Many RTA users express concerns about traveling I-90 across Montana during winter as their expectations are that a more southerly route provides more assurance of avoiding ice and snow. I imagine such inquiries are included in a meaningful portion of Fall & Winter RoadTrip posts.
Experienced RoadTrippers know staying south (for example, along I-80, I-70, or even I-40) provides a largely false sense of security inasmuch as snow and ice can be found anywhere along the Continental Divide and adjacent areas simply due to the high elevations associated with the Divide and the Rocky Mountains and the Colorado and Wyoming Plateau region. In New Mexico, for example, I-40 crosses the Great Divide at some 7,700'.
I've often commented favorably about the I-90 route through Montana due to the generally modest elevations found along I-90: From Billings in the east to the ID border in the west, I-90 generally follows the Yellowstone River, the headwaters of the Missouri, or the Clark Fork River, and elevations are as low as 3,100' at Billings and 2,640' downstream from Missoula.
My favorable comments often mention "only 3 passes" provide higher elevation and higher likelihood of encountering snow and ice: Bozeman, Homestake, and Lookout Passes, and I further note the relatively short interval of roadway at higher elevation. I'm here now to elaborate a bit, add to the database, and correct some earlier statements.
Having just completed my Summer 2010 RoadTrip through the area, I decided to take a close look at the overall picture as would relate to Winter travel through Montana along I-90, which I readily admit I've never undertaken (Yet!). It's easy to argue, and I do, for a 4th pass along I-90 in Montana, although it lacks the formal name and topographic distinction as a major pass. That distinction matters little when the highway jumps up 1,300' from 4,000' to 5,300' over a several mile stretch between MP 274 at US 287 and the Cardwell exit west of there, at MP 256 and MT 2 and 359. We can refer to this high spot as the Boulder Cut-Off Pass, since I-90 jumps over a north-trending ridge between the Boulder River on the west and the Madison River on the east, and since one of the sideroads on the topos is Boulder Cut-Off Road.
I'll further elaborate on Homestake Pass and vicinity, to the east and west of Butte. Homestake Pass is the highest pass in Montana, by far, at 6,600'. Homestake Pass "features" around 12 miles of roadway at over 4,500', from the MP 241 all the way to Butte itself. In fact, one travels all the way west to the MP 208, some 20 miles west of Butte, before elevations fall below 5,000'. For the next 100 miles west to Missoula, elevation declines steadily to 3,200', then to 2,650 further west before a long climb to 4,500' at the Idaho line at Lookout Pass.
In my humble opinion, adding the Boulder Cut-Off Pass to the other 3 and an improved delineation of the distance along which the highway is above 5,000' doesn't change the overall suitability of the route as compared to I-80 across Wyoming. Wyoming's segment of I-80 enters the state at 5,050' of elevation on the Nebraska border, reaches nearly 8,600' between Cheyenne and Laramie, and from Laramie all the way to Evanston near the Utah border holds elevations of 6,500-7,500'. There are a number of long stretches of I-80 without exits or services, too. All of this probably leads to the frequent closures of I-80 during winter storms, events routinely emblazoned on the flashing overhead highway signs on I-80 east of Salt Lake City, UT, signs I see regularly in January during ski trips to Park City. The analysis of I-90 in Montana does mean, however, that the interval from around 25 miles west of Bozeman all the way to Deerlodge and a bit west of there bears a close examination for local conditions as we traverse it. All told, it's a good 100 miles at or near 5,000' which includes Homestake Pass near its east end. The good news is that there are quite a number of exits for year-round towns and cities along the segment, and that the Montana DOT webcams show the passes and roadways are kept clear during all but the worst of storms, and finally that the roads are usually cleared within a handful of hours of the "dumping" of snow.
Plus, the Wheat Montana Bakery and Deli at the east end, at the junction of I-90 and US 287 makes a delightful place to hole up for a while as the snow removal proceeds. I suggest the breakfast wrap or a fresh-baked cinnamon bun, with their bucket-sized mug of hot coffee.
Safe Travels during the Montana winters!