Is this what you're referring to? https://wiki.openstreetmap.org/wiki/Michigan/Trails#Snowmobile_Trail/Routes

If so, much of the shoulder of this highway is ATV trail that later becomes snowmobile trials. There are even bridges (e.g. 47.5301497, -94.9002472) for the ATV/snowmobile trail. Where the trail meets Frontage Road Northwest, it makes sense to tag this line as a relation. Along Voyageur Highway, would it make sense to map it as an ATV trail (i.e. highway=track??) and also add snowmobile=designated?