Quite safe. The drivers are put through a rigorous training program on how to handle different conditions. They are even trained on a skid pad to teach them how to handle spinning out. It is a 6 week training program.
The interstate highways are much better than you probably imagine in winter, particularly if based on news reporting or Hollywood characterizations which always distill the worst conditions to leave the impression that it is all horrible.I ve spent many a winter driving from ski area to ski area chasing reports and forecasts of maximum fresh snow. That should correlate to lousy driving conditions, but it doesn t. Freeways are generally bare pavement most of the winter, particularly in areas with adequate traffic, since the auto action blows snow away, and also there will be snowplows where accumulation is frequent. The snowiest part of driving to a sequence of winter ski areas is each ski area s driveway.The worst parts of your journey will be over the Washington Cascades and then the Siskiyou Mountains in Southern Oregon, a total of 30 or 40 miles of potentially snowy road. Piece of cake.