Baltimore and DC are probably the closest major markets situated next to each other in the country. A lot of times they're grouped into one mega metropolitan area for census proposes. Combined you're looking at a population of nearly 7-8 million people, ie the 4th biggest market in the country. They're closer to each other geographically then San Francisco and San Jose, which are both part of the San Francisco radio market. They're closer then Orlando is to Melbourne and Daytona Beach, which are all part of the same TV market.
Baltimore and DC could easily be turned into one radio market if there were some 100,000 watts signals to be found.