It is legal to ride a snowmobile on a road or highway as long as the road is covered with snow and has been closed to motor traffic during the winter. Snowmobiles can cross roads and highways as long as the crossing is at 90 degrees to the highway, the snowmobile makes a complete stop first, and the snowmobile yields to motor vehicles.
Check out these Washougal, WA ATV/OHV resources:
