This winter has dumped several feet of snow on Yellowstone National Park’s roads. Getting them cleared for tourists is an undertaking that is not yet done. In the winter, snowmobiling takes over, but in March, the plows start getting the roads ready for car traffic.
If you think you might want to head to Yellowstone to check out the spring flowers and the thaw, find out which roads are open. Supposedly, Mammoth to Norris Junction to Madison Junction to Old Faithful; Madison Junction to West Entrance; and Norris Junction to Canyon are already open according to Yellowstone.net.
These are the road opening dates for others if all goes well. I would call the park office to make sure the section you want to see is one you can get to.
May 2, 2008: Canyon to Lake; Lake to East Entrance.
May 9, 2008: Tower to Tower Fall; Lake to West Thumb; West Thumb to Old Faithful; South to Old Faithful.
May 23, 2008: Tower Fall to Canyon via Dunraven Pass; Long Lake Gate to Red Lodge via Beartooth Pass.