At first glance, the mile marker seems to imply that the highway is only 64 miles long but Montana's State Highway 200 startes at the Idaho border near Troy Montana and crosses through 14 Montana counties on its way from the western side of Montana to it's terminus on the eastern side at the North Dakota border just outside of Fairview Montana. At 706.624 miles, it is the longest state highway in the country, beating the second longest in California, State Route 1 (655.84) and the fourth longest in Texas, State Route 16 (541.8); the third longest route is in Oklahoma, State Highway 3 (615).
There are places to pull off the highway near the cache but it can be hard to get back on, especially with the heightened amount of work trucks and tractor trailers operating in the Bakken Oil Field. I recommend parking at the city park in Fairview and taking the short walk to the cache but staying clear of the roadway.