Heading south from the Baltimore beltway, you could take I97 south (towards Annapolis). Pick up Route 3/301 & take it south across Route 50 & into Southern Maryland. There are traffic lights, but it sure beats the Washington International Speedway - oops, I mean, the Washington Capital Beltway

. Proceed through Waldorf & LaPlata, MD & cross the Potomac River at the Potomac River Bridge into Dahlgren, VA. The bridge, I believe, is $3.oo to get out of MD

. The best prices for gas are just over the bridge in Dahlgren. Continue on 301 to, I believe, Route 207 & pick up I95 well south of DC, just north of Richmond.
If you take the Cape May ferry, then you can pick up 301 on the Eastern Shore & take it across the Chesapeake Bay Bridge, through Annapolis & head south on 301 about 10-15 miles west of Annapolis.