Culturally, the Delaware Valley is taken by various commercial media and advertising venues to mean the Philadelphia metropolitan area, but geographically, geologically, and historically the term is used to refer to the valley through which the Delaware River flows. In geology and geography, a strict sense of the term would incorporate the Delaware River's main drainage basin, and so encompass major tributaries such as the Schuylkill River and Lehigh River and their valleys or sub-basins. These extensions also apply culturally with decreasing degree gradually decreased by proximal distance because the ease of land travel enables a great deal of daily interaction; for example, the large number of commuters who travel daily 45–90 minutes creates cultural blends and parallel values.