Richard from Washington D.C. on 5/31/2018 4:20:49 PM:
National Airport parking is perfectly safe both for the car you leave there and for bike access. There are bike trails from the airport that connect to the C&O Canal. To get from Union Station to the airport upon your return, you'll have to ride on city streets for a couple of blocks to get to the National Mall where there are big gravel paths to the 14th Street Bridge. But why not park at Union Station at the beginning? It has a big elevated garage. I haven't compared the rates with those at the airport, but if you park there, you can walk your bikes to your car upon your return. It will save you a 40-minute ride to get your car.