 General aviation is additionally available at several smaller airfields, including Montgomery County Airpark (Gaithersburg, Maryland), College Park Airport (College Park, Maryland), Potomac Airfield (Friendly CDP of Prince George's County, Maryland), and Manassas Regional Airport (Manassas, Virginia).Since 2003, the general aviation airports closest to Washington, D.C. have had their access strictly limited by the implementation of the Air Defense Identification Zone (ADIZ).There is a network of dedicated bicycle lanes around Washington, D.C. and there are 1,300 bicycle racks installed on sidewalks all over the city.