There are some highways in Northern Virginia (including some that are pretty close to DCA) that are carpool only all the time. Driving in the district is easy in that the lettered and numbered streets make it easy to locate any address (paying attention to NW, SE, etc.) but the diagonal streets named after states and the traffic circles can be very disorienting.

Be aware of the Metro track work plans and resulting disruptions to service. Other than this, I would have unambiguously suggested Metro unless you have lots of luggage.