A route established between busier airports to increase system efficiency and capacity. IFR clearances are issued on the basis of such routes except when severe weather avoidance procedures or other factors dictate otherwise. Preferred IFR routes are correlated with standard instrument departures (SID) and standard terminal arrivals (STAR) and may be defined by airways, high level airways, or direct routes between NAVAIDs or waypoints, radials or distance measuring equipment (DME) fixes, or any combination thereof.