Wet ice and ice that is dry are equally slippery. Empty trucks have less wind resistance. Empty trucks have more traction. Empty trucks can bounce and lock up their wheels. They usually have as much braking power when empty as when loaded.
Empty busses have more traction.
They have no wind resistance. Thick layers of ice that contain mud and other debris. Ice that remains only when the temperature has risen to above freezing. A thin layer of ice so clear you can see the road underneath it. You may be trapped when you need to change lanes.