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Passenger Vehicle Traffic on the National Highway System: 2011
Road congestion is one of the major causes of travel time delay and negatively impacts transportation system reliability. In 2011 peak-period congestion resulted in traffic dropping below posted speed limits on 13,500 miles, or 6 percent, of the National Highway System. This congestion created stop-and-go conditions on an additional 8,700 miles of road.
U.S. Department of Transportation, Federal Highway Administration, Highway Performance Monitoring System and Freight Analysis Framework, version 3.4, 2013.