Hello Ann,
I am fortunate to spend a week to 10 days in Park City each January. Our trips always involve a number of trips to and through SLC as we fetch and deliver other visitors from and to the airport, go to the REI store, and go to the ski areas in the Cottonwood Canyons. While we've encountered plenty of traffic during rush hour, it's never risen to Los Angeles, San Francisco, DC/Baltimore, or Atlanta levels.

There is a beltway all the way around SLC--I-215. The worst of the traffic seems to be inbound and outbound from the north and south along I-15, where the geography of the valley is that it's bounded by a wall of mountains on the east and the desert to the west, so suburban sprawl has taken place to the north and south. If you can time your trip to avoid joining the fray inbound and outbound, you should be fine.

I would most certainly not detour on the route you suggested, particularly in winter. Much of that route is +2,000' in elevation above the SLC basin, and the snow flies early and often up there.

Foy