From what I remember, the Yellowstone tour busses hit the road at 8:30. Most other tourists started loading their cars about 9:30, and the traffic jams started shortly after 10:00. Four hours of clear roads in the morning if you leave early enough.

OTOH, almost everybody is off the road by 5:30. It's almost impossible to get into a restaurant between 5:30 and 8:30.