Mobility, and the transportation infrastructure needed to enable it, is foundational to American culture and
economic activity, and one of the hallmarks of modernity is the relative ease of acquiring automobiles.
Coupled with this is the effect of rapidly increasing population in major cities that has led to the development
of suburban settlements far away from centers of commercial activities. With industrialization and
localization of industries, movement of freight is mostly by trucks. The attendant outcome of all these is the
high rate of mobility, which has led to a spiraling pressure and congestion on transportation infrastructures,
especially road transportation.
Traffic congestion on the highway is occurs when traffic demand approaches or exceeds the available
capacity of the highway system. Though this concept is easy to understand, congestion can vary significantly
from day to day because traffic demand and available highway capacity are constantly changing. Traffic
demands vary significantly by time of day, day of the week, and season of the year, and subject to significant
fluctuations due to recreational travel, special events, and emergencies (e.g. evacuations). Available highway
capacity, which is often view as being fix, also varies constantly, being frequently reduced by incidents (e.g.
crashes and disabled vehicles), work zones, adverse weather, and other causes (Federal Highway
Demand for highway travel by Americans continues to grow as population increases, particularly in
metropolitan areas. Construction of new highway capacity to accommodate this growth in travel has not kept
pace. Between 1980 and 1999, route miles of highways increased 1.5 percent while vehicle miles of travel
increased 76 percent. The Texas Transportation Institute estimates that, in 2003, the 85 largest metropolitan
areas experienced 3.7 billion vehicle-hours of delay, resulting in 2.3 billion gallons in wasted fuel and a
congestion cost of $63 billion (Annual Urban Mobility Report, 2009). In addition, traffic volumes are project
to continue growing. The volume of freight movement alone is to nearly double by 2020. Congestion may be
as a big city problem, but delays are becoming increasingly common in small cities and some rural areas as
well. In 2003, over 62,800 kilometers (39,000 miles) of highways in the United States had peak period
congestion, and of these, over 10,900 kilometers (6,800 miles) were in rural areas (FHWA, 2009b).