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The DMV has a few airport options, but DCA is the best domestic choice. DCA is just a short Uber/car/metro ride away from Downtown D.C.
Flight
IAD is your best bet if coming in from abroad. Flying into IAD, you have the option for a longer metro ride into Downtown DC or a simple Uber/drive into the District.
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BWI is the furthest from D.C. (about an hour's drive from Downtown), but another option for those who need one.