Flow Map

http://www.lib.utexas.edu/maps/texas.html

(Texas - Truck Freight Flow Map (Combined Domestic and International), 1998 (U.S. Federal Highway Administration)) <- click this on the list)

A Flow map is a mixture of a map and flow charts that show the movement of objects from one location to another. Flow maps are utilized to show movement of almost anything from weather to commercial products. Flow maps indicate flow of the variable, the direction of this flow, the source of this flow and the destination of this flow. The thickness of the flow line depicts how much of the variable is being depicted.  Flow maps are very simplified and generalized as they show little else than the flow paths from one point to another. The flow map shown above shows the combined truck flow from Texas. The map shows little else, but doesn't have to because it effectively shows the combined truck flow throughout the United States.