The Washington, D.C., Metroplex will be the first section of U.S. airspace to employ three parallel satellite-based arrival routes, with one for each of the major airports serving the U.S. capital, Dulles (IAD), Reagan (DCA) and Baltimore/Washington (BWI). The parallel optimized profile descents (OPD) enable aircraft arriving from the northwest to descend to the runway using one continuous arc in place of the traditional staircase descent method. The OPD into Baltimore/Washington opened in November, joining the existing OPDs into Dulles and National. Satellite-based departures are also being rolled out at the same three airports. The OPDs help cut flight time, fuel burn and carbon dioxide emissions, as well as reducing the voice communications between air traffic controllers and pilots normally required using the staircase descent method. The Department of Transportation estimates airlines will burn at least 2.5 million fewer gallons of fuel each year in the skies above Washington, while emitting at least 25,000 fewer metric tons of carbon dioxide.