While most parks have only four seasons, Yellowstone has five. The so-called "Secret Season" happens in early springtime from about mid-March up until the third Friday in April, many of the roads throughout the park are cleared of snow, but aren't yet open to cars. This secret season gives bikers, walkers, joggers, roller-bladers and roller-skiers the chance to travel the roadways without fear of getting run over by the legions of RVs that will descend on the park in summer. Which is not to say your trip is completely without danger. It's still wise to bring along your bear spray.
For road accessbility info: 307-344-2109.