This historic route is an interesting diversion from US Highway 12 on your way to or from Helena, Montana. John Mullan is credited with discovering this pass, although it was frequently traveled by Indians before then. The pass is used today by the BNSF/Montana Rail Link lines.
The road is all gravel except for the Birdseye Road segment on the Helena side. The road offers plenty of room for two-way traffic, but typically suffers from bad washboards and an occaisional rut. With proper driving, just about any vehicle could handle this road. Never-the-less, always be aware of conditions, your vehicle's capabilities, and your driving experience before deciding to drive this road.
At the summit, plan to visit the 'open-air' Masonic Lodge and alter. In 1862, Nathaniel P. Langford and two companions started the Montana Masonic movement at this spot. Langford was also a surveyor for the Fisk Expedition and the first superintendent of Yellowstone Park.1 You will likely be sharing the open hillside here with range cattle.
On September 30, 1911, 19-year old Cromwell Dixon made the first successful flight over the Continental Divide in his Curtis bi-plane. He left Helena, and then crossed over the pass using the Northern Pacific Railroad and the Mullan Road for his bearings. He landed near Blossburg. Sadly, the young aviator died just 3 days later from an airplane crash in Spokane, Washington.2
1Sprague, Marshall. The Great Gates: The Story of the Rocky Mountain Passes. Little, Brown and Company, Boston, 1964, 430-431.
2From four Montana Daily Record articles as compiled by Coleman, Louis C. and Reiman, Leo. Captain John Mullan: His Life Building the Mullan Road as it is Today and Interesting Tales of Occurrences Along the Road. Payette Radio Limited, 1968, 454 - 456.