You probably could anyways without telling the driver, although you won't really see much except some trees and United's new hangar plus like others have said its not enough time to leave the secured area and return. Its a shame you don't have more time, there is a shuttle bus that can drive you to the National Air & Space Museum: Udvar Hazy Center, which is about 3 miles south of Dulles and it has an observation tower where you can see Dulles Airport, Blue Ridge mountains, etc.