The Steepest Highway in Montana Is a Terrifyingly Treacherous Path

Written by Kathryn Koehler
Updated: November 5, 2023
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Key Points:

  • MacDonald Pass has a very steep grade measured at 8%.
  • This particular part of US Highway 12 is very important to transportation in the area, providing much needed trucking routes through the mountains.
  • Treacherous weather conditions can also make the pass even more dangerous.

Descending a steep mountain pass can be a heart-pounding, white-knuckle experience. As you grip the wheel with sweaty palms, the road ahead twists and turns with sheer drops just inches from your tires. Your brakes are your lifeline. Your foot dances between control and caution. The engine roars in protest as you navigate hairpin turns, adrenaline surging with every curve.

The breathtaking scenery unfolds, but your focus remains locked on the winding ribbon of asphalt ahead. Cliffs, canyons, and cascading streams blur past your periphery. MacDonald Pass, the steepest highway in Montana, is a terrifyingly treacherous path. Continue reading to discover exactly how steep MacDonald Pass really is!

Heading down the west side of MacDonald Pass, Montana

The steepest highway in Montana is MacDonald Pass.

©Kevin Michael Connolly/

Where is MacDonald Pass in Montana Located on a Map?

MacDonald Pass is found at an elevation of about 6,312 feet and is a large mountain pass on the continental divide. In fact, it is directly located to the west of Helena, Montana.

It serves as a critical crossing point for U.S. Route 12.

Here it is on a map:

MacDonald Pass

MacDonald Pass is a steep mountain pass located on US Highway 12. Highway 12, also known as the Lewis and Clark Highway stretches across several states, connecting the Pacific Northwest with the Midwest. MacDonald Pass is in the Southwestern part of Montana. The road is a significant transportation route that traverses the Rocky Mountains and provides a critical link between the state capital, Helena, and the city of Missoula. The highway crosses the Continental Divide. The Continental Divide, which runs along the summits of the Rocky Mountains, separates the watersheds that flow into the Pacific Ocean from those that flow into the Atlantic Ocean. At MacDonald Pass, Highway 12 has nearly 8 miles of 8% grade, making it the steepest highway in Montana. A grade of 8% is considered extremely steep.

Continental Divide green highway sign on the side of a two lane highway.

The Continental Divide separates Pacific Ocean watersheds from those that flow into the Atlantic Ocean.

©Jeffrey M. Frank/

History of Mac

Native Americans, fur trappers, and explorers, like Lewis and Clark, used MacDonald Pass in the early 19th century. McDonald Pass is a crucial transportation route in Montana. It is the primary route for motorists traveling between Helena and Missoula, two of the state’s largest cities. The pass provides a direct route across the Continental Divide, avoiding the need to take a much longer and more circuitous route through other mountain passes. Due to its high elevation [5,425 feet (1,654 m)] and steep grade, MacDonald Pass can experience challenging winter weather conditions, including heavy snowfall and icy roads.

Making the Grade

Highway grades, also known as road gradients, refer to the steepness or slope of a section of a road or highway. Grades are expressed as a percentage. The percentage indicates how much the road ascends or descends over a certain distance. Understanding highway grades is essential for designing safe and efficient roads, especially in areas with mountainous terrain. Engineers must ensure that roads are navigable and safe for all types of vehicles, taking into account factors like visibility, braking distance, and vehicle performance capabilities.

Grading Roads

  • Ascending Grade: A positive grade, often expressed as a percentage, indicates that the road is ascending or going uphill. For example, a 5% grade means that for every 100 units of horizontal distance traveled, the road rises five units vertically.
  • Descending Grade: A negative grade, also expressed as a percentage, indicates that the road is descending or going downhill. For example, a -3% grade means that for every 100 units of horizontal distance, the road drops three units vertically.
  • Critical Grade: The critical grade is the maximum sustainable grade for a specific vehicle or road. It’s the steepest incline a vehicle can traverse without losing speed or coming to a stop.
  • Grade Break: A grade break is a transition point where the slope of the road changes. For example, a highway might go from a positive grade (ascending) to a negative grade (descending) at a grade break.
Steep Hill Descent Use Low Gear Traffic Sign on the Road in Mountain

A negative grade indicates that the road is descending or going downhill.


Even More Categories

  • Zero Grade: A zero-grade road is perfectly flat, with no incline or decline. This is rare in real-world road design, as some degree of slope is usually necessary for drainage and other factors.
  • Moderate Grade: Moderate grades have a more gradual incline or decline, usually falling between 2% and 5%. These grades are less demanding on vehicles and drivers than steep grades.
  • Steep Grade: Grades greater than 5% are generally considered steep. Steep grades can pose challenges for vehicles, particularly heavy trucks, as they may struggle to maintain speed or control on uphill or downhill sections.
  • Rolling Grade: Rolling grades describe roads that alternate between short ascending and descending segments. They are characterized by continuous changes in elevation over relatively short distances.
  • Ruling Grade: The ruling grade is the steepest continuous grade on a stretch of road. It is a critical factor in road design because it affects the selection of vehicle types, design speeds, and the need for additional lanes or climbing lanes for slow-moving vehicles on uphill sections.

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

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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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