles must descend in first gear, low range, to avoid brake failure. Driving non-4x4 vehicles into the valley can be, and has been, fatal. Survivors may also be unable to get their non-4x4 vehicles out of the valley without an extremely expensive tow. If you don't have a 4x4, there are tours that will take you into the valley and back, or you can hike on the road (a popular option, at least for the young and healthy).
To drive this road, you must have supreme confidence in your vehicle and your driving skills. The average grade is 25% and the peak grade reaches 40-45%. In some places this valley is steeper than the Baldwin Street, in Dunedin, New Zealand and Canton Avenue in Pittsburgh, which are considered as the world's steepest streets. The Waipio Valley Road looks like a real road, not a street, and offers such a great experience for cyclists. Sometimes it is not recognized as a road because it is open only to 4-wheel drive vehicles, but it is a good paved road well maintained by the government.
The zone is prone to heavy mist and can be dangerous in low visibility conditions. Do not travel this pass in severe weather conditions. Avalanches, heavy snowfalls and landslides can occur anytime, being extremely dangerous due to frequent patches of ice. It has a well-deserved reputation for being dangerous because of unpredictable snowstorms and blizzards, and driving under these conditions, can be extremely challenging.