The park is huge, it is bigger than the states of Delaware and Rhode Island combined. Besides its sheer size and thousands of things to see, another thing to be aware of and consider is the roads. They are in poor shape, although the park service is working on improving their condition. The speed limit in the park is 45 mph maximum, with many areas of 35 to 25 mph limits.
Wildlife is all along the roads and sometimes on the roads creating bear jams or buffalo jams.
With the speed limits, road conditions, construction , traffic and wildlife, do not expect to go anywhere fast in Yellowstone. The roads in the park make a large figure 8, known as the Upper Loop, the Lower Loop and the Grand Loop. The entrances are open 24 hours 7 days a week during the normal tourist season - mid Apr to the beginning of Nov. You do not need reservations to enter the park. Your park entrance pass is good for seven days and is good for both Yellowstone and Grand Teton National Park.