The Steepest Highway in Idaho Is a Terrifyingly Treacherous Path

Written by Kathryn Koehler
Updated: October 17, 2023
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Even the most experienced drivers can feel a surge of anxiety when confronted with a steep highway grade. As the road climbs or descends at a sharp angle, the sensation of the vehicle’s weight shifting and the sight of the road disappearing below or rising dramatically ahead create a palpable sense of unease. The anxiety often stems from a fear of losing control or not being able to effectively navigate the challenging terrain, making it a formidable mental obstacle. The steepest highway in Idaho is a terrifyingly treacherous path. The Old Spiral Highway has an average gradient of 5%, with a maximum 13% gradient. Continue reading to learn more about Idaho’s most challenging highway.

Steep Hill Descent Use Low Gear Traffic Sign on the Road in Mountain

Even the most experienced drivers feel a surge of anxiety when confronted with a steep highway grade.


The Old Spiral Road

The Old Spiral Highway is a historic and scenic road located in the Lewiston-Clarkston metropolitan area in Northwestern Idaho, near the Washington border. It features breathtaking views, particularly of the Snake River Valley. The Old Spiral Highway was originally constructed in 1917 as part of the Pacific Highway. This major north-south route connects the Canadian border to the Mexican border. The most distinctive feature of the highway is its design, which includes a series of tight switchback turns as it climbs the steep canyon walls. These turns were necessary to navigate the rugged terrain, hence the spiral moniker. The highway includes 64 turns in total, allowing it to gain elevation in a relatively short distance.

A New Way

In 1977, construction crews built a new, more modern highway to replace the Old Spiral Highway. This new road, which is part of U.S. Route 95, features gentler grades and is better suited for modern traffic. Though the Old Spiral Highway is no longer the major route it once was, it continues to provide a unique experience for those looking to encounter historic charm and scenic vistas.

A yellow "Truck Brake Check Area" sign on the U S 95 highway in Southern Idaho under a cloudy sky.

U.S. Highway 95 in Idaho contains several steep grades.

©Wirestock Creators/

Understanding Highway Grades

Highway grades are determined through surveying and engineering. Initially, surveyors assess the proposed highway route, taking precise measurements of the existing topography. Engineers select specific points along the highway, such as the beginning and end of steep sections or areas where grade changes are necessary. These points serve as reference markers for calculating the grade. At the chosen points, surveyors measure the vertical change in elevation (the rise) and the horizontal distance between those points (the run). The rise is typically measured in feet or meters, and the run, in miles or kilometers. To determine the grade, engineers divide the rise by the run and multiply the result by 100 to express it as a percentage. For example, if the road rises 100 feet/meters over a horizontal distance of 1,000 feet/meters, the grade would be 10%.

Categorizing Highway Grades

Highway grades, expressed as percentages, indicate the steepness of a road’s incline or decline over a given horizontal distance. Understanding these percentages is important for both road design and driving, as they can significantly affect vehicle performance and safety. Here’s a breakdown of what different highway grades mean:

  • 0% (Level Grade): A 0% grade indicates a perfectly flat road. There is no change in elevation over a specific distance. This is the easiest type of grade for vehicles to navigate.
  • 1% to 2% (Slight Grade): A grade of this caliber indicates a gradual incline or decline. It’s barely perceptible to most drivers and doesn’t pose significant challenges for vehicles. It’s common on highways and roads with gentle slopes.
  • 3% to 4% (Moderate Grade): A moderate grade represents a more noticeable incline or decline. Vehicles may experience a slight decrease in speed when ascending and a slight increase when descending, but these grades are still manageable for most vehicles.
  • 5% to 6% (Steep Grade): Steep grades are quite noticeable and can require additional effort from vehicles. Drivers may need to downshift or use lower gears to maintain speed safely. Braking control becomes more critical when descending such grades.
  • 7% to 10% (Very Steep Grade): Very steep grades present significant challenges for drivers, especially heavy trucks. Maintaining speed can be difficult when ascending, and descending can be treacherous without proper braking techniques. Drivers should exercise caution and be aware of potential overheating of brakes.
  • Over 10% (Extremely Steep Grade): Extremely steep grades are exceptionally challenging. Vehicles may struggle to ascend, and descending can be dangerous due to the risk of brake failure or overheating. Roads with extremely steep grades have runaway truck ramps for trucks whose breaks fail.

Old Spiral Road

With an average grade of 5% (steep grade) and a maximum grade of 13% (extremely steep grade), the Old Spiral Highway is a challenge to navigate. Considered a marvel in its heyday, this highway allowed automobiles to travel with ease. In the early 20th century, the automobile was swiftly replacing buggies and wagons. They required more suitable roads. Gone were the days of rutted, unpaved, muddy routes. However, what goes around, comes around. The Old Spiral Road soon became obsolete. A four-lane section of State Highway 95 now circumvents the need for the old highway. The historical highway is now a scenic route for bikers and hikers. The area hosts an annual I Made the Grade bicycle race up to the summit.

The photo featured at the top of this post is © Wirestock Creators/

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About the Author

Kathryn Koehler is a writer at A-Z-Animals where her focus is on unusual animals, places, and events. Kat has over 20 years of experience as a professional writer and educator. She holds a master's degree from Vanderbilt University. When she is not writing for A-Z-Animals, Kat enjoys puttering in her garden, baking deliciously healthful treats for her family, and playing with her two rescue mutts, Popcorn and Scooter. She resides in Tennessee.

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