A Track Density Plot graphically depicts the frequency of aircraft movements over a defined area during a specified time period.  It is an effective tool to illustrate arrival and departure corridors, or changes in flight procedures.  A grid map is defined for the region and each grid is color-coded based on the number of times (frequency) an aircraft passed through the grid.

Track Density Grid Colors are based on Aircraft Frequency Count:
            

IAD Dulles International - Quarterly Track Density Plots: