Greenbelt Road/University Blvd where it goes up and over the railroad tracks. The ROW carries the Metro Green Line, as well as MARC, Amtrak and freight traffic. There are a few alternatives to the no-shoulder bridge for me, but they are much less direct and slower.

So long as you don't mind rubbing elbows with traffic that can get up to 50mph or more, it's not really that bad--though it does have its moments...