There's some pretty big delays expected for the area between Big Sky and West Yellowstone.  Here's the official announcement.

“Road Construction has started at the Red Cliff Campground area on Highway 191 between Big Sky and West Yellowstone.  Commuters can expect delays of up to 15 minutes Monday – Saturday. Rock excavation may cause closures of 191 lasting up to one hour at milepost 42.  River closures in the Red Cliff vicinity will be in effect during rock excavation.  Construction at Red Cliff Campground to continue through the end of May.  Commuters can also expect delays of 15 minutes through the Greek Creek Campground vicinity and Moose Creek Campground while clearing operations take place.  Please watch the variable message signs located along Highway 191 for a more detailed schedule of construction activities.”