This is a fairly common instruction from a controller in my experience, at least when dealing with small aircraft that they know are flying VFR. Traditional VFR

This is a fairly common instruction from a controller in my experience, at least when dealing with small aircraft that they know are flying VFR. Traditional VFR navigation ("pilotage") is mostly based on finding landmarks anyway. Flying in another city with a runway pointed towards a river, "turn after the river," "report crossing the river" are common instructions. Other landmarks popular with controllers include the freeway, the big Marriot, the Amazon warehouse, etc. It's sort of surprisingly informal when controllers are dealing with local light aircraft pilots. But that's mostly how those pilots are navigating anyway, "follow the freeway until the bridge." There probably is a certain risk of misunderstanding here, but then pilots are expected to be reasonably familiar with the local area and its procedures, and to ask for vectors when unsure. These days DC is one of those places that it's strongly recommended (required by FARs now, I think?) to take a familiarization course because the airspace issues there are so sensitive. Plus they have the laser gun to minimize the need to send fighters.