An important difference in the nature of the ATFM problem in the United States and in Europe should also be noted. In the United States, ATFM is primarily driven by airport capacity constraints, whereas in Europe en route airspace acts as the principal “bottleneck.” Europe’s Central Flow Management Unit, located in Brussels, currently determines (heuristically) ground delays to ensure that no en route sector capacity constraints are violated. This difference may, however, become moot in the near future due to continuing progress in increasing en route airspace capacity in Europe.