Well, the name of the region (Piedmont="foot of mountain") provides a clue.
Most Virginians living just east of the Blue Ridge would recognize the term. Piedmont originally referred to the equivalent area between the Alps and the Adriatic Sea in Northern Italy. There are other regions outside Virginia that are also "foothills," including the Arctic Foothills between the Brooks Range and the Arctic Ocean.
Look at these maps, and notice how the Piedmont extends completely through Virginia. However, the northern and southern boundaries of the "cultural region" known as the Piedmont in Virginia may not coincide with the boundaries accepted by the geologists and hydrologists.
Jim Fonseca in his Nine Regions of Virginia identified a Northern Piedmont and a Southern Piedmont-Valley Industrial Zone. In his study, the northern boundary was defined by the western extent of the Northern Virginia metropolis. The James River divided the Piedmont into two zones - and Dr. Fonseca included the industrial area of Roanoke/Blacksburg in the southern unit because of common transportation corridors and economic interests that were more significant than the presence of the Blue Ridge.
The northern Piedmont is a triangle between Washington, DC, Richmond, and Charlottesville - and Virginia's fastest-growing counties are the suburbs of those urban centers. Highways connecting those cities are one way to establish boundaries for the region - I-95 on the east, I-66 on the north, US Route 29 on the west, and I-64 on the south.
Another way to define the regional boundaries is by the planning districts.