I took a trip this afternoon down to the ballpark site, and decided to walk across the Frederick Douglass (South Capitol Street) Bridge to test this whole "fans can walk across the bridge to the ballpark" concept.
I discovered that it is definitely a reasonable walk -- only took me about 10 minutes -- but its not the most pleasant stroll in the world. Cars and trucks go by pretty quickly; at one point, the side of a cement truck came within about 2 feet of my head. It was also very windy on the bridge, making it awfully unpleasant on chilly days like today.
What's more, getting across the bridge is one thing, but it's quite a bit further to get over to the Anacostia Metro station, where fans would presumably park their cars. In the future, parking at Poplar Point would make the walk a little shorter. But for now, I don't see the point of parking at Anacostia and walking when you can simply hop on Metro for one stop to the Navy Yard.
In the coming days, I'll be testing various routes to and from the stadium. I think I will test out the walk to the ballpark from the Capitol South Metro station next.